Condensed Matter Seminars

Join Zoom Meeting:

https://virginia.zoom.us/my/israel.klich

Thursday, November 5, 2020
9:30 AM
Online, Room via Zoom
Note special time.
Note special room.

"Probing the Universality of Topological Defect Formation in a Quantum Annealer"

Adolfo del Campo , Ikerbasque & DIPC
[Host: Israel Klich]
ABSTRACT:

The number of topological defects created in a system driven through a quantum phase transition exhibits a power-law scaling with the driving time. This universal scaling law is the key prediction of the Kibble-Zurek mechanism (KZM), and testing it using a hardware-based quantum simulator is a coveted goal of quantum information science. Here we provide such a test using quantum annealing. Specifically, we report on extensive experimental tests of topological defect
formation via the one-dimensional transverse-field Ising model on two
different D-Wave quantum annealing devices. We find that the quantum simulator results can indeed be explained by the KZM for open-system quantum dynamics with phase-flip errors, with certain  quantitative deviations from the theory likely caused by factors such as random control errors and transient effects. In addition, we probe  physics beyond the KZM by identifying signatures of universality in the distribution and cumulants of the number of kinks and their decay, and again find agreement with the quantum simulator results. This implies that the theoretical predictions of the generalized KZM theory, which assumes isolation from the environment, applies beyond its original scope to an open system.

We support this result by extensive numerical computations. To check whether an alternative, classical interpretation of these results is possible, we used the spin-vector Monte Carlo model, a candidate classical description of the D-Wave device. We find that the degree of agreement with the experimental data from the D-Wave annealing devices is better for the KZM, a quantum theory, than for the classical spin-vector Monte Carlo model, thus favoring a quantum description of the device. Our work provides an experimental test of quantum critical dynamics in an open quantum system, and paves the way to new directions in quantum simulation experiments.

Ref.:
Yuki Bando, Yuki Susa, Hiroki Oshiyama, Naokazu Shibata, Masayuki
Ohzeki, Fernando Javier Gómez-Ruiz, Daniel A. Lidar, Sei Suzuki, Adolfo
del Campo, and Hidetoshi Nishimori, Phys. Rev. Research 2, 033369 (2020)

Special Seminar
Meeting ID: 922 8790 9487Password: HEPseminar

Wednesday, November 4, 2020
10:00 AM
Online, Room via Zoom
Note special date.
Note special time.
Note special room.

"Crystal structure, rotational dynamics and vibrational dynamics of a two-dimensional perovskite"

Xiao Hu , University of Virginia - Department of Physics
[Host: Seung-Hun Lee]
ABSTRACT:

Metal halide perovskites (MHPs) have come to the forefront in the photovoltaic and light-emitting devices due to their attractive optoelectronic properties, such as tunable bandgaps, long charge carrier lifetime, and bright luminescence. The interactions between charge carriers and crystal lattice, such as electron-phonon coupling, polaron formation and low thermal conductivity, are proposed to be the major reasons for their extraordinary device performance. Our previous studies have demonstrated the significant role of the organic cation mobility in screening the excited charge carriers and further extending the charge carrier lifetime in three-dimensional (3D) perovskites. Compared with 3D perovskites, however, 2D perovskites exhibit a one-order-of-magnitude longer degradation time and some of them could have extremely high photoluminescence quantum yields, which make them popular in the LED field. This talk will summarize the experimental investigations on a 2D MHP and examine the relationships between the lattice dynamics and optoelectronic properties in this type of 2D perovskites.

Join Zoom Meeting:

961-0236-3843

Thursday, April 16, 2020
3:30 PM
, Room via Zoom
Note special room.

"Magnetic skyrmions and their applications"

Hamed Vakili , University of Virginia - Department of Physics
[Host: Avik Ghosh]
ABSTRACT:

Skyrmions are topologically protected magnetic quasi-particles. An isolated skyrmion is a metastable state of ferromagnet. The metastable state of skyrmions has a finite lifetime at non zero temperature which depends on energy barrier and attempt frequency. Materials with different symmetry groups can support different kinds of skyrmions (Bloch, Neel, Anti-skyrmion). We will see how these different types of symmetries can be used to control movements of a skyrmion. Skyrmion and domain wall racetracks can be used for temporal memories in race logic. Locally synchronized racetracks can spatially store relative timings of wavefronts and provide non-destructive read-out.

Join Zoom Meeting:

Meeting ID: 775-042-517

Tuesday, April 14, 2020
2:00 PM
, Room via Zoom
Note special date.
Note special time.
Note special room.

"Atomic and Electronic Correlations in the Change Density Wave phase of Dichalcogenides"

Sharon Philip , University of Virginia - Department of Physics
[Host: Despina Louca]
ABSTRACT:

Studies on transition metal dichalcogenides (TMD) is of great significance due to their interesting topological properties and remarkable electronic behavior. Among these materials,1T- TaX2 class of TMDs, where X = S, Se, has spurred considerable interest due to their multiple first order phase transitions between different charge density wave (CDW) states. The effects of CDW formation in these compounds are attributed primarily to in-plane re-orientation of Ta atoms to Star-of-David formation. But this alone doesn’t explain the notable electronic behavior of 1T-TaS2 and 1T-TaSe2 and why they differ from one another despite having the same trigonal symmetry. At very low temperatures, 1T-TaS2 undergoes a metal - insulator transition and is proposed to harbor a quantum spin liquid behavior whereas 1T-TaSe2 remains metallic. Investigating the local structure of pristine 1T-TaS2 and 1T-TaSe2 in the CDW regime could tell us the differences in local atomic correlations in these compounds.

Webinar

Join Zoom Meeting https://virginia.zoom.us/j/5140654378 Meeting ID: 514 065 4378

Thursday, March 26, 2020
11:00 AM
Online, Room via Zoom (Zoom link and meeting ID provided above)
Note special time.
Note special room.

"Investigation of the structural phase transitions in Weyl semimetal (Mo, W)Te2"

Yu Tao , University of Virginia - Department of Physics
[Host: Despina Louca]
ABSTRACT:

Mo1-xWxTe2 belongs to the family of layered transition metal dichalcogenides (TMD) that are of intense interest recently because of their fascinating topological properties. The end members of this series, MoTe2 and WTe2 are Weyl semimetals upon cooling to the orthorhombic Td phase. Mo1-xWxTe2 undergoes a structural phase transition from a high-temperature monoclinic 1T' phase, to a non-centrosymmetric orthorhombic Td phase at low temperatures through a first-order structural phase transition. Both 1T' and Td phases are comprised of weakly-bound layers, and differ mainly by shifts of the layers along the c-axis. Despite much research, the structural properties of Mo1-xWxTe2 have not been thoroughly investigated. Neutron scattering experiments at the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory were carried out on single crystals of Mo1-xWxTe2. Structural changes including changes in interlayer disorder were observed by focusing on the elastic scattering along (2, 0, L) on cooling and warming through the hysteresis loop at the transition. A Td* phase was discovered for the first time across the Td-1T’ phase boundary in Mo1-xWxTe2 with x up to ~0.2. In WTe2, a sharp transition from Td to 1T′ was observed at ambient pressure in the single crystal near ∼565 K, the transition proceeds without hysteresis. These results should clarify in microscopic detail the nature of these phase transitions.

Condensed Matter
Thursday, February 27, 2020
3:30 PM
Physics Building, Room 204

"Design principles of biological and chemical intelligence"

Zhiyue Lu , University of North Carolina at Chapel Hill
[Host: Marija Vucelja]
ABSTRACT:

Living systems respond to external stimuli by utilizing chemical reaction networks that function as cellular information processors. What can we learn from living systems as the design principle of intelligent active materials? One salient example of a biological intelligence is the single-cell circadian clock (i.e. the Kai-ABC oscillator in cyanobacteria). Such circadian clocks process external signal (sunlight intensity) and computes the time during the day/night. These microscopic computers are naturally challenged by two main sources of uncertainty the internal thermal fluctuations and the external noisy signal. To optimize its performance, we find that a clock must make a tradeoff between resisting internal thermal fluctuations and external signal noise. This noise tradeoff relation can be explained through the geometry of its energy landscape. I will also discuss the use of the energy landscape in designing intelligent responses into mechanochemical materials.

Special Seminar

Thursday, February 20, 2020
12:45 PM
Physics Building, Room 313
Note special time.
Note special room.

"Generalized 't Hooft anomalies"

Mohamed Anber , Lewis & Clark University
[Host: Peter Arnold]
ABSTRACT:

't Hooft anomalies provide a unique handle to study the nonperturbative dynamics of strongly-coupled theories.  Although this type of anomalies was known since the 80's, recently it has been realized that one can generalize them by turning on 't Hooft twists in the color, flavor, and baryon number directions.  Such generalized anomalies put severe constraints on the possible realizations of the global symmetries of a given theory in the infrared. In this talk, I will explain how one can construct such 't Hooft twists and give examples of the constrains the generalized anomalies can impose on strongly coupled gauge theories.

Special Seminar

Thursday, February 13, 2020
12:45 PM
Physics Building, Room 313
Note special time.
Note special room.

"Exact results for 5-dimensional superconformal field theories"

Christoph Uhlemann , University of Michigan
[Host: Peter Arnold]
ABSTRACT:

5-dimensional superconformal field theories play an intriguing role in the general understanding of quantum field theory. They provide strongly-coupled UV fixed points for many perturbatively non-renormalizable 5-dimensional gauge theories, and upon compactification they provide new insights into many lower-dimensional theories. This makes them both useful and interesting in their own right. In this talk I will discuss recent progress in the understanding of 5-dimensional superconformal field theories through AdS/CFT dualities and non-perturbative field theory methods, leading to interesting exact results and many cross checks.

Special Seminar

Thursday, February 6, 2020
12:45 PM
Physics Building, Room 313
Note special time.
Note special room.

"New physics in rare decays"

Julian Heeck , University of California, Irvine
[Host: Peter Arnold]
ABSTRACT:

Despite its many successes, the Standard Model of particle physics cannot be the final description of nature at the most fundamental level. Additional elementary particles and interactions are an absolute necessity but have so far evaded our experimental efforts. I will highlight the importance of searches for processes that are forbidden within the Standard Model, as these make for clean signatures of new physics. Important examples are searches for lepton flavor violation and baryon number violation, which will be tested to unprecedented levels in upcoming experiments.

Special Seminar

Thursday, January 30, 2020
12:45 PM
Physics Building, Room 313
Note special time.
Note special room.

"Collinear Superspace"

Gilly Elor , University of Washington
[Host: Peter Arnold]
ABSTRACT:

Soft Collinear Effective Theory (SCET) is a framework for modeling the infrared structure of theories whose long distance behavior is dominated by soft and collinear divergences. SCET is utilized to compute processes in Jet physics, WIMP annihilations, and more. My collaborators and I showed that SCET can be made compatible with supersymmetry, and that such a theory can be conveniently formulated in "Collinear Superspace". In this talk I will introduce a new set of effective field theory rules for constructing Lagrangians in collinear superspace. This new formalism represents a general way to derive on-shell superspace Lagrangians directly from the symmetries of the theory. However, I will also demonstrate how the non-propagating off shell degrees of freedom i.e. F and D terms, can be reintroduced into the theory. This framework paves the way to constructing theories with N > 1 supersymmetry directly from low-energy considerations, and has potential implications for supergravity, the Scattering Amplitudes program, and more.

Special Seminar

Thursday, January 23, 2020
12:45 PM
Physics Building, Room 313
Note special time.
Note special room.

"Duality between Space-like and Time-like Processes in Gauge Theory"

Duff Neill , Los Alamos National Lab
[Host: Peter Arnold]
ABSTRACT:

Since the original papers of Drell-Levy-Yan,  there has been a desire to unite the process of production of hadrons in the final state to the process of probing the structure of a hadronic initial state. Specifically, one wants to relate the process of single inclusive annihilation (SIA) to deep in-elastic scattering (DIS) via crossing and analyticity. It has long been known that any straightforward relation between the two processes fails, however, pursuing this relation has lead to a deeper and richer connection between SIA and DIS, now known as the space-time reciprocity relation (arXiv hep-th/0612247). I shall give an introduction to this space-time reciprocity relation, and argue that the relationship is a consequence of the deeper connection between final and initial state dynamics governed by an underlying conformal symmetry which maps between the two, up-to anomalous terms which should cancel as regulators are removed, but from which one is never free due to the initial conditions of the scaling evolution. As a consequence, I will give a time-like BFKL equation that resums the soft region of the fragmentation function of QCD, which maps to the space-like BFKL equation that governs the soft region of the parton distribution function. Time permitting, I will discuss both possible formal implications and phenomenological applications. This will be a blackboard talk.

Special Seminar

Thursday, January 16, 2020
12:45 PM
Physics Building, Room 313
Note special time.
Note special room.

"From hydrodynamics to quantum chaos"

Saso Grozdanov , MIT
[Host: Peter Arnold]
ABSTRACT:

Hydrodynamics is a theory of the collective properties of fluids and gases that can also be successfully applied to the description of the dynamics of quark-gluon plasma. It is an effective field theory formulated in terms of an infinite-order gradient expansion. For any collective physical mode, hydrodynamics will predict a dispersion relation that expresses this mode’s frequency in terms of an infinite series in powers of momentum. By using the theory of complex spectral curves from the mathematical field of algebraic geometry, I will describe how these dispersion relations can be understood as Puiseux series in (fractional powers of) complex momentum. The series have finite radii of convergence determined by the critical points of the associated spectral curves. For theories that admit a dual gravitational description through holography, the critical points correspond to level-crossings in the quasinormal spectrum of a dual black hole. Interestingly, holography implies that the convergence radii can be orders of magnitude larger than what may be naively expected. This fact could help explain the “unreasonable effectiveness of hydrodynamics” in describing the evolution of quark-gluon plasma. In the second part of my talk, I will discuss a recently discovered phenomenon called “pole-skipping” that relates hydrodynamics to the underlying microscopic quantum many-body chaos. This new and special property of quantum correlation functions allows for a precise analytic connection between resummed, all-order hydrodynamics and the properties of quantum chaos (the Lyapunov exponent and the butterfly velocity).

Condensed Matter
Thursday, December 5, 2019
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Monopole Superconductivity and Density-Wave Order in Weyl Semi-metals"

Professor Yi Li , Johns Hopkins University
[Host: Dima Pesin]
ABSTRACT:

Although the existence of magnetic monopoles is admitted by the fundamental laws, the real monopoles in nature remain elusive. Nevertheless, variations of monopoles appear in realistic condensed matter systems, from quantum Hall effects to ​​topological superconductivity, which spur a race to discover new exotic topological phases of matter. In this talk, we will present a dramatic effect arising from topological Fermi surfaces -- a novel topological class of superconductivity and density-wave ordersWhen the ordered pairs acquire non-trivial two-particle Berry phases, their pairing phases cannot be globally well-defined in the momentum space. Therefore, the conventional description of superconducting pairing symmetries in terms of spherical harmonics (e.g. s-, p-, d-waves) ceases to apply. Instead, they are characterized by topologically protected nodal gap functions represented by monopole harmonic functions. This so-called “monopole harmonic order” is expected to be realized and detected in Weyl semimetal materials.

Condensed Matter
Thursday, November 14, 2019
3:30 PM
Physics Building, Room 313
Note special room.

"Mapping TASEP back in time"

Leonid Petrov , UVa, Department of Mathematics
[Host: Marija Vucelja]
ABSTRACT:

We obtain a new relation between the distributions μ_t at different times t ≥ 0 of the continuous-time TASEP (Totally Asymmetric Simple Exclusion Process) started from the step initial configuration. Namely, we present a continuous-time Markov process with local interactions and particle-dependent rates which maps the TASEP distributions μ_t backwards in time. Under the backwards process, particles jump to the left, and the dynamics can be viewed as a ver- sion of the discrete-space Hammersley process. Combined with the forward TASEP evolution, this leads to a stationary Markov dynamics preserving μ_t which in turn brings new identities for expectations with respect to μ_t. Based on a joint work with Axel Saenz.

Condensed Matter
Thursday, August 22, 2019
2:30 PM
Physics Building, Room 204
Note special time.

"TBA"

Paul Fendley , Oxford University
[Host: Israel Klich]
ABSTRACT:

TBA

Condensed Matter
Thursday, May 30, 2019
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Two-dimensional magnetism and spintronics"

Adam Wei Tsen , University of Waterloo
[Host: Seunghun Lee]
ABSTRACT:

The recent discoveries of ferromagnetism in single atomic layers have opened a new avenue for two-dimensional (2D) materials research. Not only do they raise fundamental questions regarding the requirements for long-range magnetic order in low-dimensional systems, but they also provide a new platform for the development of spintronic devices. In this talk, I will present a series of studies on the family of layered ferromagnetic semiconductors, CrX3 (X = I, Br, Cl), in the atomically thin limit. By incorporating these materials as tunnel barriers between graphene electrodes, we are able to achieve extremely large tunnel magnetoresistance as well as robust memritive switching that is tunable with magnetic field. Tunneling spectroscopy further allows for direct observation of their spin wave excitations, or magnons, from which we are able to derive a simple microscopic Hamiltonian for all three spin systems. These results show that strong exchange anisotropy is not necessary to stabilize ferromagnetism in the monolayer limit.

Condensed Matter
Tuesday, May 28, 2019
11:00 AM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"Flexible Sganac interferometers for the neophytes"

Joseph Avron , Technion
[Host: Israel Klich ]
ABSTRACT:

I shall review the history of Sagnac interferometers and give a geometric description of light rays propagation in flexible optical fibers in Minkowski space. Based on joint works with Amos Ori and Oded Kenneth.

Condensed Matter
Thursday, May 16, 2019
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Tunneling-induced restoration of classical degeneracy in quantum kagome ice"

Professor Ying-Jer Kao , National Taiwan University
[Host: Gia-Wei Chern]
ABSTRACT:

Quantum effect is expected to dictate the behavior of physical systems at low temperature. For quantum magnets with geometrical frustration, quantum fluctuation usually lifts the macroscopic classical degeneracy, and exotic quantum states emerge. However, how different types of quantum processes entangle wave functions in a constrained Hilbert space is not well understood. Here, we study the topological entanglement entropy (TEE) and the thermal entropy of a quantum ice model on a geometrically frustrated kagome lattice. We find that the system does not show a Z2 topological order down to extremely low temperature, yet continues to behave like a classical kagome ice with finite residual entropy. Our theoretical analysis indicates an intricate competition of off-diagonal and diagonal quantum processes leading to the quasi-degeneracy of states and effectively, the classical degeneracy is restored.

Condensed Matter
Thursday, May 9, 2019
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Exciton polarons in two-dimensional organic-inorganic hybrid perovskites"

Professor Carlos Silva , Georgia Tech
[Host: Seunghun Lee]
ABSTRACT:

Owing to both electronic and dielectric confinement effects, two-dimensional organic-inorganic hybrid perovskites sustain strongly bound excitons at room temperature. In this seminar, we demonstrate that there are non-negligible contributions to the excitonic correlations that are specific to the lattice structure and its polar fluctuations, both of which are controlled via the chemical nature of the organic counter-cation. In these systems, organic cations not only serve as spacers between slabs consisting of corner-sharing metal-halide octahedra, but also determine lattice structure by inducing varying degree of distortion of the octahedra via the organic-inorganic interactions. We present a phenomenological yet quantitative framework to simulate excitonic absorption line shapes in single-layer organic-inorganic hybrid perovskites, based on the two-dimensional Wannier formalism. We include four distinct excitonic states separated by 35±5 meV, and additional vibronic progressions. Intriguingly, the associated Huang-Rhys factors and the relevant phonon energies show substantial variation with temperature and the nature of the organic cation. This points to the hybrid nature of the line shape, with a form well described by a Wannier formalism, but with signatures of strong coupling to localized vibrations, and polaronic effects perceived through excitonic correlations. Furthermore, by means of high-resolution resonant impulsive stimulated Raman spectroscopy, we identify vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various low-frequency (≲50 cm−1) optical phonons involving motion in the lead iodide layers. This supports our conclusion that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. Excitonic correlations (exciton and biexciton binding energies) and exciton dynamics (e.g. uni- and bimolecular population decay mechanisms, pure dephasing processes, excitation-induced dephasing, etc.) reflect the polar solvation-like processes induced by organic cation components of the hybrid lattice in a broad structural space. I will address how ultrafast nonlinear spectroscopies yield deep insight on the multiparticle properties in compelx semiconductor materials.

Condensed Matter
Thursday, May 2, 2019
3:30 PM
Physics Building, Room 313
Note special room.

"Worldline approach"

Yuchen Du , University of Virginia - Physics
[Host: Diana Vaman]
ABSTRACT:

Through string theory, people found interesting relations in particle theory. For example, Kawai-Lewellen-Tye (KLT) relation relates the scattering amplitudes of QCD and gravity. However, these kinds of relation are completely mysterious from the point view of Quantum Field Theory since the gravity Lagrangian seems totally unrelated to the Yang-Mills Lagrangian. On the other hand, these kinds of relations are nevertheless true and can be checked by computing the amplitudes using Feynman diagrams order by order. Thus the Feynman diagrammatic expansion does not capture everything of interest, there are still hidden relations between different field theories. Worldline approach, born as a first quantized approach to calculate amplitudes, shares a lot of similarities to string theory.  In this talk, I will show how worldline approach works and how it helps shed some light on the problems we are interested in. I will also discuss the subtlety and limitation of the approach and the possibility of generalizing it to "worldgraph approach".

Condensed Matter
Wednesday, May 1, 2019
1:00 PM
Physics Building, Room 314
Note special date.
Note special time.
Note special room.

"Thermoelectric transport properties of topological Bi-Sb cryogenic materials"

Xixiao Hu , University of Virginia - Physics
[Host: Joe Poon]
ABSTRACT:

Bi-Sb alloys have shown promising thermoelectric (TE) properties at cryogenic temperature (<200 K). Over six decades, the figure of merit zT of n-type polycrystalline Bi-Sb has plateaued at ~0.4, while its p-type counterpart has remained even lower at ~0.1. We have studied the TE properties of melt-spun and spark plasma sintered (SPS) Bi-Sb alloys. We obtained a zT of 0.55 @100-150 K for n-type undoped Bi85Sb15 based on a low thermal conductivity 1.5 W/(m*K) measured with the hot-disk method. For p-type Bi-Sb, doping effects of Ge, Sn, and Pb were investigated. A high doping level of Ge and a high doping efficiency of Pb were obtained with the help of a low-temperature SPS processing. The transport properties (resistivity and Seebeck coefficient) of n-type undoped and p-type doped Bi85Sb15 were analyzed using the two-band effective mass model within the Boltzmann transport theory. A band gap decreasing phenomenon was observed which poses challenges to the improvement of p-type Bi-Sb’s zT.

Condensed Matter
Friday, April 26, 2019
1:00 PM
Physics Building, Room 205
Note special date.
Note special time.
Note special room.

"Topological phases with two-fold spatial antiunitary symmetries"

Meng Hua , University of Virginia - Physics
[Host: Jeffrey Teo]
ABSTRACT:

An interesting theme in topological materials has been classification and prediction of symmetry protected topological(SPT) phases. Despite the Altland-Zirnbauer(AZ) classification under time-reversal symmetry, particle-hole symmetry and chiral symmetry, a system can also be invariant under a combined symmetry composed by two distinct operations. In this talk I will discuss the classification of nodal topological phases under two-fold spatial antiunitary symmetries. We also generalize SPT phases to non-Hermitian system with two-fold spatial antiunitary symmetries and give an example of dissipative topological superconductors.

Condensed Matter
Thursday, April 25, 2019
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"The three-body problem: periodic solutions, topological classification"

Milovan Suvakov , Institute of Physics Belgrade
[Host: Marija Vucelja ]
ABSTRACT:

The three-body problem dates back to the 1680s. Isaac Newton had
already shown that his law of gravity could always predict the orbit
of two bodies held together by gravity, such as a star and a planet,
with complete accuracy. The periodic two-body orbit is always an
ellipse (circle). For two centuries, scientists tried different tacks
to find similar solution for three-body problem, until the German
mathematician Heinrich Bruns pointed out that the search for a general
solution for the three-body problem was futile, and that only specific
solutions that work only under particular conditions, were possible.
Only three families of such collisionless periodic orbits were known
until recently: 1) the Lagrange-Euler (1772); 2) the Broucke-Henon
(1975); and 3) Cris Moore's (1993) periodic orbit of three bodies
moving on a "figure-8" trajectory. Few years ago we reported the
discovery of 13 new families of periodic orbits. Meanwhile, hundreds
of new topologically different solutions have been reported by our and
other groups. We discuss the numerical methods used to find orbits and
to distinguish them from others. Additionally, we found that period T
of an orbit depends on its topology. This dependence is a simple
linear one, when expressed in terms of appropriate variables,
suggesting an exact mathematical law. This is the first known relation
between topological and kinematical properties of three-body systems.

Condensed Matter
Wednesday, April 24, 2019
10:00 AM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"Magnetic Skyrmions on a racetrack"

Hamed Vakili , University of Virginia - Physics
[Host: Avik Ghosh]
ABSTRACT:

Skyrmions are topologically protected magnetic quasi-particles. An isolate skyrmion is a metastable state of ferromagnet. The metastable state of skyrmions have a finite lifetime at non zero temperature which depends on energy barrier and attempt frequency. I will talk about how we are trying to calculate lifetime of skyrmion in candidate Heuslers compounds. Materials with different symmetry groups can support different kind of skyrmions (Bloch, Neel, Anti-skyrmion). We will see how this different types of symmetries can be used to control movements of a skyrmion. Also, we will look at how presence of point defects can effect dynamics of skyrmion, either for movement or nucleation. The ultimate goal is to figure out a compact analytical model for describing skyrmion movement and critical spin current needed for nucleation.

Condensed Matter
Thursday, April 18, 2019
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Spin dynamics in two distinct types of classical spin liquids "

Preetha Saha , University of Virginia - Physics
[Host: Gia-Wei Chern]
ABSTRACT:

Unconventional magnetic states such as spin liquids and spin glasses continue to attract the interest of researchers in magnetism.These materials retain their magnetic disorder even at zero temperatures. We study two different cases of frustrated systems  1)In the case of Kitaev-type models frustration originates from highly anisotropic exchange interactions. We report a new classical spin liquid in which the collective flux degrees of freedom break the translation symmetry of the honeycomb lattice. This exotic phase exists in frustrated spin-orbit magnets where a dominant off-diagonal exchange, the so-called Γ term, results in a macroscopic ground-state degeneracy at the classical level [1]. We show that this phase transition actually corresponds to plaquette ordering of hexagonal fluxes. We also study the dynamical behavior of fluxes.  2) We study the deterministic spin precession dynamics using energy conserving Landau-Lifshitz equation on a geometrically frustrated magnet. The lattice constitutes of a triangular arrangement of bipyramids with classical antiferromagnetic Heisenberg interaction. Such a lattice structure is realized in frustrated SrCr9Ga12-9pO19 [SCGO(p)] compounds [2]. Monte Carlo simulations are used to thermalize the system, which is then used as the initial state for the dynamical studies. We explore the temperature, wave vector and frequency dependence in the dynamical structure factor and the corresponding time dependent correlation functions of the model. Dynamics simulations is further used to estimate the extent to which transport of spin excitations in the lattice conform with phenomenological concept of spin diffusion [1]I. Rousochatzakis and N. B. Perkins Phys. Rev. Lett. 118, 147204 (2017). [2]T. Arimori and H. Kawamura J. Phys. Soc. Jpn. 70, 3695 (2001)

Special Presentation

Thursday, April 4, 2019
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Introduction to Rivanna"

UVa's Advanced Research Computing Services , University of Virginia
[Host: Bryan Wright]
ABSTRACT:

Members of UVa's Advanced Research Computing Services group will be presenting the second of two information/Q&A sessions about rivanna (UVa's supercomputing cluster) on Thursday, April 4 at 11am in Physics 313.

Rivanna is a 7,000-core cluster with more than a petabyte of storage.  It includes a subset of GPU-equipped nodes. Please drop in if you have any interest in high-performance computing.

Slides from the talk can be found here:

http://galileo.phys.virginia.edu/compfac/faq/IntroductionToRivanna.pdf

Condensed Matter
Thursday, March 28, 2019
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Quantum gas microscopy of many-body dynamics in Fermi-Hubbard and Ising systems"

Peter Schauss , University of Virginia - Physics
[Host: Dmytro Pesin]
ABSTRACT:

The ability to probe and manipulate cold atoms in optical lattices at the atomic level using quantum gas microscopes enables quantitative studies of quantum many-body dynamics. While there are many well-developed theoretical tools to study many-body quantum systems in equilibrium, gaining insight into dynamics is challenging with available techniques. Approximate methods need to be benchmarked, creating an urgent need for measurements in experimental model systems. In this talk, I will discuss two such measurements.
First, I will present a study that probes the relaxation of density modulations in the doped Fermi-Hubbard model. This leads to a hydrodynamic description that allows us to determine the conductivity. We observe bad metallic behavior that we compare to predictions from finite-temperature Lanczos calculations and dynamical mean field theory.
Second, I introduce a new platform to study the 2D quantum Ising model. Via optical coupling of atoms in an optical lattice to a low-lying Rydberg state, we observe quench dynamics in the resulting Ising model and prepare states with antiferromagnetic correlations.

Condensed Matter
Thursday, March 21, 2019
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Metal-Insulator Transition in Phase Change Material Ge2Sb2Se5xTe5-5x"

Zhenyang Xu , University of Virginia - Physics
[Host: Despina Louca]
ABSTRACT:

Ge2Sb2Te5 (GST-225) is a phase change material which has wide use in fabricating random access memories. With different quenching process, the GST-225 could have three different phases: an amorphous phase, an intermediate cubic phase, a crystalline hexagonal phase. The fast transition between amorphous and crystalline phase makes GST-225 an ideal material for RAM with fast speed. In our project, the Se-doped GST-225 materials are grown and studied. At x=0.9 in liquid nitrogen quenched samples, we observe a phase transition from crystalline to amorphous. The transport measurement also confirm that there are metal-insulator transition happen for both furnace cooled samples and liquid N2 quenched samples at this limit. A tentative hypothesis is proposed to explain this metal-insulator transition.

Special Condensed Matter Seminar

Wednesday, March 13, 2019
11:00 AM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"Walks, tiles, and zippers: exact holographic tensor networks for Motzkin spin chains"

Rafael Alexander , UNM
[Host: Israel Klich]
ABSTRACT:

The study of low-dimensional quantum systems has proven to be a particularly fertile field for discovering novel types of quantum matter. The tensor network's utility in studying short range correlated states in 1D have been thoroughly investigated. Yet, despite the large number of works investigating these networks and their relations to physical models, examples of exact correspondence between the ground state of a quantum critical system and an appropriate scale-invariant tensor network have eluded us so far. Here we show that the features of the quantum-critical Motzkin model can be faithfully captured by an analytic tensor network that exactly represents the ground state of the physical Hamiltonian. In particular, our network offers a two-dimensional representation of this state by a correspondence between walks and a type of tiling of a square lattice. We discuss connections to renormalization and holography.

Condensed Matter
Thursday, February 14, 2019
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Thermodynamic properties of disordered unconventional superconductors with competing interactions"

Maxim Dzero , Kent State
[Host: Israel Klich]
ABSTRACT:

A topic of interplay between disorder and competing electronic phases in multiband superconductors have recently got renewed interest in the context of iron-based superconductors. In my talk I will present a theory of disordered unconventional superconductor with competing magnetic order. My discussion will be based on the results obtained for on a two-band model with quasi-two-dimensional Fermi surfaces, which allows for the coexistence region in the phase diagram between magnetic and superconducting states in the presence of intraband and interband scattering induced by doping. Within the quasi-classical approximation I will present the analysis of the quasi-classical Eilenberger’s equations which include weak external magnetic field. I will demonstrate that disorder has a crucial effect on the temperature dependence of the magnetic penetration depth as well as critical current, which is especially pronounced in the coexistence phase.

Condensed Matter
Thursday, November 8, 2018
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Fractionalized excitations towards a non-Abelian phase in a Kitaev honeycomb magnet"

Arnab Banerjee , Oak Ridge National Laboratory
[Host: Bellave Shivaram]
ABSTRACT:

The Kitaev model on a honeycomb lattice predicts a special quantum spin liquid (QSL) ground state with excitations resembling Majorana Fermions and gauge flux excitations. These emergent features are exciting prospects to both basic physics and applications towards a lossless technology for quantum qubits.  In this talk, I will describe our recent range of experiments on the magnetic Mott insulator alpha-RuCl3 which has honeycomb layers held together with weak van-der-Waals  interactions. A strong spin-orbit coupling and an octahedral crystal field makes the Kitaev interactions arguably the leading order term in the Hamiltonian. Prominently, despite a long-range ordered ground state, our neutron scattering measurements reveal a continuum of fractionalized excitations resembling predictions from Majorana Fermions, confirming that the material is proximate to a QSL. In a 8T  magnetic field the long-range order vanishes and the continuum becomes gapped, supporting a state where a direct evidence of non-Abelian excitations can be measured. I will describe the present and future endeavors that may help to stabilize the coherent quantum excitations allow a better understanding of the underlying physics, as well as experiments to complete the understanding of the phase diagram of this material.

Condensed Matter
Thursday, October 18, 2018
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Competing orders in a quantum spin model with long-range interactions"

Erhai Zhao , George Mason University
[Host: Bellave Shivaram]
ABSTRACT:

Quantum spin liquids evade long-range magnetic order down to absolute zero temperature. These anarchic, yet highly entangled states break no symmetry but have remarkable properties such as fractional excitations. In this talk, I will first give an example of spin liquid using  a compass model relevant to recently discovered honeycomb antiferromagnet NaNi2BiO6. Then I will introduce a new model, the dipolar Heisenberg model, motivated by recent experiments on artificial many-spin systems based on interacting dipoles. I will argue that long-range magnetic order can be suppressed by simply tuning the direction of the dipoles using an external field. The classical, semiclassical, and quantum phase diagram of this frustrated spin model will be presented to show an extended region where the ground state is a quantum paramagnet. By comparing to DMRG, I will argue that it is likely a quantum spin liquid.

Condensed Matter
Thursday, October 11, 2018
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Transport in Strongly Correlated 2D Electron Fluids"

Alex Levchenko , University of Wisconsin-Madison
[Host: Dmytro Pesin]
ABSTRACT:

In this talk I plan to overview measured transport properties of the two dimensional electron fluids in high mobility semiconductor devices with low electron densities with an emphasis on magnetoresistance and drag resistance. As many features of the observations are not easily reconciled with a description based on the well understood physics of weakly interacting quasiparticles in a disordered medium we will concentrate on physics associated with strong correlation effects and develop hydrodynamic theory of transport. We will apply these ideas to composite fermions of quantum Hall bilayers in hydrodynamic regime.

Condensed Matter
Thursday, September 27, 2018
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Toward the next quantum revolution: controlling physical systems and taming decoherence"

Ed Barnes , Virginia Tech
[Host: Israel Klich]
ABSTRACT:

Recent years have witnessed enormous progress toward harnessing the power of quantum mechanics and integrating it into novel technologies capable of performing tasks far beyond present-day capabilities. Future technologies such as quantum computing, sensing and communication demand the ability to control microscopic quantum systems with unprecedented accuracy. This task is particularly daunting due to unwanted and unavoidable interactions with noisy environments that destroy quantum information in a process known as decoherence. I will present recent progress in understanding and modeling the effects of multiple noise sources on the evolution of a quantum bit and show how this can be used to develop new ways to slow down decoherence. I will then describe a new general theory for dynamically combatting decoherence by driving quantum bits in such a way that noise effects destructively interfere and cancel out, enabling the high level of control needed to realize quantum information technologies.

Condensed Matter
Thursday, September 20, 2018
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

""A materials-driven approach to the novel topological states of matter""

Nirmal Ghimire , George Mason University
[Host: Bellave Shivaram]
ABSTRACT:

Materials in condensed matter have recently been testbeds for several exotic particles, predicted but never realized, in high energy physics. The examples are skyrmions observed in magnetic textures. Weyl fermions in the low energy electronic excitations of Weyl semimetals and Majorana fermions in topological superconductors. These discoveries have not only allowed access to the fundamental physics of the rare particles but also driven large interest in the application of such exotic states to future technologies such as spin based electronics and quantum computation. Discoveries of topological states in materials have largely benefited from the precision of the electronic structure calculations in the weakly correlated systems. In the first part of this talk, I will discuss our resent results on two such predicted materials – 1) NbAs, one of the first generation Weyl semimetals [1-3] and 2) Pd3Pb, a novel topological material hosting multiple Dirac points and surface states [4]. While calculations are pretty accurate in weakly correlated systems, the topological states in presence of strong electron correlations are still not well understood. As such, materials can take a lead in this field. In the second part of the talk, I will briefly highlight our recent efforts in this area, driven by specific materials design criteria. As an illustration, I will discuss our study on the chiral-lattice antiferromagnet CoNb3S6 that has topological character in the electronic band structure, and manifests an unusually large anomalous Hall effect [5].

[1] N. J. Ghimire et al. J. Phys.: Condens. Matter 27, 152201 (2015).

[2] Y. Luo et al. Phys. Rev. B 92, 205134 (2015)

[3] P. J. W. Moll et al., Nat. Communs. 7, 12492 (2016).

[4] N. J. Ghimire et al., Phys. Rev. Materials 2, 081201(R) (2018)

[5] N. J. Ghimire et al., Nat. Communs. 9, 3280 (2018)

Special Condensed Matter Seminar

Friday, August 31, 2018
3:30 PM
Physics Building, Room 204
Note special date.

"The mother of all states of the kagome quantum antiferromagnet"

Hitesh J. Changlani , Florida State University
[Host: Bellave Shivaram]
ABSTRACT:

Strongly correlated systems provide a fertile ground for discovering exotic states of matter, such as those with topologically non-trivial properties. Among these are geometrically frustrated magnets, which harbor spin liquid phases with fractional excitations.

On the experimental front, this has motivated the search for new low dimensional quantum materials and on the theoretical front, this area of research has led to analytical and numerical advances in the study of quantum many-body systems.

I present aspects of our theoretical and numerical work in the area of frustrated magnetism, focusing on the frustrated kagome geometry, which has seen a flurry of research activity owing to several near-ideal material realizations. On the theoretical front, the kagome problem has a rich history and poses multiple theoretical puzzles which continue to baffle the community. First, I present a study of the spin-1 antiferromagnet, where our numerical calculations indicate that the ground state is a trimerized valence bond (simplex) solid with a spin gap [1], contrary to previous proposals. I show evidence from recent experiments that support our findings but also pose new questions. The second part of the talk follows from an unexpected outcome of my general investigations in the area for the well-studied spin-1/2 case [2]. I explain the existence of an exactly solvable point in the XXZ-Heisenberg model for the ratio of Ising to transverse coupling $J_z/J=-1/2$ [3]. This point in the phase diagram, previously unreported in the literature, has "three-coloring" states as its exact quantum ground states and is macroscopically degenerate. It exists for all magnetizations and is the origin or "mother" of many of the observed phases of the kagome antiferromagnet. I revisit aspects of the contentious and experimentally relevant Heisenberg case and discuss its relationship to the newly discovered point [3,4].

[1] H. J. Changlani, A.M. Lauchli, Phys. Rev. B 91, 100407(R) (2015).
[2] K. Kumar, H. J. Changlani, B. K. Clark, E. Fradkin, Phys. Rev. B 94, 134410 (2016).

[3] H. J. Changlani, D. Kochkov, K. Kumar, B. K. Clark, E. Fradkin, Phys. Rev. Lett. 120, 117202 (2018).

[4] H. J. Changlani, S. Pujari, C.M. Chung, B. K. Clark, under preparation.

Condensed Matter
Thursday, May 3, 2018
10:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Gravitational Weyl Anomalies in 1+1-Dimensional Lifshitz Field Theories"

Amr Ahmadain , UVA-Department of Physics
[Host: Israel Klich ]
ABSTRACT:

In this talk, I will explain the notion of anisotropic gravitational Weyl anomalies in Lifshitz field theories and their potential applications. Anomalies are quantum violations of classical symmetries, in this case under a non-relativistic (non-Lorentz-invariant) symmetry group. More specifically, I will focus on the only anomaly found in 1+1-dimensional Lifshitz field theories that arise after coupling to Newton-Cartan geometry with torsion, and it's relation to the Lorentz (diffeomorphism) anomaly in the quantum effective action of a 1+1 CFT. At the heart of this discussion, I will emphasize the role of temporal torsion in generating this anomaly and show how such an anomaly can be derived from a 2+1-dimensional Weyl-invariant Chern-Simons Horava-Lifshitz theory of gravity. I will finally comment briefly on how the 1+1 Lifshitz Weyl anomaly can potentially be connected to thermal Hall transport and some other potential applications.

Condensed Matter
Thursday, May 3, 2018
3:30 PM
Physics Building, Room 204

"Magnetic-Field Induced Weyl Semimetal from Wannier-Function-based Tight-Binding Model"

Kyungwha Park , Virginia Tech
[Host: Jeffrey Teo]
ABSTRACT:

Weyl semimetals (WSMs) have a three-dimensional (3D) bulk band structure in which the conduction and valence bands meet at discrete points, i.e. Weyl nodes. Projections of Weyl points with opposite chirality can be connected by Fermi arcs at a surface. Topological Dirac semimetals (DSMs) have 3D Dirac points which can be viewed as superimposed copies of Weyl points and are stabilized by rotational symmetry. When an external magnetic field is applied to a DSM, Dirac points can be separated into multiple Weyl points and so a WSM phase can be driven. DSMs and WSMs have received a lot of attention because of the chiral anomaly and novel magneto-transport signatures. We develop a tight-binding model based on Wannier functions directly from density functional theory (DFT) calculations for a topological DSM Na3Bi. We add spin-orbit coupling and Zeeman terms in the tight-binding model. Upon magnetic field along the rotational axis, we find that each Dirac node splits into two single Weyl nodes and two double Weyl nodes with opposite chirality, in contrast to  common belief. Our calculations also reveal an interesting evolution of Fermi-arc surface states and other topological surface states as a function of chemical potential in the presence of the external magnetic field.

Condensed Matter
Thursday, April 26, 2018
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Within & Beyond the Realm of KPZ"

Timothy Halpin-Healy
[Host: Genya Kolomeisky]
ABSTRACT:

We discuss significant events in the recent Renaissance triggered by the enigmatic and elusive, but rich stochastic nonlinear PDE of Kardar, Parisi & Zhang,^1 a celebrated equation whose reach far exceeds its grasp, touching such diverse phenomena as non-equilibrium stochastic growth, optimal paths in ill-condensed matter, as well as the extremal statistics of random matrices & increasing subsequences in random permutations.

1. J. Stat. Phys. 160, 794 (2015).

Condensed Matter
Tuesday, April 24, 2018
11:00 AM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"Frustrated Kondo chains and glassy magnetic phases on the pyrochlore lattice"

Jing Luo , UVA-Department of Physics
[Host: Gia-Wei Chern]
ABSTRACT:

We present an extensive numerical study of a new type of frustrated itinerant magnetism on the pyrochlore lattice. In this theory, the pyrochlore magnet can be viewed as a cross-linking network of Kondo or double-exchange chains. Contrary to models based on Mott insulators, this itinerant magnetism approach provides a natural explanation for several spin and orbital superstructures observed on the pyrochlore lattice. Through extensive Monte Carlo simulations, we obtain the phase diagrams at two representative electron filling fractions $n = 1/2$ and 2/3. In particular, we show that an intriguing glassy magnetic state characterized by ordering wavevectors $\mathbf q = \langle \frac{1}{3},\frac{1}{3}, 1\rangle$ gives a rather satisfactory description of the low temperature phase recently observed in spinel~GeFe$_2$O$_4$.

Condensed Matter
Thursday, April 19, 2018
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Crystal Structures and Photoluminescence of a Two-Dimensional Perovskite"

Depei Zhang , UVA - Department of Physics
[Host: Seunghun Lee]
ABSTRACT:

Arguably the biggest challenge of the high-efficiency perovskite solar cells, such as CH3NH3PbI3 and CH(NH2)2PbI3, is their device instability. A recent study of 2D perovskite compounds, butylammonium methylammonium lead iodide perovskite, [CH3(CH2)3NH3]2(CH3NH3)n-1PbnI3n+1, proposed a solution to this problem. This class of materials shows a maximum photovoltaic efficiency of 12.52%, without any obvious degradation over thousands of hours under standard light illumination and humidity test. This talk focuses on the study of temperature-dependent crystal structures, along with the photovoltaic properties of the 2D 1-layer (n = 1) perovskite material. We have performed elastic and inelastic neutron scattering, Raman scattering, and photoluminescence measurements on a powder sample of the 1-layer system ([CH3(CH2)3NH3]2PbI4). Our analysis of the data illuminates the evolution of the lattice structure, rotational and vibrational dynamics with temperature, and their connection to the charge carrier lifetime of the solar cell will be discussed.

Condensed Matter
Thursday, April 12, 2018
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Opportunities in Quantum Materials Research using Neutrons"

Clarina Dela Cruz , Oak Ridge National Laboratory
[Host: Despina Louca]
ABSTRACT:

Quantum materials will arguably be the key materials to push forward the forefront energy relevant technologies of the future. Having two powerful neutron sources at the Oak Ridge National Laboratory, enables us to be positioned strongly to use neutron scattering in unveiling the structure of, and dynamics in quantum systems that lead to fundamental understanding and control of quantum phenomena such as coherence, entanglement and novel emergent states. quantum critical phenomena among others, with a range of correlation strength in them. With the increasing progress in instrumentation, the instruments at ORNL are able to study systems in extreme environments of ultra-low temperatures, high magnetic field as well as high pressure. In this talk, I will give several examples of quantum materials problems where neutron scattering played a crucial role with some prospects for possible studies in molecular magnets in particular.

Condensed Matter
Thursday, March 29, 2018
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"New Directions in Theoretical Studies of High Tc Superconductors"

Adriana Moreo , University of Tennessee
[Host: Despina Louca]
ABSTRACT:

The discovery of high critical temperature superconductivity in iron-based pnictides and chalcogenides brought to the  forefront the need to develop efficient theoretical procedures to treat multiorbital models of interacting electrons. Among the many challenges, we need to clarify the role that the orbital degree of freedom plays in pairing and how its interaction with magnetic and lattice degrees of freedom leads to the stabilization of exotic phases such as the nematic state. Theoretical studies in the strong and weak coupling limits cannot address the physically relevant intermediate regime, with a mixture of itinerant and localized degrees of freedom. Traditional numerical methods, such as Lanczos or quantum Monte Carlo, have either a too rapidly growing Hilbert space with increasing size or sign problems. For this reason, it is necessary to develop new models and techniques, and also better focus on systems where both experiments and accurate theory can be used in combination to reach a real understandingof iron pairing tendencies. Examples of recent advances along these directions that will be discussed in this talk include:
i) The development of spin-fermion models [1] that allow studies in the difficult nematic regime with a finite
short-range antiferromagnetic correlation length above the ordering critical temperatures. This type of studies
also allow the inclussion of doping, quenched disorder, and the study of transport and real-frequency responses;
ii) The application of the Density Matrix Renormalization Group (DMRG) approach to multi-orbital Hubbard
models in chain and ladder structures [2] triggered by the discovery of superconductivity at high pressure in ladder
iron-based compounds such as BaFe2S3 and BaFe2Se3. In this context,
the recently reported [2] pairing tendencies unveiled at intermediate Hubbard U will be discussed;
iii) Results for a newly developed multi-orbital spin-fermion model for the CuO2 planes in high Tc cuprates.[3]

[1] S.Liang {\it et al.}, Phys.Rev.Lett.{\bf 109}, 047001 (2012) and Phys. Rev. Lett. {\bf 111} 047004 (2013); Phys. Rev. B{\bf 92} 104512 (2015); C. Bishop {\it et al.}, Phys. Rev. Lett. {\bf 117} 117201 (2016); Phys. Rev. B{\bf 96} 035144 (2017). [2] N.D. Patel {\it et al.}, Phys. Rev. B{\bf 96}, 024520(2017). See also  N.D. Patel {\it et al.}, Phys. Rev. B{\bf 94}, 075119(2016). [3] Mostafa Hussein et al., in preparation.

Condensed Matter
Thursday, March 1, 2018
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Local structure and the Jahn-Teller effect in TiSe2-xTex charge density waves"

Aaron Wegner , UVA- Department of Physics
[Host: Despina Louca]
ABSTRACT:

Transition metal dichalcogenide (TMDC) materials exhibit a wide variety of interesting physical phenomena. This diverse family of materials forms a quasi-two dimensional layered hexagonal structure of X-M-X sandwiches (M= Ti, Mo, Hf, W, etc.., X= S, Se, Te) that, depending on composition, may be semiconducting, metallic, or superconducting and many undergo charge density wave transitions. As the materials are layered and can be exfoliated, interest in the TMDCs has increased due to the search for graphene-like materials and the importance of thin film applications. One particularly interesting material is TiSe2, which forms a prototypical commensurate CDW that occurs in the vicinity of superconductivity. The origin of this CDW phase is controversial and has alternatively been attributed to exciton condensation or several possible Jahn-Teller type mechanisms. I will discuss how neutron scattering and local structure refinements give insight into the effect of the lattice on CDW formation in TiSe2 and the doping series in which Te is substituted for Se.

Condensed Matter
Thursday, February 1, 2018
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Phase transitions and mode coupling in PbZrO3 and Zr-rich PbZr1-xTixO3 solid solutions"

Sergey Vakhrushev , Ioffe Institute, St. Petersburg, Russia
[Host: Despina Louca]
ABSTRACT:

(PbZrO3)1-x-(PbTiO3)x (PZT) solid solutions probably represent the most studied group of functional dielectrics materials. For many years the main efforts were devote to the study of the PZT compounds at the morphotropic boundary region (x≈0.5). However recently Zr-rich (x<0.06) compounds attracted new attention due to their potential for the electric energy storage and electricaloric application. Beside possible application related interest these crystals demonstrate extremely reach phase diagram including antiferroelectric, ferroelectric and incommensurate phases.

In my presentation I would like to concentrate on the dynamical features related to the phase transitions in the Zr-rich PZT (including PbZrO3 itself) and in the PbHfO3. In the papers [1,2] we demonstrated that the antiferroelectric phase transition In PbZrO3 with order parameter described by the wavevector qAFE=(¼ ¼ 0) can be considered as a missed incommensurate transition with some arbitrary wavevector, corresponding to the flat part of the dispersion curve of the TA mode. Later in PbHfO3 and PbZrO3 at high pressure the minima at the TA dispersion curves were found [3,4], resulting in the realization of the incommensurate phases.

Extremely complicated diffraction pattern is observed in the PZr1-xTixO3 crystals with x<0.06. In the intermediate phase between the paraelectric and antiferroelectric phases incommensurate phase is sometime observed similar to that in the PbZrO3 under high presuure. And in addition complicated system of the satellite peaks in the vicinity of the qM=(½ ½ 0) including first order and second order satellites exists. In addition to the satellite peaks near the M-points we found second order satellites near the main Bragg peaks.

Observed diffraction pattern can be fully described by the incommensurate structure determined by 2 wavevectors from the same star: q1=(0.5+δ 0.5-δ -δ)  and q2 =(0.5-δ δ 0.5+ δ). Combination of the q1 and q2 describes all observed superstructure peaks.

Creation of the true incommensurate phase can be attributed to the mode softening not at qM, but at a position shifted from the zone boundary. Such unusual soft mode can be described in terms of the coupling of 2 modes in the vicinity of M-point, namely TA mode and oxygen tilt mode. Such coupling is forbidden at qM but became allowed aside of it. Proposed model provides qualitative agreement with the results of the inelastic and diffuse X-ray scattering measurements

References [1] A. K. Tagantsev et al., Nat. Commun., 4, 2229 (2013) [2]R. G. Burkovsky, et al. Phys. Rev. B 90, 144301 (2014) [3] R.G. Burkovsky, et al.. J. Phys.: Condens. Matter,  27, 335901 (2015) [4] R.G. Burkovsky, et al.., Sci. Reports, 7, 41512 (2017)

Special Nuclear Seminar

Thursday, January 25, 2018
3:30 PM
Physics Building, Room 204

"Searching the origin of the knee of cosmic ray spectrum -LHAASO experiment"

Cunfeng Feng , Shandong University
[Host: Xiaochao Zheng]
ABSTRACT:

The curve of cosmic ray energy spectrum is turned around PeV region, which is called "the knee" of the cosmic ray spectrum. The origin of the PeV knee of cosmic ray spectrum remains a puzzle since its first discovery near 70 years ago. Searching for the origin of the knee is one of main aims of LHAASO experiment, a hybrid cosmic ray observatory, which is building on Haizi mountain, south of China. In this talk, the puzzle of the knee of cosmic ray spectrum will be introduced briefly together with the LHAASO experiment introduction. Then I will focus on the technique of scintillator detector of LHAASO and the photomultiplier tube used in this detector. The PMT test bench of Shandong University will also will be introduced.

Condensed Matter
Thursday, November 16, 2017
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Interface symmetry and non-helical states in topological insulator-semiconductor heterostructures"

Ilya Vekhter , Louisiana State University
[Host: Utpal Chatterjee]
ABSTRACT:

Heterostructures combining topological and non-topological materials constitute the next frontier in the effort to incorporate topological insulators (TIs) into functional electronic devices. I show that the properties of the interface states appearing at the planar boundary between a topologically-trivial semiconductor (SE) and a TI are qualitatively different from those at the vacuum surface, and are controlled by the symmetry of the interface. In contrast to the well-studied helical Dirac surface states, SE-TI interface states exhibit elliptical contours of constant energy and complex spin textures with broken helicity. Experimental signatures include out of plane spin accumulation under a transport current and the opening of a spectral gap that depends on the direction of an applied in-plane magnetic field. I will also discuss how symmetry breaking at the interface controls proximity-induced superconductivity of the TI surface state.

Condensed Matter
Thursday, November 9, 2017
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Many Faces of Carbon"

Puru Jena , VCU
[Host: Utpal Chatterjee]
ABSTRACT:

Carbon is one of the most fascinating elements in the periodic table. It not only forms the basis of all life on the Earth but also it is important to technology. The unique properties of carbon emerge from its ability to form diverse spn (1 < n < 3) bonds. Until 1960’s graphite with sp2 and diamond with sp3 bonding were the most common forms of carbon known. The discovery of one-dimensional (1D) chain-like polymer called “carbyne” in 1960 and later zero-dimensional (0D) carbon fullerenes, 1D carbon nanotube, and two-dimensional (2D) graphene, all with novel properties characteristic of their reduced dimensionality and size, has ushered a new era in carbon science. In recent years many new meta-stable forms of carbon exhibiting a mixture sp1, sp2 and/or sp3 bonding pattern have also emerged.  In this talk I will focus on the carbon allotropes that have been studied in our group1-7. These include functionalized C60 fullerenes for hydrogen storage1, 2, semi-hydrogenated graphene for metal-free ferromagnet3, metal-organic complexes with large electron affinity4, 3D metallic carbon made of hybridized sp2 and sp3 bonded atoms5, a Cairo-tilling inspired quasi-2D penta-graphene made of only carbon pentagons,6 and its thermal conductivity7.   All calculations have been carried out using gradient corrected density functional theory. Thermodynamic stability of the above carbon allotropes is confirmed by total energy calculations as well as quantum molecular dynamics. Potential applications of some of these carbon allotropes will be discussed.

1. Sun, Q., Jena, P., Wang, Q., and Marquez, M.: “First-principles study of hydrogen storage on Li12C60”, J. Am. Chem. Soc. 128, 9741 (2006).
2. Berseth, P. A., Harter, A. G., Zidan, R., Blomqvist, A., Araujo, C. M., Scheicher, R. H., Ahuja, A., and Jena, P.: “Carbon Nanomaterials as Catalysts for Hydrogen Uptake and Release in NaAlH4”, Nano Letters. 9, 1501 (2009).
3. Zhou, J., Wang, Q., Sun, Q., Chen, X. S., Kawazoe, Y., and Jena, P.: “Ferromagnetism in semihydrogenated graphene”, Nano Letters 9, 3867 (2009).
4. Giri, S., Child, B., Zhou, J., and Jena, P: “Unusual Stability of Multiply Charged Organo-metalic Complexes”, RSC Advances 5, 44003 (2015).
5. Zhang, S., Wang, Q., Chen, X., and Jena, P.: “Stable Metallic 3D Metallic Phase of Carbon with Interlocking Hexagons”, Proc. Nat. Acad. Sci. 110, 18809 (2013).
Condensed Matter
Thursday, November 2, 2017
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Real-space condensation in mass transport models: statics, dynamics, and large deviations"

Ori Hirschberg
[Host: Israel Klich ]
ABSTRACT:

The formation of traffic jams on highways, the clustering of particles in shaken granular gases, and the emergence of macroscopically-linked hubs in complex networks are all examples of real-space condensation. This real-space analogue of Bose-Einstein condensation is rather ubiquitous in nonequilibrium systems. In this talk, I shall present some of the insights into this phenomenon garnered from the study of prototypical toy models. After reviewing static properties of the condensation phase transition, I shall focus on two unexpected features recently discovered: (1) Spatial correlations, which generically exist in driven systems, may give rise to a collective motion of the condensate through the system. Using simplified models, the mechanism behind this motion is explained and shown to be rather robust. (2) Rare fluctuations with extremely atypical currents may lead to condensate formation in systems that otherwise do not condense. I will present microscopic and macroscopic approaches to analyze this novel scenario of condensation.

Condensed Matter
Thursday, October 26, 2017
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Dissipation bounds on far-from-equilibrium fluctuations"

Jordan Horowitz , MIT
[Host: Marija Vucelja]
ABSTRACT:

Near equilibrium, linear response theory has proven to be a powerful tool.  At its core is the fluctuation-dissipation theorem, which dictates that the variance of small fluctuations is intimately related to dissipation.  However, far from equilibrium no such equality exists.  I will show that arbitrarily far from equilibrium dissipation still plays a dominant role in shaping fluctuations, both small and large, through some novel inequalities.  In particular, I will discuss the thermodynamic uncertainty relation and its variants, which are universal nonequilibrium constraints between the variance of fluctuations and dissipation.  These predictions offer general design principles for engineering artificial and natural nano-devices under energy restrictions.

http://jordanmhorowitz.mit.edu/

Condensed Matter
Thursday, October 19, 2017
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Skyrmion Lattices in Random and Ordered Potential Landscapes"

Charles Reichhardt , Los Alamos National Lab
[Host: Gia-Wei Chern]
ABSTRACT:

Since the initial discovery of skyrmion lattices in chiral magnets [1], there has been a tremendous growth in this field as an increasing number of compounds are found to have extended regions of stable skyrmion lattices [2] even close to room temperature [3].  These systems have significant promise for applications due to their size scale and the low currents or drives needed to move the skyrmions [4].  Another interesting aspect of skyrmions is that the equations of motion have significant non-dissipative terms or a Magnus effect which makes them unique in terms of collective driven dynamics as compared to other systems such as vortex lattices in type-II superconductors, sliding charge density waves, and frictional systems.  We examine the driven dynamics of skyrmions interacting with random and periodic substrate potentials using both continuum based modelling and particle based simulations. In clean systems we examine the range in which skyrmion motion can be explored as a function of the magnetic field and current and show that there can be a current-induced creation or destruction of skyrmions.  In systems with random pinning we find that there is a finite depinning threshold and that the Hall angle shows a strong dependence on the disorder strength. We also show that features in the transport curves correlate with different types of skyrmion flow regimes including a skyrmion glass depinning/skyrmion plastic flow region as well as a transition to a dynamically reordered skyrmioncrystal at higher drives. We find that increasing the Magnus term produces a low depinning threshold which is due to a combination of skyrmions forming complex orbits within the pinning sites and  skyrmion-skyrmion scattering effects.  If the skyrmions are moving over a periodic substrate, with increasing drive the Hall angle changes in quantized steps which correspond to periodic trajectories of the skyrmion that lock to symmetry directions of the substrate potential.

[1] S. Muhlbauer et al Science 323 915 (2009).
[2] X. Z.  Yu et al. Nature 465, 901–904 (2010).
[3] X.Z. Yu et al Nature Materials, 10, 106 (2011).
[4] A. Fert, V. Cros, and J. Sampaio Nature Nanotechnology 8, 152 (2013).

Condensed Matter
Thursday, October 5, 2017
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Topological mechanics and hidden symmetry in rigid origami"

Bryan Chen , University of Pennsylvania
[Host: Jeffrey Teo]
ABSTRACT:

The study of topological invariants in band theory has led to many insights into the behavior of electrons in insulators and superconductors. In 2013, Kane and Lubensky pointed out that certain "isostatic" mechanical systems can also admit topological boundary modes [1].  This has led to the design and realization of several families of "topological mechanical metamaterials". In my talk I will introduce the "topological polarization" of Kane and Lubensky and then explain how it can be realized in certain mechanical structures, called rigid origami and kirigami, which consist of rigid plates joined by hinges meeting at vertices [2]. Mysteriously, we found in [2] that triangulated origami structures always seem to be unpolarizable, that is, despite the lack of any apparent symmetry, all of these structures have a vanishing polarization invariant. I will describe recent work with Zeb Rocklin (Georgia Tech), Louis Theran (St. Andrews) and Chris Santangelo (UMass Amherst) which explains this via a "motion to stress" correspondence, that generalizes the 19th-century Maxwell-Cremona correspondence in several directions.

[1] C.L. Kane and T.C. Lubensky, Nat Phys 10, 39–45 (2014).

[2] B.G. Chen, B. Liu, A.A. Evans, J. Paulose, I. Cohen, V. Vitelli, and C. Santangelo, Phys Rev Lett 116, 135501 (2016).

Condensed Matter
Thursday, September 28, 2017
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Geometric theory of nonlocal transport in metals"

Dmytro Pesin , University of Utah
[Host: Gia-Wei Chern]
ABSTRACT:

I will discuss the topological and geometric aspects of optical
and transport phenomena in metals with nontrivial band geometry, and outline
the full theory of linear-in-q contribution to the non-local conductivity in
a disordered metal. Physical applications of the theory include the natural
optical activity of metals and the dynamic chiral magnetic effect, as well
as the kinetic magnetoelectric effect/the current-induced magnetization in
metallic systems. The theory is similar in spirit to the one of the
anomalous Hall effect in metals, and can be used for the analysis of the
typical optical and transport measurements (e.g. Faraday rotation,
current-induced magnetization) in the THz frequency range.

Condensed Matter
Thursday, September 7, 2017
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Structure and dynamics of hydrogen in nanocrystalline palladium"

Maiko Kofu , JPARC, Japan
[Host: Despina Louca]
ABSTRACT:

The behavior of hydrogen in metals has attracted much attention in fundamental and applied research areas. Palladium hydride (PdHx) is a typical metal-hydrogen system and has been studied for many decades. Pd has remarkable abilities to absorb plenty of H atoms and the H atoms are highly mobile in the Pd lattice. It is interesting to examine how the properties are changed as the particle size is reduced to a nanometer-scale. We have investigated structure, diffusion and vibrational dynamics by means of neutron scattering techniques for both bulk and nanocrystalline PdHx with a diameter of 8nm. Neutron diffraction work on nanocrystalline sample demonstrated that some of hydrogen atoms are accommodated at the tetrahedral (T) sites. This is in contrast to bulk PdHx with octahedral (O) occupation. In quasielastic scattering measurements, we found an additional fast diffusion process with a small potential barrier in nanocrystalline PdHx. Furthermore, our inelastic scattering works revealed that nanocrystalline PdHx exhibits two distinct vibrational excitations; one resembles that observed in bulk PdHx and the other is the excitations appeared at higher energies. The additional diffusion process and vibrational states are attributed to the H atoms at T sites near the surface of nanoparticles. The potential shape around the T site near the surface will be discussed in the seminar.

Condensed Matter
Thursday, April 27, 2017
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"SUBATOMIC STATES IN CONDENSED MATTER AND MAGNETORESISTANCE OF SUPERCONDUCTORS"

Boris I. Ivlev , Landau Institute and Instituto de Fisica, UASLP
[Host: Genya Kolomeisky]
ABSTRACT:

In the recent experiments, PRB 94, 054504 (2016), the unusual oscillatory magnetore- sistance of superconductors was discovered with a periodicity essentially independent on magnetic ﬁeld direction and even material parameters. The nearly universal period points to a subatomic mechanism of the phenomenon. This mechanism is related to formation inside samples of subatomically thin (1011cm size) threads in the form of rings of the interatomic radius. Electron states of rings go over into conduction electrons which carry the same spin imbalance in energy as the ring. The imbalance occurs due to the spin interaction with the orbital momentum of the ring. The conductivity neat Tc is determined by ﬂuctuating Cooper pairs consisting of electrons with shifted energies. Due to diﬀerent angular momenta of rings these energies periodically depend on magnetic ﬁeld resulting in the observed oscil- latory magnetoresistance. Calculated universal positions of peaks (n + 1)∆H (∆H '"" 0.18T and n = 0, 1, 2, ...) on the R(H) curve are in a good agreement with measurements.

Condensed Matter
Wednesday, April 26, 2017
5:00 PM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"Surfaces and Slabs of Fractional Topological Insulators"

Alexander Sirota , UVA-Department of Physics
[Host: Jeffrey Teo]
ABSTRACT:

Fractional topological insulators (FTI) are electronic topological phases in (3 + 1) dimensions enriched by time reversal (TR) and charge U(1) conservation symmetries. We focus on the simplest series of fermionic FTI, whose bulk quasiparticles consist of decon ned partons that  carry fractional electric charges in integral units of e = e=(2n + 1) and couple to a discrete Z2n+1 gauge theory. We propose massive  symmetry preserving or breaking FTI surface states. Combining the long-ranged entangled bulk with these topological surface states, we  deduce the novel topological order of quasi-(2 + 1) dimensional FTI slabs as well as their corresponding edge conformal eld theories. We will also describe some past work on coupled wire constructions of Weyl/Dirac semimetals, and some new work about mechanical topology and conformal field theory.

Condensed Matter
Thursday, April 13, 2017
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Unusual magnetic order in some frustrated spin systems: "

Bhanu Mahanti , Michigan State University
[Host: Utpal Chatterjee]
ABSTRACT:

Classical and quantum spin systems have played an enormously important role in the study of magnetism. They provide excellent models to understand the ground state magnetic structure and low energy excitations of magnetic systems. In addition, they provide a rich arena for studying the physics of interacting many-body systems. In this talk I will discuss a very specific problem, namely the observation of an unusual ordering of spins, the so called uudd states, in quasi one-dimensional Heisenberg spin chains and anisotropic two-dimensional Heisenberg spin systems. Theoretical work to understand this and some related issues using both model spin Hamiltonians with competing interactions, exchange anisotropy, biquadratic exchange etc. and ab initio electronic structure calculations, will be discussed.

Condensed Matter
Thursday, April 13, 2017
3:30 PM
Physics Building, Room 204

"Symmetry preserving gapping of Topological Weyl and Dirac semimetals"

Syed Raza , UVA- Physics Department
[Host: Jeffrey Teo]
ABSTRACT:

Topological  Weyl and Dirac semimetals in three dimensions have gapless (massless) Weyl fermions. These semimetals were predicted theoretically in 2011 (Vishwanath et al, Burkov and Balents) and have been experimentally discovered recently in TaAs, TaP, NbAs, NbP (Weng et al., Huang et al., Shekhar et al., 2015). Although these gapless systems can be gapped (given mass) trivially by symmetry breaking terms, a more interesting problem is if they can be gapped without breaking symmetries? We show that you can indeed do this by introducing many body interactions. We also show that this symmetry-preserving gapping gives point-like and line-like anyonic excitations in the gapped bulk.

Condensed Matter
Thursday, April 6, 2017
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Understanding glassy systems using population annealing"

Jon Machta , University of Massachusetts
[Host: Marija Vucelja]
ABSTRACT:

Population annealing is a sequential Monte Carlo method for studying the equilibrium properties of glassy systems characterized by rough free energy landscapes.  In this talk I will introduce two glassy systems, the Ising spin glass and a glass-forming binary mixture of hard spheres. I will describe population annealing and some of its useful features, and then present results from simulations of these glassy systems.

http://people.umass.edu/machta/

Condensed Matter
Thursday, March 30, 2017
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Understanding and Controlling Organic Molecule and Metal Organic Framework Crystallization "

Gaurav Giri , University of Virginia, Chemical Engineering
[Host: Seunghun Lee]
ABSTRACT:

Small organic molecules have had a dramatic impact on our health and daily life over the past century. Small molecule pharmaceuticals have increased human lifespans, and organic pigments have expanded in use in textiles and displays. This impact is set to accelerate in the near future, as small molecules are explored for novel applications such as organic electronics, or metal-organic frameworks for chemical separations, catalysis and sensing. One of the major barriers for using small organic molecules for new applications is the limited understanding we possess on how molecules aggregate together to form different crystal habits and phases. Different crystal structures and morphologies can have wildly varying physical, chemical and physiological properties. Thus, if we do not control the crystallization organic molecules, we cannot predict its behavior for the aforementioned applications. Understanding the crystallization process can also help form metastable phases. These metastable phases can be more useful than the equilibrium phase for many applications. Metastable phases permit tunable optical bandgap for optoelectronics, control over pore size and shape in metal organic frameworks (MOF), and increased bioavailability in pharmaceuticals. General methods used to create metastable phases, like confinement or rapid cooling, require small length scales and extreme rates of heat and mass transfer. Moreover, these processes need precise control to get reliable results. This talk will focus on flow coating and microfluidic methods of controlling organic molecule and MOF crystallization characteristics, and the use of these materials for various applications.

Condensed Matter
Friday, March 24, 2017
9:00 AM
Physics Building, Room 205
Note special date.
Note special time.
Note special room.

"Energetics of a heat engine: a molecular dynamics simulation study"

Mulugeta Bekele , Addis Ababa University, Ethiopia
[Host: Bellave Shivaram]
ABSTRACT:

I first consider an elevator as a system that lifts a load from a ground floor to a top floor by consuming an input energy. I ask: what amount of load enables the system to perform maximum amount of power? Can I design a mode of operation where I can utilize good enough amount of the input energy better than that of energy utilized at maximum power?

I will then go over to a molecular dynamics simulation study of a heat engine where the working substance is a real gas and try to address similar questions as the elevator system.

Condensed Matter
Thursday, March 23, 2017
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"The Haldane phase in spin chains with SU(N) symmetry"

Abhishek Roy , Institute for Theoretical Physics, University of Cologne
[Host: Jeffrey Teo]
ABSTRACT:

Spin chains with PSU(n) symmetry are known to have symmetry protected topological phases distinguished by boundary states. We determine the parent Hamiltonians of some of these phases at the AKLT point. In the adjoint representation we use a graphical method that produces expressions for arbitrary N. Finally we discuss the fate of the Haldane conjecture.

Condensed Matter
Thursday, February 23, 2017
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Dynamics of Kitaev-Heisenberg model on a Honeycomb lattice in the classical limit"

Anjana Samarakoon , UVA-Department of Physics
[Host: Seunghun Lee]
ABSTRACT:

Quantum spin liquids (QSLs) have achieved great interest in both theoretical and experimental condensed matter physics due to their remarkable topological properties. Among many different candidates, the Kitaev model on the honeycomb lattice is a 2D prototypical QSL which can be experimentally studied in materials based on iridium or ruthenium.. Here we study the spin-1/2 Kitaev model using classical Monte-Carlo and semiclassical spin dynamics of classical spins on a honeycomb lattice. Both real and reciprocal space pictures highlighting the differences and similarities of the results to the linear spin wave theory will be discussed in terms dispersion relations of the pure-Kitaev limit and beyond. Interestingly, this technique could capture some of the salient features of the exact quantum solution of the Kitaev model, such as features resembling the Majorana-like mode comparable to the Kitaev energy, which is spectrally narrowed compared to the quantum result, can be explained by magnon excitations on fluctuating one-dimensional manifolds (loops). Hence the difference from the classical limit to the quantum limit can be understood by the fractionalization of a magnon to Majorana fermions. The calculations will be directly compared with our neutron scattering data on α-RuCl3 which is a prime candidate for experimental realization of Kitaev physics.

Condensed Matter
Thursday, February 2, 2017
3:30 PM
Physics Building, Room 204

"Between topological strings and topological phases"

Jeffrey Teo , UVA - Department of Physics
[Host: Eugene Kolomeisky]
ABSTRACT:

Topological phases in two and three dimensions can be theoretically constructed by coupled-wire models whose fundamental constituents are electronic channels along strings. On the other hand the collective topological phases support further fractionalized emergent quasi-string excitations or defects such as flux vortices. In this talk I will describe topological superconductors and Dirac (or Weyl) semimetals using coupled-wire models, and discuss the fractional behavior of emergent topological strings.

Condensed Matter
Thursday, January 19, 2017
11:00 AM
Physics Building, Room 204
Note special time.

"Resonant inelastic X-ray scattering response of the Kitaev spin liquid"

Gabor Halasz , KITP
[Host: Marija Vucelja]
ABSTRACT:

We propose that resonant inelastic X-ray scattering (RIXS) is an effective probe to detect spin-liquid character in potential material incarnations of the Kitaev spin liquid (such as the honeycomb iridates and ruthenium chloride). Calculating the exact RIXS response of the Kitaev honeycomb model, we find that the fundamental RIXS channels, the spin-conserving (SC) and the non-spin-conserving (NSC) ones, can probe the fractionalized excitations of the Kitaev spin liquid separately. In particular, SC RIXS picks up the gapless Majorana excitations with a pronounced momentum dispersion, while NSC RIXS creates immobile flux excitations, thereby rendering the response weakly momentum dependent.

Condensed Matter
Tuesday, January 17, 2017
11:00 AM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"Formation probabilities, Post-measurement entanglement entropy and Casimir effect "

Mohammed Ali Rajabpour , Fluminense Federal University
[Host: Israel Klich ]
ABSTRACT:

I will first introduce formation probability as a quantity which can determine the universality class of a quantum critical system. In other words, by calculating this quantity one can find the central charge and critical exponents of the quantum system. We will show that calculating this quantity boils down to finding Casimir energy of two needles. Then we will  briefly talk about Shannon mutual information as another quantity which can play similar role. Finally, we will introduce post-measurement entanglement entropy as a tripartite measure of entanglement. We will show that this quantity is related to the Casimir energy of needles on Riemann surfaces and can be calculated exactly for conformal field theories.

Ref:
1: MAR, Europhysics Letters, 112, 66001 (2015), J. Stat. Mech. (2016) 123101 and K. Najafi, MAR, Phys. Rev. B 93, 125139 (2016).
2: F. C. Alcaraz, MAR, Phys. Rev. Lett. 111, 017201(2013), Phys. Rev. B, 90, 075132 (2014).
3: MAR, Phys. Rev. B 92, 075108 (2015) , J. Stat. Mech. (2016) 063109 and MAR,
K. Najafi, MAR, JHEP12(2016)124.

Condensed Matter
Thursday, December 1, 2016
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Spheres form strings, and a swimmer from a spring"

Daphne Klotsa , University of North Carolina at Chapel Hill
[Host: Marija Vucelja]
ABSTRACT:

Rigid spherical particles in oscillating fluid flows form interesting patterns as a result of fluid mediated interactions. Here, through both experiments and simulations, we show that two spheres under horizontal vibration align themselves at right angles to the oscillation and sit with a gap between them, which scales in a non-classical way with the boundary layer thickness. A large number of spherical particles form strings perpendicular to the direction of oscillation. Investigating the details of the interactions we find that the driving force is the nonlinear hydrodynamic effect of steady streaming. We then design a simple swimmer (two-spheres-and-a-spring) that utilizes steady streaming in order to propel itself and discuss the nature of the transition at the onset of swimming as the Reynolds number gradually increases. We discuss implications and connections to biological systems, motility, and collective behavior of swimmers.

http://dklotsa.wixsite.com/dklotsa

Condensed Matter
Thursday, November 17, 2016
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"OPTICS IN FLATLAND: RAMAN SPECTROSCOPIC PROBES OF STRUCTURAL SYMMETRY"

Patrick Vora , George Mason University
[Host: Utpal Chatterjee]
ABSTRACT:

The progress of modern technology is increasingly driven by the development and evaluation of novel materials. Layered two-dimensional (2D) materials, collectively referred to as van der Waals solids, are receiving intense interest in the community due to their unconventional and diverse electronic behaviors. Transition metal dichalcogenides (TMDs) are a class of 2D material following the basic chemical formula MX2, where M = (Mo, W, Nb, Re, …) and X = (S, Se, Te, …). Varying the chemical composition allows access to semiconducting, semi-metallic, superconducting, or magnetic behaviors in the 2D limit. Of recent interest are telluride based TMDs such as MoTe2 and WTe2 which are predicted exhibit controllable structural phase transitions appropriate for phase change memory applications as well as topologically protected and spin polarized electronic states.

In this colloquium, I will present our efforts to understand the temperature-dependent optical properties of MoTe2 and MoxW1-xTe2 using temperature-dependent and polarization-resolved Raman spectroscopy. We have used this technique to identify the anharmonic contributions to the optical phonon modes in bulk MoTe2 occupying the distorted orthorhombic (T­d) lattice structure. At temperatures ranging from 100 K to 200 K, we find that all modes redshift linearly with temperature however, below 100 K we observe nonlinear frequency shifts in some modes. We show that this anharmonic behavior is consistent with the decay of an optical phonon into multiple acoustic phonons. Furthermore, the highest frequency Raman modes show large changes in intensity and linewidth near 250 K that correlate well with a structural phase transition.

We also explore the composition-dependent optical properties of MoxW1-xTe2 alloys. Our observations identify signatures of the hexagonal (H), monoclinc (1T’) and Td structural phases. Polarization-resolved Raman measurements allow for the assignment of all vibrational modes as well as the evolution of mode symmetry and frequency with composition. We discover a previously unobserved WTe2 mode as well as a Raman-forbidden MoTe2 mode that is activated by compositional disorder. The primary WTe2 Raman peak is asymmetric for x <= 0.1, and is well fit by the spatial correlation model. From these fits, we extract the spatial phonon correlation length which serves as an indirect measure of the WTe2 domain size. Our study is foundational for future studies of MoxW1-xTe2 and provides new insights into the impact of disorder in transition metal dichalcogenides.

Condensed Matter
Friday, November 11, 2016
3:30 PM
Physics Building, Room 204
Note special date.

"Conductance of a superconducting Coulomb blockade nanowire at finite temperature"

Ching-Kai Chiu , University of Maryland
[Host: Jeffrey Teo]
ABSTRACT:

By applying a magnetic field, a superconducting proximity nanowire in the presence of spin-orbital coupling can pass through topological phase transition and possesses Majorana bound states on the ends. One of the promising platforms to detect the Majorana modes is a coulomb blockade island by measuring its two-terminal conductance (S. M. Albrecht et al., Nature (London) 531, 206 (2016)). Here, we study the transportation of a single electron across the superconducting Coulomb blockade nanowire at finite temperature to obtain the generic conductance equation. By considering all possible scenarios that Majorana modes appear in the nanowire, we compute the nanowire conductance as the magnetic field and the gate voltage of the nanowire vary. The oscillation behavior of the conductance peak is temperature independent and the  oscillation amplitude of the conductance peak spacings increases as the magnetic field increases.

Condensed Matter
Thursday, November 10, 2016
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Interacting helical liquids in bilayer graphene"

Chaoxing Liu , Penn State University
[Host: Jeffrey Teo]
ABSTRACT:

A grand challenge in the field of topological physics is to understand the role of interaction and to realize interacting topological phases in realistic materials. In this talk, we will discuss the possible realization of interacting topological states in bilayer graphene under a strong magnetic field. We start from a fermonic two-channel quantum spin Hall state with two copies of helical edge states, which have been demonstrated experimentally in bilayer graphene. By introducing interaction into two channel helical liquids, we demonstrate that all the fermion degrees are gapped out and only one bosonic mode remains, thus yielding a bosonic version of topological insulator, dubbed “bosonic symmetry protected topological state”. Physically, the two dual boson fields of this bosonic mode carry charge-2e and spin-1, respectively, due to the helical nature. Thus, we dubbed them “bosonic helical liquids”. We further study the transport of a quantum point contact for bosonic helical liquids and compare them to fermonic two-channel helical liquids. A novel charge insulator/spin conductor phase is identified in the weak repulsive interaction regime for bosonic helical liquids while charge insulator/spin insulator or charge conductor/spin conductor phase is present for fermonic two-channel helical liquids. Thus, a quantum point contact experiment will allow us to identify the bosonic symmetry protected topological states unambiguously. Similar physics can also emerge in topological mirror Kondo insulators, such as SmB6.

Reference:

[1] Bilayer Graphene as a platform for Bosonic Symmetry Protected Topological States, Zhen BiRuixing ZhangYi-Zhuang YouAndrea YoungLeon BalentsChao-Xing LiuCenke Xu, arXiv:1602.03190v1, 2016

[2] Interacting topological phases in thin films of topological mirror Kondo insulators, Rui-Xing ZhangCenke XuChao-Xing Liu, arXiv: 1607.06073, 2016

[3] Fingerprints of bosonic symmetry protected topological state in a quantum point contact, Rui-xing Zhang, Chao-xing Liu, arxiv: 1610.01236, 2016

Condensed Matter
Thursday, October 27, 2016
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Spin and orbital interactions in complex oxide heterostructures and transient states probed by x-rays"

Mark Dean , Brookhaven National Lab
[Host: Israel Klich ]
ABSTRACT:

The spin and orbital degrees of freedom play a crucial role in determining the remarkable properties of transition metal oxide materials. In this talk, I will describe how resonant inelastic x-ray scattering (RIXS) opens up important new possibilities for measuring these degrees of freedom even in challenging cases such a heterostructures and transient states [1]. This includes determining how orbitals are modified within LaNiO3-based heterostructures [2] and characterizing the spin behavior within the ultra-fast transient state of photo-doped Sr2IrO4 [3].

References

1. M. P. M. Dean et al., Nature Materials 11, 850 (2012); M. P. M. Dean et al., Nature Materials 12, 1019–1023 (2013)

2. G. Fabbris et al., Phys. Rev. Lett. 117, 147401 (2016)

3. M. P. M. Dean et al., Nature Materials 15, 601-605 (2016)

Condensed Matter
Monday, October 24, 2016
11:00 AM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"First-principles predictions of thermodynamically stable two-dimensional electrides"

Mina Yoon , Center for Nanophase Materials Sciences, Oak Ridge National Laboratory
[Host: Seunghun Lee]
ABSTRACT:

Two-dimensional (2D) electrides, emerging as a new type of layered material whose electrons are confined in interlayer spaces instead of at atomic proximities, are receiving interest for their high performance in various (opto)electronics and catalytic applications. Experimentally, however, 2D electrides have been only found in a couple of layered nitrides and carbides. Here, we report new thermodynamically stable alkaline-earth based 2D electrides by using a first-principles global structure optimization method, phonon spectrum analysis, and molecular dynamics simulation. The method was applied to binary compounds consisting of alkaline-earth elements as cations and group VA, VIA, or VIIA nonmetal elements as anions. We revealed that the stability of the layered 2D electride structure is closely related to the cation/anion size ratio; stable 2D electrides possess a sufficiently large cation/anion size ratio to minimize electrostatic energy among cations, anions, and anionic electrons. Our work demonstrates a new avenue to the discovery of thermodynamically stable 2D electrides beyond the material database and provides new insight into the principles of electride design.

Condensed Matter
Thursday, October 20, 2016
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"The Causes of Metastability and Their Effects on Transition Times"

Katie Newhall , University of North Carolina at Chapel Hill
[Host: Marija Vucelja]
ABSTRACT:

Many experimental systems can spend extended periods of time relative to their natural time scale in localized regions of phase space, transiting infrequently between them.  This display of metastability can arise in stochastically driven systems due to the presence of large energy barriers, or in deterministic systems due to the presence of narrow passages in phase space.  To investigate metastability in these different cases, I take a Langevin equation and determine the effects of small damping, small noise, and dimensionality on the dynamics and mean transition time.  Of particular interest is what happens in the infinite dimensional limit, a stochastic partial differential equation, and the question of what ensemble this system appears to sample over time.  Both analytical and numerical results will be presented.

http://knewhall.web.unc.edu/

Condensed Matter
Thursday, October 13, 2016
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Anomalous cooling and heating – the Mpemba effect and its inverse"

Oren Raz , University of Maryland
[Host: Israel Klich ]
ABSTRACT:

Under certain conditions, it takes a shorter time to cool a hot system than to cool the same system initiated at a lower temperature. This phenomenon – the “Mpemba Effect” – is well known in water, and has recently been observed in other systems as well. However, there is no single generic mechanism that explains this counter-intuitive behavior. Using the theoretical framework of non-equilibrium thermodynamics, we present a widely applicable mechanism for this effect, derive a sufficient condition for its appearance in Markovian dynamics, and predict an inverse Mpemba effect in heating: under proper conditions, a cold system can heat up faster than the same system initiated at a higher temperature. Our results suggest that it should be possible to observe the Mpemba effect and its inverse in a variety of systems, where they have never been demonstrated before.

Condensed Matter
Thursday, October 6, 2016
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Multiferroics by design with frustrated molecular magnets"

Yoshitomo Kamiya , RIKEN
[Host: Gia-Wei Chern]
ABSTRACT:

Geometric frustration in Mott insulators permits perturbative electron fluctuations controlled by local spin configurations [1]. An equilateral triangle (“trimer”) of spins with S = 1/2 is the simplest example, in which low-energy degrees of freedom consist of built-in magnetic and electric dipoles arising from the frustrated exchange interaction. Such trimers can be weakly coupled to make multiferroics by design [2]. An organic molecular magnet known as TNN [3], with three S = 1/2 nitronyl nitroxide radicals in a perfect C3 symmetric arrangement, is an ideal building block as demonstrated by recent experiments on a single crystal comprising TNN and CH3CN. The fascinating thermodynamic phase diagram of this molecular crystal, TNN·CH3CN, is in excellent agreement with our theory, which predicts multiferroic behavior and strong magnetoelectric effects arising from an interplay between magnetic and orbital degrees of freedom [4]. Our study thus opens up new avenues for designing multiferroic materials using frustrated molecular magnets.

References:

[1] L. N. Bulaevskii, C. D. Batista, M. V. Mostovoy, and D. I. Khomskii, Phys. Rev. B 78, 024402 (2008).

[2] Y. Kamiya and C. D. Batista, Phys. Rev. Lett. 108, 097202 (2012).

[3] Y. Nakano et al., Polyhedron 24, 2147 (2005).

[4] Y. Kamiya et al.,  in preparation.

Condensed Matter
Thursday, September 15, 2016
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Exploring metal-insulator transitions with optical microscopy and spectroscopy"

Mumtaz Qazilbash , William and Mary
[Host: Utpal Chatterjee]
ABSTRACT:

The study of metal-insulator, structural and magnetic phase transitions in materials with strongly interacting electrons is a challenging frontier of condensed matter physics. The challenge is to disentangle the contributions of charge, lattice, spin and orbital degrees of freedom to phase transitions. I will report on the optical properties of two materials that undergo thermally-induced metal-insulator transitions accompanied by structural and/or magnetic instabilities: vanadium dioxide (VO2) and the manganite La0.67Sr0.33MnO3. Infrared micro-spectroscopy and micro-ellipsometry measurements on crystals of VO2 reveal that its insulating phases are Mott-Hubbard insulators, not Peierls insulators. Scanning near-field infrared microscopy (SNIM) allows us to directly image nano-scale metallic puddles that appear at the onset of the first-order metal-insulator transition (MIT) in VO2 films. We find that the patterns of metallic domains are reproducible upon repeated thermal cycles across the MIT and point to the important role of imperfections in films.  In contrast to VO2, recent SNIM data shows time dependence of near-field infrared amplitude in a La0.67Sr0.33MnO3 film which we attribute to fluctuating conductivity in the vicinity of its second order metal-insulator transition.

Condensed Matter
Thursday, September 8, 2016
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Topological Phase Transitions in Line-nodal Superconductors"

Gil Young Cho , Advanced Institute of Science and Technology, Korea
[Host: Jeffrey Teo]
ABSTRACT:

Fathoming interplay between symmetry and topology of many-electron wave-functions has deepened understanding of quantum many body systems, especially after the discovery of topological insulators. Topology of electron wave-functions enforces and protects emergent gapless excitations, and symmetry is intrinsically tied to the topological protection in a certain class. Namely, unless the symmetry is broken, the topological nature is intact. We show novel interplay phenomena between symmetry and topology in topological phase transitions associated with line-nodal superconductors. The interplay may induce an exotic universality class in sharp contrast to that of the phenomenological Landau-Ginzburg theory. Hyper-scaling violation and emergent relativistic scaling are main characteristics, and the interplay even induces unusually large quantum critical region. We propose characteristic experimental signatures around the phase transitions in three spatial dimensions, for example, a linear phase boundary in a temperature-tuning parameter phase-diagram.

Condensed Matter
Tuesday, May 17, 2016
2:00 PM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"Gutzwiller paradigm and its first ab initio implementation in molecular and lattice systems"

Jun Liu , Ames Lab
[Host: Gia-Wei Chern]
ABSTRACT:

Predicting material-specific physical properties is always a fascinating goal of physics research. The current state-of-the-art methods for doing this are the density function theory (DFT) and its derivatives for lattice systems and Quantum Chemistry approaches for molecular systems. However, their performance is hindered when being applied to strongly correlated systems either due to their intrinsic limitations of being only valid in weakly correlated systems or not truly ab initio, or limited by system and basis set sizes. Thus it is highly expected to come up with new ideas to help tackle this research goal. In this talk, I will discuss the possibility of having such an alternative line of thought based on Gutzwiller wavefunction and effective Kohn-Sham Hamiltonian, and introduce an implementation of this idea in our recent work. Discussion on alternative implementations, generality to different phases and interactions will be provided.

Nuclear Seminar

Thursday, April 28, 2016
1:00 PM
Physics Building, Room 313
Note special time.
Note special room.

"GEANT 4 simulation prep for Bethe-Heitler experiment"

Haoyu Chen , UVA- Department of Physics
ABSTRACT:

A measurement of charge asymmetry in Bethe-Heitler (BH) pair production using the High Intensity Gamma-Ray Source at the Tri-Universities Nuclear Laboratory (TUNL) has been approved and is undergoing preparation.  These preparations include theoretical computation, support and target construction, as well as detector testing and simulation.  A GEANT4 simulation has been created in order to optimize the experimental setup, to make predictions of statistics, and to identify systematic uncertainties. Following a brief introduction about the physics and the experimental preparations, the following will be discussed: 1) relationship of the simulation to the BH experiment and their difference, 2) basic simulation process for the experiment, 3) simulation results for various virtual detectors, 4) results including the charge asymmetry computed from simulations, 5) unresolved questions regarding the simulation results.

Condensed Matter
Tuesday, April 26, 2016
1:00 PM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"Aging and memory effects in the spin jam states of densely populated frustrated magnets"

Anjana Samarakoon , UVA - Department of Physics
ABSTRACT:

Defects and randomness has been largely studied as the key mechanism of glassiness find in a dilute magnetic system. Even though the same argument has also been made to explain the spin glass like properties in dense frustrated magnets, the existence of a glassy state arise intrinsically from a defect free spin system, far from the conventional dilute limit with different mechanisms such as quantum fluctuations and topological features, has been theoretically proposed recently. We have studied field effects on zero-field cooled and field cooled susceptibility bifurcation and memory effects below freezing transition, of three different densely populated frustrated magnets which glassy states we call spin jam, and a conventional dilute spin glass. Our data show common behaviors among the spin jam states, which is distinct from that of the conventional spin glass. We have also performed both Neutron scattering experiments and Monte Carlo simulations to understand the nature of their energy landscapes.

Condensed Matter
Thursday, April 21, 2016
1:00 PM
Physics Building, Room 313
Note special time.
Note special room.

"Critical mechanical structures: topology and entropy"

Xiaoming Mao , University of Michigan
[Host: Marija Vucelja]
ABSTRACT:

Critical mechanical structures are structures at the verge of mechanical instability. These structures are characterized by their floppy modes, which are deformations costing little energy. On the one hand, numerous interesting phenomena in soft matter are governed by the physics of critical mechanical structures, because they capture the critical state between solid and liquid. On the other hand, the design of mechanical metamaterials (i.e., engineered materials that gain their unusual mechanical properties, such as negative Poisson's ratio, from their structures) often rely on floppy modes to realize novel properties, and the floppy modes in this situation are called "mechanisms". This talk focuses on the intersection between the research of soft matter and mechanical metamaterials. I will propose a new design principle, for mechanical metamaterials that are transformable between states with dramatically different properties. These different properties are highly robust because they are topologically protected. Then I will discuss entropic effects on floppy modes (mechanisms), the interplay of which with topology leads to fascinating phenomena that need be considered when machines and metamaterials are made at small scales.

http://www-personal.umich.edu/~maox/

Condensed Matter
Thursday, April 14, 2016
1:00 PM
Physics Building, Room 313
Note special time.
Note special room.

"Spin-Orbit Coupled Double Perovskites: Topology and Magnetism"

Arun Paramekanti , University of Toronto
[Host: Israel Klich ]
ABSTRACT:

Double perovskites are a broad class of transition metal oxides which exhibit a wide range of phenomena - high temperature ferromagnetism, half-metallicity, and Mott insulators with geometrically frustrated magnetism. I will discuss how spin-orbit coupling leads to new physics in these systems. Specifically, I will discuss how ultrathin double perovskite films can support Chern bands and quantum anomalous Hall insulators. Turning to spin dynamics, I will present our work on understanding neutron scattering experiments in half-metallic double perovskites as well as bulk iridium-based Mott insulators where we argue for dominant Kitaev exchange interactions.

Nuclear Seminar

Thursday, April 14, 2016
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Experimental study of the 3He and neutron spin structure at low Q2 using a polarized 3He target"

Nguyen Ton , UVA- Department of Physics
ABSTRACT:

I will report on the analysis status of the Jefferson Lab (JLab) Hall A E97110 experiment. The experiment performed a precise measurement of the neutron spin structure functions at low Q2 by using a polarized 3He target as an effective polarized neutron target. The goals of the experiment are to make a bench-mark test of Chiral Perturbation Theory calculations and to check the Gerasimov-Drell-Hearn (GDH) sum rule by extrapolating the integral to the real photon point. The data were taken in two experimental run periods. The first period covered the lowest Q2 points but with a defective equipment which complicate the data analysis. The second period covered higher Q2 point, with a properly working equipment. The raw asymmetry analysis and elastic carbon cross section measurement will be discussed for the fi run period along with the future plans for the analysis. I will also discuss the progress and the tests in the polarized 3He Target Lab at JLab. Results of Pulse NMR (nuclear magnetic resonance) will be presented.

SLIDESHOW:
Condensed Matter
Thursday, April 7, 2016
1:00 PM
Physics Building, Room 313
Note special time.
Note special room.

"Spin-1/2 Heisenberg antiferromagnet in one dimension"

Yasu Takano , University of Florida
[Host: Seunghun Lee]
ABSTRACT:

The one-dimensional spin-1/2 Heisenberg model is one of the few exactly solvable models in physics. When the interaction is antiferromagnetic, the ground state of this model is a Tomonaga-Luttinger liquid, in which dynamic and static properties are inextricably linked. Low-energy excitations are spinons, which are fermions, instead of bosonic magnons, with a unique gapless dispersion. These and other properties of the model have been extensively studied since the pioneering work by Bethe, published in 1931. This seminar describes our recent experiment that puts some of the theoretical predictions to tests.

Condensed Matter
Thursday, March 24, 2016
1:00 PM
Physics Building, Room 313
Note special time.
Note special room.

"Simulating frustrated itinerant magnets at the mesoscale"

Kipton Barros , LANL
[Host: Gia-Wei Chern]
ABSTRACT:

Models of classical fields coupled to itinerant fermions appear commonly in condensed matter. We present an efficient technique to sample these classical fields. Instead of expensive direct diagonalization, we use the Kernel Polynomial Method (KPM) to stochastically estimate electron states and relevant observables. Our extension of KPM significantly improves stochastic convergence by leveraging locality, e.g. spatial decay of 2-point correlations. A GPU/MPI implementation enables practical treatment of tens of thousands of lattice sites. To demonstrate the method, we study complex spin textures in the Kondo lattice model.  We observe mesoscale chiral domain coarsening and Z2 vortex dynamics. Other emergent phenomena includes metastable skyrmions and a chiral stripe phase that arises due to instability of standard helical order.

Finally, we discuss a generic spin liquid state that may explain the experimentally observed resistivity minima in compounds with large local magnetic moments, e.g. the pyrochlore oxides Pr2Ir2O7, Nd2Ir2O7 under pressure, and RInCu4 (R=Gd, Dy, Ho, Er and Tm).

Condensed Matter
Friday, March 18, 2016
1:00 PM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"Gauging and Orbifolding the topological phases in (2+1) and (3+1) dimensions"

Xiao Chen , University of Illinois at Urbana-Champaign
[Host: Jeffrey Teo]
ABSTRACT:

Topological phases of matter are commonly equipped with global symmetries, such as electric-magnetic duality in gauge theories and bilayer symmetry in fractional quantum Hall states. Gauging these symmetries into local dynamical ones is one way of obtaining exotic phases from conventional systems. We first study this using the bulk-boundary correspondence in the (2+1) dimensional topological phases and orbifolding the (1+1) dimensional edge described by a conformal field theory (CFT). Our procedure puts twisted boundary conditions into the partition function, and predicts the fusion, spin and braiding behavior of anyonic excitations after gauging. We demonstrate this for the twofold-symmetric D(Z_N ) quantum double model, SU(3)_1, and the S_3-symmetric SO(8)_1 state. Later on, we generalize this idea to (3+1) dimensional topological phases and study the bulk-boundary correspondence there

Condensed Matter
Thursday, March 3, 2016
1:00 PM
Physics Building, Room 313
Note special time.
Note special room.

"Complex Magnetic Phase Diagrams of Models for Iron Based Superconductors"

Elbio Dagotto , University of Tennessee/Oak Ridge National Laboratory
[Host: Despina Louca]
ABSTRACT:

high temperature superconductors, emphasizing that these materials appear to
be located in an intermediate Hubbard coupling regime [1] which is difficult
to study with canonical analytical techniques. Moreover, competing
ferro- and antiferro-magnetic tendencies are present, leading to frustration.
For these reasons computational studies are important. After the introduction,
and time allowing, I will focus on three main areas of recent active research:
(i) the widely discussed spin nematic state analyzed from the perspective
of the spin fermion model [2];
(ii) the several zero temperature competing magnetic states that appear
in quasi one-dimensional two-leg ladder geometries  [3];
(iii) the rich phase diagrams of multiorbital Hubbard models varying
couplings and temperatures from the perspective of a recently developed
technique that combines mean field and Monte Carlo aspects [4].

References:

[1] P. Dai, JP Hu, and E.D., Nat. Phys. 8, 709 (2012); E.D., RMP 85, 849 (2013).

[2] S. Liang et al., PRL 111, 047004 (2013); S. Liang et al., PRB 90, 184507 (2014).

[3] Q. Luo et al., PRB 84, 140506(R) (2011); Q. Luo et al., PRB 87, 024404 (2013); S.
Dong et al, PRL 113, 187204 (2014); J. Rincon et al., PRL 112, 106405 (2014).

[4] A. Mukherjee et al., PRB 90, 205133 (2014), and references therein;
A. Mukherjee et al., in preparation.

Condensed Matter
Thursday, February 25, 2016
1:00 PM
Physics Building, Room 313
Note special time.
Note special room.

"Jamming and glassy behavior in dense biological tissues"

Max Dapeng Bi , Rockefeller University
[Host: Marija Vucelja]
ABSTRACT:

Cells must move through tissues in many important biological processes, including embryonic development, cancer metastasis, and wound healing. Often these tissues are dense and a cell's motion is strongly constrained by its neighbors, leading to glassy dynamics. Although there is a density-driven glass transition in particle-based models for active matter, these cannot explain liquid-to-solid transitions in confluent tissues, where there are no gaps between cells and the packing fraction remains fixed and equal to unity. I will demonstrate the existence of a new type of rigidity transition that occurs in confluent tissue monolayers at constant density.  The onset of rigidity is governed by a model parameter that encodes single-cell properties such as cell-cell adhesion and cortical tension. I will also introduce a new model that simultaneously captures polarized cell motility and multicellular interactions in a confluent tissue and identify a glassy transition line that originates at the critical point of the rigidity transition. This work suggests an experimentally accessible structural order parameter that specifies the entire transition surface separating fluid tissues and solid tissues. Finally I will present my collaboration work with the Fredberg group (Harvard School of Public Health), where these predictions have been successfully tested in bronchial epithelial cells from asthma patients, explaining pathologies in lung epithelia.

Condensed Matter
Thursday, February 18, 2016
1:00 PM
Physics Building, Room 313
Note special time.
Note special room.

"Phase Diagram and Quantum Order by Disorder in the Kitaev K1-K2 Honeycomb Model"

Natalia Perkins , University of Minnesota
[Host: Gia-Wei Chern]
ABSTRACT:

The search for experimentally tangible scenarios of spin liquids as topologically ordered quantum states of matter is one of the most vibrant subfields of contemporary condensed matter research. Honeycomb iridates and related materials have originally been suggested as possible candidates for hosting a spin-liquid state. Intriguingly, no quantum paramagnetic ground state has been discovered so far in these materials, posing fundamental challenges to determining an accurate underlying microscopic spin model. In our study we  show that the second-neighbor Kitaev coupling is an important ingredient to such a microscopic description for the strong spin-orbit  transition-metal oxide Na2IrO3.

We analyze the K1-K2 model from a variety of methodological perspectives. As a coherent picture emerges from the investigation, the K1-K2 model allows us to explain the onset of zigzag magnetic order that is also found experimentally. Furthermore, we find that the K1-K2 model is a suitable minimal description for resolving the substantially different nature of quantum and thermal fluctuations originating from such Kitaev couplings.

Condensed Matter
Thursday, February 4, 2016
12:30 PM
Physics Building, Room 313
Note special time.
Note special room.

"Resonant inelastic x-ray scattering as a probe of band structure"

Marton Kanasz-Nagy , Harvard University
[Host: Israel Klich ]
ABSTRACT:

Resonant inelastic x-ray scattering (RIXS) has emerged as a powerful tool for studying high-temperature superconducting materials. In the talk, I will show how a theory based on non-interacting quasi-particles can describe recent experimental data on optimally doped and overdoped YBa2Cu3O6+x [Minola et al., Phys. Rev. Lett. 114, 217003 (2015)]. Surprisingly, the RIXS signal is qualitatively different from those measured using a different cuprate material, Bi2Sr2CuO6+x [Guarise et al., Nat. Commun. 5, 5760 (2014)], a feature originally attributed to collective magnetic excitations. I will demonstrate that this discrepancy can be explained by the sensitivity of RIXS to details of the band structures of these materials, especially at energies well above the Fermi surface. This energy range is inaccessible to traditionally used band structure probes, such as angle-resolved photemisson spectroscopy, making RIXS a powerful band structure probe, potentially applicable to a wide range of materials.

Condensed Matter
Thursday, January 21, 2016
12:30 PM
Physics Building, Room 313
Note special time.
Note special room.

"Extracting Hidden Hierarchies in Complex Spatial Networks"

Carl Modes , Rockefeller University
[Host: Marija Vucelja]
ABSTRACT:

Natural and man-made transport webs are frequently dominated by dense sets of nested cycles. The architecture of these networks -- the topology and edge weights -- determines how efficiently the networks perform their function. Yet, the set of tools that can characterize such a weighted cycle-rich architecture in a physically relevant, mathematically compact way is sparse. In order to fill this void, this seminar presents a new characterization that rests on an abstraction of the physical tiling' in the case of a two dimensional network to an effective tiling of an abstract surface in space that the network may be thought to sit in. Generically these abstract surfaces are richer than the plane and upon sequential removal of the weakest links by edge weight, neighboring tiles merge and a tree characterizing this merging process results. The properties of this characteristic tree can provide the physical and topological data required to describe the architecture of the network and to build physical models. This new algorithm can be used for automated phenotypic characterization of any weighted network whose structure is dominated by cycles, such as mammalian vasculature in the organs, the root networks of clonal colonies like quaking aspen, and the force networks in jammed granular matter.

Condensed Matter
Thursday, December 3, 2015
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Local probe investigation of interfaces in two-dimensional materials"

Prof. Chenggang Tao , Virginia Tech
[Host: Utpal Chatterjee]
ABSTRACT:

Emerging two-dimensional (2D) materials, such as graphene and atomically thin transition metal dichalcogenides, have been the subject of intense research efforts for their fascinating properties and potential applications in future electronic and optical devices. The interfaces in these 2D materials, including domain boundaries and edges, strongly govern the electronic and magnetic behavior and can potentially host new states. On the other hand these interfaces are more susceptible to thermal fluctuation and external stimuli that enable mass displacement and generate disorder. In this talk I will present our scanning tunneling microscopy (STM) and spectroscopy (STS) explorations of edges of few layered MoS2 nanostructures with unique structural and electronic properties and show how step edges on TiSe2 surfaces change dynamically due to electrical fields. I will also discuss temperature evolution of quasi-1D C60 nanostructures on graphene.

Condensed Matter
Thursday, November 12, 2015
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Magnetic orderings of XY-pyrochores"

Prof. Haidong Zhou , University of Tennessee
[Host: Seunghun Lee]
ABSTRACT:

Due to the geometrically frustrated lattice and effective spin-1/2, the XY pyrochlores Er2Ti2O7 and Yb2Ti2O7 exhibit exotic magnetic ground states related to quantum spin fluctuations. In this talk, we presented our recent studies on the magnetic orderings of new XY pyrochlores Er2Ge2O7 and Yb2Ge2O7. Then we tried to unify the magnetic orderings of all studied XY pyrochlores Er2B2O7 and Yb2B2O7 (B = Sn, Ti, and Ge) through the chemical pressure effects based on the theoretically proposed phase diagram.

Condensed Matter
Thursday, November 5, 2015
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Topological superconductivity in Metal/Quantum-Spin-Ice heterostructure"

Prof. Eun-Ah Kim , Cornell University
[Host: Seunghun Lee]
ABSTRACT:

Achieving predictive power is the central problem faced by the field of unconventional superconductivity. One long standing proposal by Anderson predicts that a quantum spin liquid(QSL) will give way to a superconductor upon doping. However, to the best of our knowledge no QSL has been successfully doped into becoming a superconductor. In this talk, I will discuss our proposal of a conceptually new framework. We propose to exploit spin entanglement in QSL to drive superconductivity without doping and hence destroying QSL: a heterostructure consisting of a QSL and a metal. In this new proposal, the conduction electrons in metal will "borrow" the spin correlation in QSL to pair leaving QSL itself intact. To aid materialization of the proposed setup we focus on using quantum spin ice as the QSL layer and establish the guideline for finding a suitable compound to be grown on a QSL substrate. I will present our prediction for a topological superconductivity in one such setup: Y2Sn2-xSbxO7 grown on the (111) surface of Pr2Zr2O7. The predicted order parameter symmetry is analogous to that of 2D superfluid He3-B phase and the realization of the proposal will amount to the first solid-state realization of such pairing state.

Condensed Matter
Thursday, October 29, 2015
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Bulk Thermoelectric and Thermodynamic Probes of non-Abelian Anyons in Topological States of Matter"

Kun Yang , FSU
[Host: Israel Klich ]
ABSTRACT:

Topological states of matter support quasiparticle excitations with fractional charge and possibly exotic statistics of the non-Abelian type, known as non-Abelian anyons. Most current experimental attempts to reveal such exotic statistics focus on interference involving edge transport. After a brief introduction of topological states (mostly in the context of fractional quantum Hall effect) in general, in this talk we will discuss how one can reveal the non-Abelian quasiparticle statistics using bulk probes. We show that bulk thermopower is a promising way to detect their non-Abelian nature, and measure the quantum dimension (a key parameter that quantifies non-Abelian statistics) of these anyons. This method is particularly effective in the Corbino geometry. We also demonstrate a novel cooling effect associated with them. We discuss application of these ideas to the specific candidate system of fractional quantum Hall liquid at filling factor 5/2, and topological insulator-superconductor hybrid systems. Some of the predicted behavior has been observed in recent experiments, which will also be discussed.

Condensed Matter
Thursday, October 15, 2015
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Phononics in liquids and supercritical fluids"

Dima Bolmatov , Brookhaven National Lab
[Host: Genya Kolomeisky]
ABSTRACT:

Abstract: While physicists have a good theoretical understanding of the heat capacity of both solids and gases, a general theory of the heat capacity of liquids has always remained elusive. Apart from being an awkward hole in our knowledge of condensed-matter physics, heat capacity – the amount of heat needed to change a substance's temperature by a certain amount – is a technologically relevant quantity that it would be nice to be able to predict1. I will introduce a phonon-based approach to liquids2 and supercritical fluids3, and describe its thermodynamics in terms of phonon excitations. I will show that the effective Hamiltonian4 has a transverse phononic gaps and explain their evolution with temperature variations. I will explain how the introduced formalism covers the Debye theory of solids, the phonon theory of liquids, and thermodynamic limits such as the Delong-Petit and the ideal gas thermodynamic limits. The experimental evidence for the new thermodynamic boundary (the Frenkel line) on the pressure-temperature phase diagram will be demonstrated5. Finally, I will discuss the phonon propagation and localization effects in liquids above and below the Frenkel line, and outline new directions towards phonon band gaps engineering and sound manipulation.

Condensed Matter
Thursday, October 8, 2015
11:00 AM
Physics Building, Room 313*
Note special time.
Note special room.

"Quantum dissection of a covalent bond with the entanglement spectrum"

Norman Tubman , UIUC
[Host: Israel Klich]
ABSTRACT:

Theoretical analysis of molecular bonding is quite successful in many regards, but leaves open many mysteries, even for simple diatomic molecules. Such mysteries have been highlighted in several recent theoretical studies in which there have been claims of a new type of bond identified in the carbon dimer.  These studies have been quite controversial.    To address these claims directly from the quantum mechanical wave function, we developed a new approach to describe chemical bonds from ideas originating in the field of quantum entanglement. This approach is based on breaking up a many body wave function into real space pieces, which is formally known as the entanglement spectrum.  We are able to address the controversial C2 molecule with these tools, and demonstrate its bonding properties, which includes an inverted fourth bond.  The ideas considered in this work provide a significantly more detailed picture of bonding than allowed by previous techniques.

Condensed Matter
Thursday, October 1, 2015
3:30 PM
Physics Building, Room 204

"A counterexample to the area law for quantum matter"

Ramis Movassagh , IBM
[Host: Israel Klich]
ABSTRACT:

Entanglement is a quantum correlation which does not appear classically, and it serves as a resource for quantum technologies such as quantum computing. The area law says that the amount of entanglement between a subsystem and the rest of the system is proportional to the area of the boundary of the subsystem and not its volume. A system that obeys an area law can be simulated more efficiently than an arbitrary quantum system, and an area lawprovides useful information about the low-energy physics of the system. It was widely believed that the area law could not be violated by more than a logarithmic factor (e.g. based on critical systems and ideas from conformal field theory) in the system’s size. We introduce a class of exactly solvable one-dimensional models which we can prove have exponentially more entanglement than previously expected, and violate the area law by a square root factor.  We also prove that the gap closes as n^{-c}, where c \ge 2, which rules out conformal field theories as the continuum limit of these models. It is our hope that the mathematical techniques introduce herein will be of use for solving other problems.

(Joint work with Peter Shor).

References:

Phys. Rev. Lett. 109, 207202

Condensed Matter
Thursday, September 3, 2015
3:30 PM
Physics Building, Room 204

"Topological line nodes in Ca3P2 and other semi-metals"

Ching-Kai Chiu , University of Maryland
[Host: Jeffrey Teo]
ABSTRACT:

In an ordinary three-dimensional metal the Fermi surface forms a two-dimensional closed sheet separating the filled from the empty states. Topological semimetals, on the other hand, can exhibit protected one-dimensional Fermi lines or zero-dimensional Fermi points, which arise due to an intricate interplay between symmetry and topology of the electronic wavefunctions. Here, we study how reflection symmetry, time-reversal symmetry, and inversion symmetry leads to the topological protection of line nodes in three-dimensional semi-metals. We derive the Z- and Z2-type invariants that guarantee the stability of the line nodes and lead to the appearance of protected surfaces states. As a representative example of a topological semimetal with line nodes, we consider Ca3P2 and discuss the topological properties of its Fermi line in terms of a tight-binding model, derived from ab initio DFT calculations. We show that due to a bulk-boundary correspondence, Ca3P2 displays nearly dispersionless surface states, which take the shape of a drumhead. These topological surface states give rise to the charge polarization.

Condensed Matter
Thursday, April 23, 2015
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Magnetic Nematicity in the Hidden Ordered Material URu2Si2"

Peter Riseborough , Temple University
[Host: Bellave Shivaram]
ABSTRACT:

The compound URu2Si2 is a heavy-fermion compound with a linear T term in the low temperatures electronic specific heat with a coefficient of 155 mJ/mole/K2. At T=1.5 K the system undergoes a transition to a superconducting state but exhibits another transition at the higher temperature of T=17.5 K. The 17.5 K transition is marked by a large lambda-like anomaly in the specific heat and was originally thought to be a second-order transition to a spin density wave state. The change in entropy associated with the specific heat anomaly is of the order of 0.3 kB ln(2). Also, at the transition, the system loses 90% of the carriers and develops a gap of the order of 7 meV over about 60% of the Fermi-surface. However, neutron scattering experiments have demonstrated that spin density wave order is absent below the 17.5 K transition and, likewise, x-ray scattering experiments have indicated the absence of charge or orbital density wave ordering. Despite over 30 years of intensive study, the nature of the ordering is still unknown. Hence, the transition is now known as the Hidden Order transition.

We propose that the 17.5 K transition is due to a combined spin and orbital ordering which can only be probed by measurements that are both charge and spin sensitive. We have developed a model in which the Hund’s rule exchange interaction stabilizes this novel state. Furthermore, we show that the material breaks spin-rotational invariance at low temperatures, but does not develop a static magnetic moment. In particular, the model shows that the magnetic susceptibility becomes anisotropic below the ordering temperature. Our calculations are compared with the results of the magnetic torque measurements of Okazaki et al. that found the four-fold symmetry of the magnetic susceptibility in the a-b plane is broken as the susceptibility develops a two-fold axis below the transition.

SLIDESHOW:
Condensed Matter
Friday, April 17, 2015
11:00 AM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"In-plane Charge Fluctuations in Bismuth Sulfide Superconductors"

Anushika Athauda , University of Virginia
[Host: Despina Louca]
Condensed Matter
Thursday, April 16, 2015
3:30 PM
Physics Building, Room 204

"Entanglement entropy from thermodynamic entropy"

[Host: Israel Klich]
ABSTRACT:

In recent years, entanglement has become a main frontier with applications across several fields in physics. Nevertheless, simple conceptual pictures and practical ways to compute the entanglement, especially in many-body systems, have remained elusive. In this talk, I will consider a simple setup where a dispersive medium becomes entangled with zero-point fluctuations in the vacuum. I show that the entanglement entropy of the material body with the vacuum can be viewed as the classical thermodynamic entropy in one higher dimension. Such mapping allows us to immediately verify the quantum version of the strong subadditivity property. As a byproduct of our formalism, I show that the entanglement finds a simple pictorial interpretation in terms of phantom polymers.

Condensed Matter
Tuesday, April 7, 2015
3:30 PM
Physics Building, Room 204
Note special date.

"Length Scale Dependent of Thermal Conductivity of Si-Ge Alloys"

Long Chen , University of Virginia
[Host: Joe Poon]
Condensed Matter
Thursday, April 2, 2015
3:30 PM
Physics Building, Room 204

"Skyrmions in Helimagnets"

Jiadong Zang , Johns Hopkins University
[Host: Jeffrey Teo]
ABSTRACT:

A Skyrmion is a topological configuration in which local spins wrap around the unit sphere for an integer number of times. After decades of theoretical discussions in high energy physics, it has been recently observed in a series of non-centrosymmetric chiral magnets. Several experiments by neutron scattering or transmission electron microscopy confirm the presence of skyrmions in a crystalline state at a finite window of magnetic field and temperature. Skyrmions show various novel properties inherent to its topological nature, such as topological Hall effect, topological stability, and ultralow critical current for movement, which offer the skyrmion promising prospects for next generation spintronic devices and information storage.

In this talk, I will explain the physical origin of skyrmions in chiral magnets, and discuss their dynamics under electric current or temperature gradient, where an emergent electromagnetism plays an important role. Furthermore, several routes to single skyrmions will be presented, and new chiral magnet materials are discussed.

Condensed Matter
Thursday, March 26, 2015
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Micro-machined probes for the study of quantum fluids"

Yoonseok Lee , University of Florida
[Host: Seunghun Lee]
ABSTRACT:

In the superfluid phases of liquid 3He with p-wave spin triplet pairing, all of the continuous symmetry except for the translational symmetry are broken, exhibiting extraordinarily rich physical phenomena. In particular, the surface scattering in unconventional superfluids/superconductors induces quasi-particle sub-gap bound states spatially localized near the surface within the coherence length xo, called the Andreev surface bound states (ASBS). This generic nature of the unconventional order parameter combined with the exotic symmetries in the superfluid phases of 3He lends an unfathomable source of fascinating physical phenomena predicted to exist in confined geometry: crystalline superfluid phase, Majorana fermionic excitations, and helical spin current in the B-phase, and the edge current and the chiral edge state in the A-phase, many of which are of topological origin. We have developed micro-machined probes or micro-electro-mechanical system (MEMS) devices for this purpose. In this talk, we will discuss the design and the operation of the device, and the results obtained using these devices in air, liquid 3He and also in liquid 4He in a wide range of temperature down to submillikelvin range. Our work demonstrates great potential of the device in a wide range of experiments in quantum fluids.

SLIDESHOW:
Condensed Matter
Wednesday, March 18, 2015
3:30 PM
Physics Building, Room 204
Note special date.

"Magneto-transport and domain wall scattering in the epitaxial L10 MnAl thin films"

Linqiang Luo , University of Virginia
[Host: Stu Wolf]
ABSTRACT:

L10 MnAl films with perpendicular magnetic anisotropy were synthesized by a bias target ion beam deposition technique. XRD results showed that MnAl thin film was expitaxially grew on Cr buffered MgO(001) substrate with tetragonal distortion c/a ration ~1.3. A Cr seeding layer optimized the magnetic anisotropy and saturation magnetization. The magneto-transport(MR) properties of MnAl were investigated by using a Hall bar structure. From 320K to 175K, the MR curve peaked exactly at coercivity field and was suggested to be a manifestation of domain wall(DW) scattering. Maze-like strip domains were observed by MFM after demagnetization. The difference of in-plane and out-of-plane resistance showed evidence of domain effect. The quantitative analysis for domain density in comparison with that of resistance implied the contribution of domain wall scattering. Below 150K, the effect of DW scattering on the MR was very small compared to the Lorentz MR.

Condensed Matter
Thursday, February 26, 2015
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Problems in Human Motion Planning"

Brian Skinner , Argonne National Laboratory
[Host: Genya Kolomeisky]
ABSTRACT:

Moving through a densely-populated environment can be surprisingly hard, owing to the problem of congestion. Learning to deal with congestion in crowds and in networks is a long-standing and urgently-studied problem, one that can be equally well described at the level of dense, correlated matter or at the level of game-theoretical decision making. In this talk I describe two related problems associated with motion planning in congested environments. In the first part I consider a description of pedestrian crowds as densely-packed repulsive particles, and I address the question: what is the form of the pedestrian-pedestrian interaction law?  In the second part of the talk I examine a simple model of a traffic network and study how inefficiency in the traffic flow arises from "selfish" decision-making. Analysis of the model reveals a surprising connection between Nash equilibria from game theory and percolative phase transitions from statistical physics.

SLIDESHOW:
Condensed Matter
Thursday, February 12, 2015
3:30 PM
Physics Building, Room 204

"Mechanics of strong DNA bending"

Harold Kim , Georgia Tech
[Host: Seunghun Lee]
ABSTRACT:

Bending of double-stranded DNA plays an essential role in genome structure and function. However, the energetic cost of sharp DNA bending is difficult to determine. In this talk, I will explain how we study the energetics of sharp DNA bending using small DNA loops. Using single-molecule Fluorescence Resonance Energy Transfer (FRET), we measure the lifetime of a DNA loop as a function of loop size. Above a critical loop size, the loop lifetime changes with loop size in a manner consistent with elastic bending stress, but below it, becomes less sensitive to loop size, indicative of DNA softening. Our result is in quantitative agreement with the kinkable worm-like chain model. Time permitting, I will also present a new method to compute the equilibrium distribution of forces exerted by a semi-flexible polymer loop.

Condensed Matter
Wednesday, February 11, 2015
3:30 PM
Physics Building, Room 204
Note special date.

"Tensor network renormalization"

Glen Evenbly , Caltech
[Host: Israel Klich]
ABSTRACT:

I will describe how to define a proper RG flow in the space of tensor networks, with applications to the evaluation of classical partition functions, euclidean path integrals, and overlaps of tensor network states.

Condensed Matter
Thursday, February 5, 2015
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Quantum quenches in 2D via arrays of coupled chains"

Andrew James , University College of London
[Host: Israel Klich]
ABSTRACT:

Matrix product state (MPS) methods, while highly effective when applied to the study of quantum systems in 1D, stumble in higher dimensions due to the 'area law' growth of entanglement entropy. This growth of entanglement can be mitigated in 2D by studying anistropic systems composed of coupled integrable chains, because the required area' is reduced. As a specific example I will describe the implementation of the time evolving block decimation algorithm to study quantum quenches in a system of coupled quantum Ising chains.

SLIDESHOW:
Condensed Matter
Thursday, January 29, 2015
3:30 PM
Physics Building, Room 204

"Metamagnetism"

Bellave Shivaram & Vittorio Celli , University of Virginia
[Host: Utpal Chatterjee]
ABSTRACT:

A metamagnet (MM) is characterized by the sharp rise of the low-temperature magnetization at a critical field Bc. Other MM properties that become singular at T=0 and B= Bc include specific heats, magnetostriction, and sound velocities. Metamagnetism occurs in single molecules as well as in solids, and is not a phase transition in the classical sense; however, distinctive MM features are also seen at finite T for all B. We will present new data on macromolecules and heavy-fermion metals, such as UPt3, and show how these systems are well described by a simple two-levels model, which appears to be applicable to all MMs, when suitably extended.

Condensed Matter
Thursday, January 15, 2015
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Uniaxial Pressure on Strongly Correlated Materials"

Rena Zieve , University of California, Davis
[Host: Bellave Shivaram]
ABSTRACT:

We describe a helium-activated bellows cell for measurements under uniaxial pressure. The apparatus can be useful for various reasons: monitoring asymmetric behavior with pressure applied along different axes; breaking the symmetry of a tetragonal crystal; changing pressure at low temperature to cross pressure-induced phase transitions directly; and making smaller pressure adjustments than possible with typical hydrostatic pressure arrangements. Several families of unconventional superconductors, including high-Tc materials, the 115 family, and iron pnictides and chalcogenides, have a layered structure and notably anisotropic behavior, making them good candidates for uniaxial pressure measurements. We present data on 115 and pnictide materials highlighting the influence of geometry on their phase diagrams.

SLIDESHOW:
Joint Nuclear/Condensed Matter Physics Seminar

Thursday, December 4, 2014
3:30 PM
Physics Building, Room 204

"Optimal number of terms in QED series and its consequence in condensed matter implementations of QED"

Genya Kolomeisky , University of Virginia
[Host: Oscar Rondon]
ABSTRACT:

In 1952 Dyson put forward a simple and powerful argument indicating that the perturbative expansions of QED are asymptotic. His argument can be related to Chandrasekhar's limit on the mass of a star for stability against gravitational collapse. Combining these two arguments we estimate the optimal number of terms of the QED series to be approximately 5000. For condensed matter manifestations of QED in narrow band-gap semiconductors and Weyl semimetals the optimal number of terms is around 80 while in graphene the utility of the perturbation theory is severely limited.

Condensed Matter
Thursday, November 20, 2014
3:30 PM
Physics Building, Room 204

"Anyonics: Designing exotic circuitry with non-Abelian anyons"

Kiril Shtengel , University of California, Riverside
[Host: Israel Klich]
ABSTRACT:

Non-Abelian anyons are widely sought for the exotic fundamental physics they harbour as well as for their possible applications for quantum information processing. Currently, there are numerous blueprints for stabilizing the simplest type of non-Abelian anyon, a Majorana zero energy mode bound to a vortex or a domain wall. One such candidate system, a so-called "Majorana wire" can be made by judiciously interfacing readily available materials; the experimental evidence for the viability of this approach is presently emerging. Following this idea, we introduce a device fabricated from conventional fractional quantum Hall states, s-wave superconductors and insulators with strong spin-orbit coupling. Similarly to a Majorana wire, the ends of our “quantum wire” would bind "parafermions", exotic non-Abelian anyons which can be viewed as fractionalized Majorana zero modes.

I will briefly discuss their properties and describe how such parafermions can be used to construct new and potentially useful circuit elements which include current and voltage mirrors, transistors for fractional charge currents and "flux capacitors".

Condensed Matter
Monday, November 17, 2014
3:30 PM
Physics Building, Room 204
Note special date.

"Impact of electronic nematicity on the unconventional superconductivity of the iron pnictides"

Rafael Fernandes , University of Minnesota
[Host: Despina Louca]
ABSTRACT:

Five years after their discovery, much of the interest in the iron pnictides remains in understanding not only their high-temperature superconducting phase, but also the nature of their normal state. In this context, recent experiments have provided strong evidence for the existence of an unusual correlated state in the phase diagram of these materials, dubbed electronic nematic. Below the nematic transition temperature, the tetragonal symmetry of the system is broken down to orthorhombic not by lattice vibrations, but by electronic degrees of freedom. However, two questions remain open: What is the origin of this nematic state? What is its relationship to the superconducting state? In this talk we will explore these two issues via a microscopic electronic model in which the nematic instability is caused by magnetic fluctuations arising from a degenerate ground state. A key consequence of this model is that lattice fluctuations and magnetic fluctuations are not independent. Instead, they follow a simple scaling relation, which we will show to be satisfied by elastic modulus and NMR experimental data. We will also demonstrate that, in general, nematic order competes with the unconventional sign-changing s+- superconducting state, although they may coexist under certain conditions. When the s+- instability is in close competition with a d-wave instability – as it has been suggested in several iron pnictides – we will show that nematic and superconducting degrees of freedom are strongly coupled. As a result, not only Tc can be significantly enhanced by nematic order, but also nematicity itself can be used as a diagnostic tool to search for more exotic superconducting states – such as states that spontaneously break time-reversal or tetragonal symmetries.

Condensed Matter
Thursday, November 6, 2014
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Negative Casimir Entropies in Nanoparticle Interactions"

Kimball Milton , University of Oklahoma
[Host: Israel Klich]
ABSTRACT:

Negative entropy has been known in Casimir systems for some time. For example, it can occur between parallel metallic plates modeled by a realistic Drude permittivity. Less well known is that negative entropy can occur purely geometrically, say between a perfectly conducting sphere and a conducting plate. The latter effect is most pronounced in the dipole approximation, which occurs when the size of the sphere is small compared to the separation between the sphere and the plate. Therefore, here we examine cases where negative entropy can occur between two electrically and magnetically polarizable nanoparticles or atoms, which need not be isotropic, and between such a small object and a conducting plate.

Negative entropy can occur even between two perfectly conducting spheres, between two electrically polarizable nanoparticles if there is sufficient anisotropy, between a perfectly conducting sphere and a Drude sphere, and between a sufficiently anisotropic  electrically polarizable nanoparticle and a transverse magnetic conducting plate.

SLIDESHOW:
Condensed Matter
Monday, October 13, 2014
3:30 PM
Physics Building, Room 204
Note special date.

"Spin dynamics in a rare-earth based single molecule magnet"

Maiko Kofu , Institute of Solid State Physics, University of Tokyo
[Host: Despina Louca]
ABSTRACT:

Single-molecule magnets (SMMs) are a class of metalorganic compounds in which each constituent molecule, containing magnetic atoms, possesses a giant and isolated resultant spin. Given that the giant spin exhibits easy-axis magnetic anisotropy, the magnetization reversal between the ground states is hindered by the potential barrier, yielding a slow magnetic relaxation that is characteristic of SMMs. In the beginning of SMM researches, SMMs containing multiple transition metal atoms such as Mn, Fe, and Ni, have been intensively studied. Recently, however, a new series of rare-earth based SMMs attracts much attention. In this talk, I will show our recent results on Tb-based SMMs. We have investigated the energy scheme by inelastic neutron scattering (INS) and the relaxation phenomena by quasi-elastic neutron scattering (QENS) measurements. The mechanism of magnetization reversal through quantum tunneling in the Tb-based SMM will be discussed.

Condensed Matter
Thursday, September 18, 2014
3:30 PM
Physics Building, Room 204

"Hybrid inorganic-organic nanomaterials as building blocks for designer solids and their applications in optoelectronic devices"

Joshua Choi , University of Virginia
[Host: Seunghun Lee]
ABSTRACT:

Tunable optoelectronic properties of hybrid inorganic-organic nanomaterials provide unique opportunities for science and technology. The degree of quantum confinement and collective electronic interaction in these materials can be readily tuned by controlling their synthesis and self-assembly processes. This enables fabrication of 'designer solids' with programmable optoelectronic properties tailored for specific scientific studies and technological applications. In this talk, I will discuss colloidal quantum dots and metal-organic perovskites, two of the most promising building blocks for designer solids. Both material systems exhibit intriguing properties tunable by design while looking set to revolutionize the field of solution processed optoelectronic devices.

Condensed Matter
Thursday, April 24, 2014
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Amplification, entanglement and storage of microwave radiation using superconducting circuits"

Jean-Damien Pillet , Columbia University
[Host: Tom Gallagher]
ABSTRACT:
Microwave signals have appeared during the last decade as a powerful and versatile platform to investigate a wide variety of quantum phenomena, from fundamental quantum optics to more oriented researches toward quantum information processing. This specific place originates from the fact that microwave signals can on one hand be precisely tailored and controlled with standard commercial electronics. On the other hand they can easily be processed at the single photon level in the quantum regime by superconducting circuits cooled down to dilution fridge temperatures thanks to a unique component, the Josephson junction, an intrinsic non dissipative lumped non-linear inductor.

I will present how we designed and built a superconducting circuit, based on a Josephson ring modulator (JRM), a ring of 4 Josephson junctions in a Wheatstone bridge configuration, allowing non-degenerate three wave-mixing. I will show that, when pumped at the appropriate frequency, this single circuit behaves as a tunable beam splitter with frequency conversion, a quantum limited amplifier or an EPR states generator. Using frequency conversion, we demonstrate on demand capture, storage and release of microwave radiations with approx. 80% catching efficiency and about 30 storage operations per memory lifetime. We then demonstrate entanglement generation between a propagating microwave mode and a localized mode in the cavity.

SLIDESHOW:
Condensed Matter
Wednesday, April 23, 2014
11:00 AM
Physics Building, Room 210
Note special date.
Note special time.
Note special room.

"Effect of surface disorder on transport in topological insulators"

Kun Woo Kim , Caltech
[Host: Israel Klich]
ABSTRACT:
What is the effect of surface-only disorder on the electronic states of a 3d TI? The layers in the clean bulk parallel to surface probe the surface impurities as they hop in and out of the surface layer. A recursive treatment of the impurity effects is made possible through successive elimination of the lattice layer by layer. This leads to non-linear renormalization group flow of an effective surface impurity potential. We found an exact mapping between the recursion relation and Schrodinger equation along the layers, therefore the modified self energy due to surface impurity could be simply obtained from the transfer matrix method. As a concrete example of 2d topological insulator, we found the exact expression of on-layer self energy for a clean system and an asymptotic expression that captures a general behavior of layers deep in the bulk.
Condensed Matter
Thursday, April 17, 2014
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Looking beyond the spin-orbit Mott phase"

Stephen Wilson , Boston College
[Host: Seunghun Lee]
ABSTRACT:
An unusual manifestation of Mott physics dependent on strong spin-orbit interactions has recently been identified in a growing number of classes of 5d transition metal oxides built from Ir4+ ions. Instead of the naively expected increased itinerancy of these iridates due to the larger orbital extent of their 5d valence electrons, the interplay between the amplified relativistic spin-orbit interaction (intrinsic to large Z iridium cations) and their residual on-site Coulomb interaction U, conspires to stabilize a novel class of spin-orbit assisted Mott insulators with a proposed Jeff=1/2 ground state wavefunction. The identification of this novel spin-orbit Mott state has been the focus of recent interest due to its potential of hosting a variety of new phases driven by correlated electron phenomena (such as high temperature superconductivity or enhanced ferroic behavior) in a strongly spin-orbit coupled setting. Currently, however, there remains very little understanding of how spin-orbit Mott phases respond to carrier doping and, more specifically, how relevant U remains for the charge carriers of a spin-orbit Mott phase once the bandwidth is increased. Here I will present our group’s recent experimental work exploring carrier doping and the resulting electronic phase behavior in one such spin-orbit driven Mott material, Sr3Ir2O7, with the ultimate goal of determining the relevance of U and electron correlation effects within the doped system’s ground state. Our results reveal the stabilization of an electronically phase separated ground state in B-site doped Sr3Ir2O7, suggestive of an extended regime of localization of in-plane doped carriers within the spin-orbit Mott phase. This results in a percolative metal-to-insulator transition with a novel, global, antiferromagnetic order. The electronic response of B-site doping in Sr3Ir2O7 will then be compared with recent results exploring A-site doping of electrons into the system and the resulting electronic phase diagrams discussed.
SLIDESHOW:
Condensed Matter
Thursday, April 10, 2014
3:30 PM
Physics Building, Room 204

"The internal structure of a vortex and its exited states, in a two dimensional superfluid"

Oded Agam , Hebrew University
[Host: Israel Klich]
ABSTRACT:
The motion of quantum vortex in a two-dimensional spinless superfluid is analyzed within Popov's hydrodynamic description. In the long healing length limit (where a large number of particles are inside the vortex core) the superfluid dynamics is determined by saddle points of Popov's action, which allows for weak solutions of the Gross-Pitaevskii equation. The resulting equations are solved for a vortex moving with respect to the superfluid. It is found that the vortex core is reconstructed in a non-analytic way. The response of the vortex to applied force produces an anomalously large dipole moment of the vortex and, as a result, the spectrum associated with the vortex motion exhibits narrow resonances lying within the phonon part of the spectrum, contrary to traditional view.
Condensed Matter
Monday, February 24, 2014
3:30 PM
Physics Building, Room 204
Note special date.

Slideshow (PDF)

"Matrix Product States for the Fractional Quantum Hall Effect"

Roger Mong , Caltech
[Host: Paul Fendley]
ABSTRACT:

The quantum Hall effect is an exotic phenomena found in nature. At fractional filling, these phases support fractional charge excitations with anyonic braiding statistics, which may be suitable for topological quantum computation. I will discuss the recent progress on studying these phases using matrix product states (MPSs) and the density-matrix-renormalization group (DMRG) technique. On one hand the MPS structure for quantum Hall reveals deep connection between quantum entanglement, conformal field theory, and topological field theory. Pragmatically, an understanding of the MPS structure allows us to perform efficient DMRG simulations for physical quantum Hall systems. There, the key advance is to reliably get the ground state wavefunction from a microscopic Hamiltonian, and to be able to identify the braiding statistics of the excitations solely from the ground state. Finally, I will also discuss various applications of these numerical methods.

SLIDESHOW:
Condensed Matter
Thursday, February 20, 2014
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"From Topological Insulators to Majorana Fermions"

Fan Zhang , University of Pennsylvania
[Host: Paul Fendley]
ABSTRACT:
The discovery of topological insulators has created a revolution in condensed matter science that has far ranging implications over coming decades. I will introduce a simplest way to understand various topological phases that can fit into an elegant periodic table, and apply these ideas to semimetals, insulators, and superconductors. In the case for a time-reversal-invariant topological superconductor, a Majorana Kramers pair may appear on the boundary and induces unprecedented fractional Josephson effects. These effects indicate the existence of a periodic building for topological phases, with the aforementioned table being its ground floor. Strong connections will be made to ongoing experiments and related topics.
SLIDESHOW:
Condensed Matter
Monday, February 17, 2014
3:30 PM
Physics Building, Room 204
Note special date.

Slideshow (PDF)

"Exotic Zero Energy Modes at Topological Defects in Crystalline Superconductors"

Jeffrey Teo , University of Illinois at Urbana-Champaign
[Host: Paul Fendley]
ABSTRACT:
Majorana bound states (MBS) are zero energy point-like excitations that support robust non-local storage of quantum information and non-abelian unitary operations. These exotic properties are essential requirements of a topological quantum computer. MBS were predicted to be present at magnetic flux vortices in spinless chiral p-wave superconductors. We show that MBS can also arise at gravitational" vortices in the form of lattice defects, such as dislocations, disclinations and corners, in a wider class of topological crystalline superconductors. We completely classify these BCS superconductors in two dimensions with discrete crystalline symmetries, and propose a criterion that determines the existence of MBS at a defect. This predicts the appearance of MBS at lattice defects in superconducting strontium ruthenate, regardless of its controversial pairing nature, and also doped graphene or silicene, which are single layers of carbon or silicon.
SLIDESHOW:
Condensed Matter
Thursday, February 6, 2014
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"The Bulk-Edge Correspondence in Abelian Fractional Quantum Hall States"

Michael Mulligan , Station Q/UCSB
[Host: Paul Fendley]
ABSTRACT:
It is commonly assumed that a given bulk quantum Hall state and its low energy edge excitations are in one-to-one correspondence. In this talk, I will explain, contrary to this conventional wisdom, how a given bulk state may host multiple, distinct edge phases. I will describe a few surprising examples of this phenomenon and discuss experimental consequences.
SLIDESHOW:
Condensed Matter
Monday, February 3, 2014
3:30 PM
Physics Building, Room 204
Note special date.

"Controlling Atomic Movement on the Nanoscale"

Sinisa Coh , University of California, Berkeley
[Host: Paul Fendley]
ABSTRACT:
Some of the grand challenges in nanoscience are the ability to control movement of atoms either to propel nanometer-sized machines, or to synthesize novel electronic devices and materials. To that end, electrical current can be used to move a wide range of metals (Fe, Cu, W, In, Ga) along the outside and inside of a carbon nanotube. In this talk I will present a peculiar mechanism in which these metals move. For example, this mechanism allows an iron nanocrystal to pass through a constriction in the carbon nanotube with a smaller cross-sectional area than the nanocrystal itself. Remarkably, while passing through a constriction, the nanocrystal remains largely solid and crystalline and the carbon nanotube is unaffected. This behavior is accounted for by a pattern of iron atom motion and rearrangement on the surface of the nanocrystal. The nanocrystal motion can be described with a model whose parameters are nearly independent of the nanocrystal length, area, temperature, and electromigration force magnitude. I will also discuss implications of this work on synthesis of nanocomposite materials, and on the stability of carbon-based electronic devices.

More details can be found in these publications:

Phys. Rev. Lett. 110, 185901 (2013)

Phys. Rev. B 88, 045424 (2013)

Condensed Matter
Friday, January 24, 2014
2:30 PM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"Casimir forces and torques. from quantum fluctuations to self assembly"

Raul Esquivel-Sirvent , Universidad Nacional Autónoma de México
[Host: Israel Klich]
Condensed Matter
Thursday, January 23, 2014
3:30 PM
Physics Building, Room 204

"Simulating condensed matter systems with tensor network states and discovery of algebraic decoherence"

Thomas Barthel , Ludwig-Maximilians-Universität Munich
[Host: Paul Fendley]
ABSTRACT:
The non-locality of quantum many-body systems can be quantified by entanglement measures. Studying the scaling behavior of such measures, one finds that the entanglement in most states of interest (occurring in nature) is far below the theoretical maximum. Hence, it is possible to describe such systems with a reduced set of effective degrees of freedom. This is exploited in simulation techniques based on so-called tensor network states (MPS, PEPS, or MERA). I will describe how this approach can be employed to simulate systems of all particle statistics in order to study ground states, thermal states, and non-equilibrium phenomena. Besides explaining the main ideas, I will highlight some applications.

The second part of the talk focuses on an application to the decoherence in systems that are coupled to an environment. Until our recent study, it was assumed that, as long as the environment is memory-less (i.e. Markovian), the temporal coherence decay is always exponential -- to a degree that this behavior was synonymously associated with decoherence. However, the situation can change if the system itself is a many-body system. For the open spin-1/2 XXZ model, we have discovered that the interplay between dissipation and internal interactions can lead to a divergence of the decohernce time. The quantum coherence then decays according to a power law. To complement the quasi-exact numerical simulation, I will explain the result on the basis of a perturbative treatment.

Condensed Matter
Tuesday, January 21, 2014
3:30 PM
Physics Building, Room 204
Note special date.

"Equilibration and Thermalization in Quantum Many-Body Systems"

Alioscia Hamma , Tsinghua University
[Host: Paul Fendley]
ABSTRACT:
A closed quantum mechanical many-body system evolves unitarily. Therefore, in a strict sense, it should not equilibrate or thermalize. However, thermalization occurs, as it is revealed in experiments in ultra cold atom gases with very long coherence times. How do equilibration and thermalization occur then? Even if equilibration cannot happen at the level of the wave-function, it may happen in the expectation value of typical observables, as Von Neumann first pointed out. Recently, it has been understood that equilibration and thermalization are due to the typicality of Entanglement. In this talk, I will review some of the recent theoretical progress made in this field. Typically, quantum states are very entangled, and this implies that locally they look like thermal states. A long-standing problem is the meaning and role of integrability for thermalization. Recent novel results show that the structure of Entanglement reveals the connection between non-integrability, irreversibility, and thermalization. Finally, I will describe how some exotic states of matter also show exotic ways of equlibrating, which is very promising for quantum information processing.
Condensed Matter
Thursday, January 16, 2014
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"What would you do with the exact functional? Probing the limits of density functional theory (DFT)"

Miles Stoudenmire , Perimeter Institute
[Host: Paul Fendley]
ABSTRACT:
Density functional theory (DFT) is a very successful method for computing properties of realistic many-electron systems applied across condensed matter and materials physics. It is based on exact principles, but using it in practice requires approximations. Unfortunately, present approximations have systematic drawbacks, such as failing to produce accurate charge gaps for strongly correlated systems or predicting spurious magnetic order.

To understand DFT’s potential and limitations, we have extended the powerful density matrix renormalization group (DMRG) technique to solve one-dimensional continuum electron systems with realistic interactions. Such systems are also interesting in their own right (in the context of cold atom experiment, for example). With our ability to solve these systems essentially exactly, we have even implemented the exact functional at the heart of DFT.

I will discuss our results on computing gaps within DFT (both exact and approximate) and a recent proof from our group that Kohn-Sham DFT always converges when using the exact functional. We find that while some drawbacks of DFT can be blamed on approximations, other limitations are fundamental. Yet our work suggests that great progress is possible for applying DFT to strongly correlated systems.

SLIDESHOW:
Condensed Matter
Thursday, December 12, 2013
3:30 PM
Physics Building, Room 313
Note special room.

"Monte Carlo Studies of Spin Ice"

Roger Melko , Perimeter Institute/Waterloo
[Host: Paul Fendley]
ABSTRACT:
The name Spin Ice refers to a class of materials where rare-earth magnetic moments reside on a pyrochlore lattice of corner sharing tetrahedron. Since their discovery in 1997 by Harris and colleagues, these materials (such as Dy2Ti2O7 and Ho2Ti2O7) have been a mainstay of classical frustrated magnetism research. The analogy to ice arises because the magnetic groundstate is actually a degenerate manifold of equal-energy states, which leads to the same ground-state entropy as the proton-positional disorder in ice water, calculated by Linus Pauling. The highly-constrained spin ice groundstate is an example of a classical "spin liquid", which harbors a fascinating array of phenomena, such as emergent monopole excitations and topological winding number sectors. In this talk, I will give a basic introduction to the physics of spin ice from the perspective of Monte Carlo simulations, including the necessity of considering dipolar interactions, its behavior in magnetic fields, and topological order in ice systems with reduced dimensionality.
Condensed Matter
Thursday, December 5, 2013
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Elasticity and Mixing on Random Graphs"

Marija Vucelja , Rockefeller University
[Host: Joe Poon]
ABSTRACT:
I will discuss the physics of mixing and elasticity on random graphs. The results I will show are important step toward understanding on the macroscopic properties of amorphous solids, spin glasses and alike and are also closely related to fields of interest of computer science. The first part of the talk is about elasticity. The physical properties of amorphous solids are far less understood than those of crystalline solids. The analysis of these systems is complicated due to disorder and vastly different interaction strengths present in these materials. I will look at spectral properties of random elastic networks and argue in which sense they provide a good toy-model of disordered solids. Using the Cavity method, a sort of Bethe-Peierls iterative method, I will derive the analytical expressions for the spectral density of such graphs. I will conclude this part by pointing out implications of these results on the physics of amorphous solids. Next I plan to talk about a new type of numerics for mixing on random graphs. These results are about Markov Chain Monte Carlo algorithms and have potential applications in studying granular materials, colloids, protein folding, etc. Most implementations of these algorithms use Markov Chains that obey detailed balance, even though this not a necessary requirement for converging to a steady state. I plan to overview several examples that utilize irreversible Markov Chains, where violating detailed balance has improved the convergence rate. Finally I will pose some open questions and discuss attempts to use non-equilibrium dynamics for efficient sampling.
SLIDESHOW:
Condensed Matter
Thursday, November 21, 2013
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Full counting statistics for matrix product states"

Yifei Shi , University of Virginia
[Host: Israel Klich]
SLIDESHOW:
Condensed Matter
Thursday, November 7, 2013
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Theory of Resonant X-Ray Scattering with Applications to high-Tc Cuprates"

David Benjamin , Harvard University
[Host: Israel Klich]
ABSTRACT:
Resonant x-ray scattering has recently become an exceptionally powerful and versatile technique. However, its theoretical foundation lags behind experiments. In particular, experiments on conducting phases are often interpreted in terms of local models based on Mott insulators. I will discuss a method that handles arbitrary band structures and includes the effect of the positively-charged core hole. Next, I will compare our results to experiments on conducting and charge density wave cuprate phases and show how simple quasiparticle physics yields signatures previously considered evidence of more exotic behavior.
SLIDESHOW:
Condensed Matter
Thursday, October 31, 2013
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Studying Topological Insulators via time reversal symmetry breaking and proximity effect"

Roni Ilan , UC Berkeley
[Host: Israel Klich]
ABSTRACT:
Topological Insulators (TI) have been at the focus of immense theoretical and experimental studies in the past few years. They represent accessible topological phases, where time reversal symmetry plays a key role. These systems provide a platform for the emergence of many exciting physical phenomena, such as helical transport in one and two dimensions, physics of Dirac fermions and the formation of Majorana fermion states. While the theoretical understanding of these materials progresses, it is crucial that theorists make concrete proposals for simple and informative ways to detect all the properties attributed to them. In this talk I will describe our proposals for transport experiments, the most common way to probe physics in condensed matter systems. I will present a few interesting questions concerning the current challenges transport experiments in TI are facing, discuss which physical phenomena are yet to be confirmed, and suggest two experimental schemes meant to detect the physics of the surface states in both two and three dimensional TI.
SLIDESHOW:
Condensed Matter
Thursday, October 24, 2013
3:30 PM
Physics Building, Room 204

"Microcavity Exciton-Polariton Condensates Physics and Applications"

Na Young Kim , Stanford University
[Host: Seunghun Lee]
ABSTRACT:
Microcavity exciton-polaritons are hybrid light-matter quasi-particles as an admixture of cavity photons and quantum well excitons. The inherent light-matter duality provides experimental advantages to form coherent condensates at high temperatures (e.g. 4 K in GaAs and room temperature in GaN materials), and to access the dynamics of exciton-polaritons.

I will first discuss the characteristics of exciton-polariton condensates with emphasis on their intrinsic open-dissipative nature. I will present exciton-polariton-lattice systems, where we explore the non-zero momentum condensate order. We envision that the polariton-lattice systems would serve as a solid-state platform to investigate strongly correlated materials. Finally, I will show our recent progress on electrically pumped exciton-polariton coherent matter waves towards the development of novel coherent light sources operating at low threshold powers and at high temperatures.

Condensed Matter
Thursday, October 17, 2013
3:30 PM
Physics Building, Room 204

"The Interplay of Correlation, Electron-Phonon Coupling, Magnetism, and Superconductivity in High Tc Superconductors"

Zhiping Yin , Rutgers University
[Host: Joe Poon]
ABSTRACT:
Strong electronic correlation lies at the heart of modern condensed matter and material physics. An adequate treatment of the electronic correlation beyond standard band theory is a key to understand many unusual physical properties of strongly correlated materials such as Mott insulators and unconventional superconductors. In this talk, I will show how electronic correlation leads to dramatic modifications of the band theory predictions for the electronic structures in general, and the electron-phonon coupling and spin dynamics in particular in high temperature superconductors. I will demonstrate that first-principles electronic structure methods GW and dynamical mean field theory (combined with density functional theory) can account for the non-local and local electronic correlations, respectively, beyond standard band theory, and are useful tools to understand the microscopic mechanisms of high temperature superconductivity in (Ba,K)BiO3 and iron-based superconductors. For the former, electronic correlation leads to a strong enhancement of the electron-phonon coupling while for the latter, electronic correlation significantly strengthens the low-energy spin excitation, both of which favor higher superconducting temperature. In the end, I will discuss how we may use these first-principles methods to accelerate the discoveries of new functional materials with desired properties.

References:
[1] Z. P. Yin, A. Kutepov, and G. Kotliar, Phys. Rev. X 3, 021011 (2013).
[2] Z. P. Yin, K. Haule, and G. Kotliar, Nat. Mater. 10, 932-935 (2011).
[3] Z. P. Yin, K. Haule, and G. Kotliar, Nat. Phys. 7, 294-297 (2011).
[4] Z. P. Yin, K. Haule, and G. Kotliar, Phys. Rev. B 86, 195141 (2012).
[5] Z. P. Yin, K. Haule, and G. Kotliar, to be published (2013).
[6] Z. P. Yin and G. Kotliar, EPL 101, 27002 (2013).
[7] M. Retuerto et al. Chemistry of Materials, 10.1021/cm402423x (2013).

Condensed Matter
Thursday, September 26, 2013
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Gauge dynamics of kagome antiferromagnets"

Michael Lawler , Binghamton University
[Host: Israel Klich]
ABSTRACT:
How to describe the semi-classical dynamics of spins in kagome lattice Heisenberg antiferromagnetic insulators remains a unsolved problem. In essence, the largest term in their classical Hamiltonian merely selects out a low energy sector leaving a great freedom for them to roam around within it. To make progress on the problem, I have employed the "Dirac constraint" method used to study problems in constrained Hamiltonian mechanics. This method is excellent at identifying the "physical modes" of a system, particularly when there is an unexpected gauge redundancy. Remarkably, for the spin waves of kagome antiferromagnets in their low energy sector, I found an extreme reduction in their number with it growing at best with the size of the boundary of the system. We have also carried out numerical simulations on the Heisenberg model with additional Dzyaloshinskii-Moriya interactions and found strong evidence for both, even though no specific restriction was made to the low energy sector. These results make significant progress on the solution to the problem of spin dynamics in kagome antiferromagnets and we hope similar progress could be made on a wide variety of other condensed matter systems.
SLIDESHOW:
Condensed Matter
Thursday, September 5, 2013
3:30 PM
Physics Building, Room 204

"Metal insulator transitions in Ir oxides with strong SOC: a case of SrIrO3 and related compounds"

Yoon H. Jeong , Pohang University of Science and Technology
[Host: Seunghun Lee]
ABSTRACT:
Motivated by the rich physics anticipated from strong spin orbit coupling, we have investigated various 5d Ir oxides. We will first describe the general physics of Ir compounds arising from strong SOC, and then focus on the metal insulator transitions occurring in perovskite SrIrO3 and related compounds.
Condensed Matter
Thursday, August 15, 2013
3:30 PM
Physics Building, Room 204

"Fock Parafermions and Self-Dual Representations of the Braid Group"

Emilio Cobanera , Leiden University
[Host: Israel Klich]
ABSTRACT:
Because of potential relevance to topological quantum information processing, we introduce and study the self-dual family of representations of the braid group. Self-dual representations are physically motivated by strong-coupling/weak-coupling dualities in clock models and include as special cases the Majorana and Gaussian (metaplectic) representations. To show that self-dual representations admit a particle interpretation, we introduce and describe in second quantization a family of particle species with (p=2,3,dots) exclusion and ( heta=2pi/p) exchange statistics. We call these anyons Fock parafermions, because they are the particles naturally associated to the parafermionic zero-energy modes potentially realizable in mesoscopic arrays of fractional topological insulators. Self-dual representations are local combinations of either parafermions or Fock parafermions, an important requisite for the potential physical implementation of (semi-)topologically protected quantum logic gates. The second-quantization description of Fock parafermions entails the concept of Fock algebra, i.e., a Fock space endowed with a statistical multiplication that captures and logically correlates these anyons' exclusion and exchange statistics. As a consequence, normal-ordering remains a well-defined operation.
Condensed Matter
Thursday, April 25, 2013
3:30 PM
Physics Building, Room 204

"Metabolic Imaging: A Novel Diagnostic Strategy for Hypertensive Heart"

Min Zhong , University of Virginia
[Host: Bijoy Kundu]
Condensed Matter
Thursday, April 18, 2013
3:30 PM
Physics Building, Room 204

"Z_2 spin liquid in the Heisenberg antiferromagnet on kagome"

Oleg Tchernyshyov , Johns Hopkins University
[Host: Paul Fendley]
ABSTRACT:
A spin liquid is a hypothetical quantum ground state of a frustrated Heisenberg magnet conjectured by P.W. Anderson in the 1970s. Its distinguishing features are (1) a lack of magnetic order and (2) a degeneracy that depends on the topology of the system. The status of spin liquids has been recently upgraded from purely speculative to plausible as these ground states were found in relatively simple two-dimensional spin models on square and kagome lattices. Experiments with herbertsmithite yielded clues of spin excitations with fractional spin 1/2 in a kagome antiferromagnet.

I will present a phenomenological Z_2 lattice gauge theory that describes low-energy properties of the spin liquid in a S=1/2 Heisenberg antiferromagnet on kagome. It reproduces many of the characteristic features observed in recent numerical studies of the model, including the ground-state degeneracy, response to quenched disorder, and spinon states near the system's edge.

Condensed Matter
Thursday, April 11, 2013
3:30 PM
Physics Building, Room 204

"Quantum Critical Behavior in a Resonant Level Coupled to a Dissipative Environment"

Gleb Finkelstein , Duke University
[Host: Israel Klich]
ABSTRACT:
We investigate tunneling through a resonant level embedded in a dissipative environment, which suppresses tunneling rates at low temperatures. Specifically, the resonant level is formed in a carbon nanotube quantum dot, and the dissipative environment is realized by fabricating resistive leads. For the symmetric coupling of the resonant level to the two leads, we find that the resonant peak reaches the unitary conductance e^2/h despite the presence of dissipative modes. Simultaneously, the width of the resonance tends to zero as a non-trivial power of temperature. We draw a connection between our system and a resonant tunneling in a Luttinger liquid and interpret the observed unitary resonance of vanishing width in terms of a quantum critical point (QCP). We further investigate an exotic state of electronic matter obtained by fine-tuning the system exactly to the QCP. Particularly striking is a quasi-linear non-Fermi liquid scattering rate found at the QCP, interpreted in terms of a Majorana resonant level model.
Quantum Information Seminar
Wednesday, April 10, 2013
3:30 PM
Physics Building, Room 313
Note special date.
Note special room.

"Classical and Quantum Annealing"

Matt Hastings , Microsoft
[Host: Paul Fendley]
ABSTRACT:
An informal talk about the quantum adiabatic algorithm for solving optimization problems using a quantum computer, and about attempts to simulate this algorithm on classical computers using quantum Monte Carlo. Interestingly, the child's puzzle in this YouTube video http://www.youtube.com/watch?v=dyWXPJSSRw8 will have direct applications to this problem.
Condensed Matter
Thursday, April 4, 2013
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Magnetic ground states and excitations in vanadates, BaV10O15 and CoV2O4"

Sachith Dissanayake , University of Virginia
[Host: Seunghun Lee]
SLIDESHOW:
Condensed Matter
Thursday, March 28, 2013
3:30 PM
Physics Building, Room 204

"Transport Phenomena in CaVO3 and SrVO3 thin films"

Mandy Gu , University of Virginia
[Host: Stu Wolf]
Condensed Matter
Thursday, March 14, 2013
11:00 AM
Physics Building, Room 204
Note special time.

"What is an essential factor for determining Tc in Iron Pnictides? - In terms of crystal structure and spin fluctuations -"

Chul-Ho Lee , National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan
[Host: Seunghun Lee]
ABSTRACT:
The discovery of superconductivity in LaFeAsO1-xFx at superconducting temperature of Tc = 26K has triggered the energetic study of searching a new superconductor. In an early stage, we reported that Tc attains maximum values when FeAs4-lattices form a regular tetrahedron [1]. Many new iron-based superconductors have been found after the report and most of them really follow the relationship. Recently, we also found that Tc depends on FeAs layer-layer distance as well as Fe-pnictogen bond length. Spin fluctuations can also be an essential factor for the superconductivity. I will summarize recent studies of spin fluctuations using inelastic neutron scattering. The reason and a strategy to get higher Tc will be discussed.
[1] C. H. Lee et al., J. Phys. Soc. Jpn. 77 (2008) 083704.
Condensed Matter
Thursday, February 14, 2013
3:30 PM
Physics Building, Room 204

"Graphene electronics and photonics "

Tony Low , IBM
[Host: Genya Kolomeisky & Avik Ghosh]
ABSTRACT:
Graphene possess unique properties for electronic and photonic applications, such as gate tunability, high carrier mobility, wide-band optical absorption extending into terahertz regime and compatibility with silicon processing technologies. Research in graphene device arena has progressed at a rapid pace, propelled by constant discoveries and maturing appreciation of the underlying graphene physics, advancement in processing technologies and most importantly, innovations. In this talk, I will review some of the exciting latest developments in graphene electronics and photonics. I would then provide a device physics perspective of what I think are some current issues limiting their performances and new opportunities going forward. Throughout this talk, I will draw extensively upon theoretical and modeling studies of our in-house experiments.
Here are some details of the talk. First, the internal and substrate polar optical phonons provide main energy dissipation pathway for optically excited carriers, and I will discuss our understanding of these energy loss channels and possibilities for more efficient graphene photodetectors and bolometers driven by hot electrons and phonons. Second, coupling of collective electronic excitations with these phonons were found leading to modified plasmon dispersions and losses, where long-lived hybrid plasmon-phonon coupled mode can be utilized in the terahertz to infrared spectrum for highly tunable plasmonic devices. Lastly, I will discuss how deformation and morphological structures found in large scale growth graphene can serve as dominant electronic scattering centers, compromising performance in high-speed electronic devices and the possibility of strain engineering for exploratory novel graphene electronics.
Condensed Matter
Tuesday, November 13, 2012
3:30 PM
Physics Building, Room 204
Note special date.

Slideshow (PDF)

"Toward a unified description of spin incoherent behavior at zero and finite temperatures"

[Host: Israel Klich]
ABSTRACT:
While the basic theoretical understanding of spin-charge separation in one-dimension, known as "Luttinger liquid theory", has existed for some time, recently a previously unidentified regime of strongly interacting one-dimensional systems at finite temperature came to light: The "spin-incoherent Luttinger liquid" (SILL). This occurs when the temperature is larger than the characteristic spin energy scale. I will show that the spin-incoherent state can be written exactly as a generalization of Ogata and Shiba's factorized wave function in an enlarged Hilbert space, using the so-called "thermo-field formalism." Interestingly, this wave-function can also describe the *ground-state* of other model Hamiltonians , such as t-J ladders, and the Kondo lattice. This allows us to develop a unified formalism to describe SILL physics both at zero, and finite temperatures.
SLIDESHOW:
Condensed Matter
Friday, November 9, 2012
2:30 PM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"Spin-lattice coupling and anomalous magnetic excitations in the triangular lattice antiferromagnet Ag2CrO2"

Masa Matsuda , Oak Ridge National Laboratory
[Host: Seunghun Lee]
ABSTRACT:
Spin-lattice coupling plays an important role in selecting the ground state in the geometrically frustrated magnets, since a small amount of structural distortion is sufficient to lift the ground state degeneracy and stabilize a long-range magnetic order. Ag2CrO2 consists of insulating triangular lattice planes of CrO2 (Cr3+ ion with S=3/2), which are separated by the metallic Ag2 layers. Interestingly, the electric transport in the Ag2 layer is strongly affected by the magnetism in the CrO2 layer. We found from their neutron diffraction experiments that a partially disordered state with 5 sublattices abruptly appears at TN=24 K, accompanied by a structural distortion [1]. The spin-lattice coupling stabilizes the anomalous state, which is expected to appear only in limited ranges of further-neighbor interactions and temperature. The nonnegligible further-neighbor interactions suggest the existence of the RKKY interaction mediated by the conduction electrons. We have also performed inelastic neutron scattering study of this material and found anomalous magnetic excitations, which cannot be explained simply by the linear spin-wave theory. The exotic behaviors in Ag2CrO2 are noteworthy. [1] M. Matsuda et al., PRB 85, 144407 (2012)
Condensed Matter
Thursday, November 8, 2012
3:30 PM
Physics Building, Room 204

"Non-abelian statistics of fractionalized Majorana Fermions"

Netanel Lindner , Caltech
[Host: Israel Klich]
ABSTRACT:
Edge states of topological insulators can be gapped out by introducing a coupling to a superconductor or by breaking time reversal symmetry. The interface between these two types of gapped regions hosts a Majorana zero mode. Here, we extend this idea to the case of edge states of fractional quantum Hall states. We show that as more interfaces are introduced, the ground state degeneracy grows with a quantum dimension of a square root of an even integer, corresponding to a new family of non-abelian anyons. Topologically protected braiding of two anyons can be achieved by a sequence of operations on the ground state manifold. We show that the unitary matrices representing these operations form representations of the braid group which are richer than that of Ising anyons. We discuss possible realizations of these ideas in experimentally accessible solid state systems.
Condensed Matter
Thursday, November 1, 2012
3:30 PM
Physics Building, Room 204

"Unconventional 'optics' with pseudospins in graphene: Metamaterials, Klein Tunneling and Subthermal switching"

Avik Ghosh , University of Virginia
[Host: Bellave Shivaram]
ABSTRACT:
A major motivation for graphene based electronics lies in its photon-like bandstructure, which makes the electron effective mass vanishingly small and mobilities much larger than their silicon counterparts. In practice however, charge puddles wash out the Dirac points and produce quasi-Ohmic current-voltage characteristics, making it hard to switch graphene electrons or to saturate their currents. Long channel devices saturate through remote optical phonon scattering, but are almost immediately compromised by band-to-band tunneling. One can open a band-gap using quantization (e.g. nanoribbons), local strains, antidot arrays or transverse fields in bilayer geometries. But the broken symmetry invariably increases the mass of the electrons and compromises mobility. This trade-off seems fundamental.

The richness of graphene electronics lies not just in its photon like eigenspectrum, but in the symmetry of its eigenvalues, specifically, the pseudospins arising from its dimer basis sets. On the one hand quasi-momentum conservation at a PN junction generates electronic analogues of Snell's law such as focusing, total internal reflection, and even negative index Veselago 'lensing'. On the other hand, the orthogonality of its pseudospins leads to Klein/antiKlein tunneling in mono/bilayer graphene, for which there seem to be experimental evidence in close agreement with atomistic models for current flow. By solving the Landauer-Keldysh quantum kinetic equations, we show that such electron 'optics' and Klein tunneling can be used to design novel low power switches that can beat the Landauer- Boltzmann thermal limit, including reconfigurable logic, metal-insulator transition switches, electron collimators and pseudospintronic analogs of electro-optic modulators.

Condensed Matter
Thursday, October 25, 2012
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Modal analysis of Casimir interactions"

Francesco Intravaia , Los Alamos National Laboratory
[Host: Israel Klich]
ABSTRACT:
While other numerical and analytic techniques exist, the modal expansion is one of the few that provides physical insight into the anatomy of the Casimir Effect. Although it is very simple in a small number of situations, this kind of analysis becomes quite involved when phenomena such as dissipation in the media or complicated geometries are taken into account for the calculation of the Casimir effect. Moreover, in this case, its connection and equivalence to other approaches becomes blurry and puzzling. The knowledge of the interplay between (quantum) thermodynamics, geometry, dissipation and mode decomposition can be important to understand the strength, the sign and, in general, the behavior of the Casimir force. Here, we investigate some aspects of the modal approach to Casimir forces, also presenting experimentally relevant examples.
SLIDESHOW:
Condensed Matter
Thursday, October 18, 2012
3:30 PM
Physics Building, Room 204

"Quantum criticality and superconducting gap evolution in Fe-based superconductors"

Kyuil Cho , The Ames Laboratory and Iowa State University
[Host: Seunghun Lee]
ABSTRACT:
The quantum critical point (QCP), where the transitions occur at absolute zero temperature, usually induces a new phase, such as unconventional superconductivity in organic, cuprate, heavy fermion and Fe-based superconductors. Whether the quantum critical point still exists beneath the superconducting phase is a long-standing question, but very tricky task since most of physical and magnetic properties are useless inside of superconducting phase. Here we have measured the zero-temperature penetration depth in BaFe2(As1-x,Px)2 by using tunnel diode resonator technique and identified the existence of the QCP beneath the superconducting dome [1]. In addition, the low-temperature analysis of the penetration depth measurements revealed that the superconducting gap structure shows a universal dome-like evolution as the dopant concentration increases. The details of experimental and theoretical consideration will be discussed.

[1] K. Hashimoto, Kyuil Cho, et al., Science 336, 1554 (2012).

Condensed Matter
Thursday, May 3, 2012
3:30 PM
Physics Building, Room 204

"Unconventional temperature-enhanced magnetism in Fe1.1Te"

Igor Zaliznyak , Brookhaven National Laboratory
[Host: Seunghun Lee]
ABSTRACT:
There are two common scenarios used to describe the magnetism in Fe-based superconductors. In one, the magnetism originates from local atomic spins, while in the other it corresponds to a cooperative spin-density-wave instability (SDW) behavior of conduction electrons. Both assume clear partition into localized electrons, giving rise to local spins, and itinerant ones, occupying well-defined, rigid conduction bands. We have used inelastic neutron scattering to characterize both the static and the dynamic magnetism in a crystal of Fe1.1Te, parent to Fe{1+y}Te{1+x}Se{x} family of superconductors [1]. In contrast to the simple pictures, we find that localized spins and itinerant electrons are coupled together. In particular, we have evaluated the effective magnetic moment by integrating both the elastic and inelastic magnetic scattering. The effective spin per Fe at T = 10 K, in the antiferromagnetic phase, corresponds to S = 1, consistent with the recent analyses that emphasize importance of Hund’s intra-atomic exchange. However, it grows to S =3/2 in the disordered phase, a result that presents a challenge to current theoretical models.
A Joint Seminar Sponsored by Electrical and Computer Engineering, Mechanical and Aerospace Engineering, and Physics
Friday, April 20, 2012
2:00 PM
, Room MEC 341
Note special date.
Note special time.
Note special room.

"Novel Semiconductor and Epitaxial Nanocomposite Materials for Energy Conversion and Optoelectronic Applications"

Joshua M. O. Zide , University of Delaware
[Host: ECE, MAE, & Physics]
ABSTRACT:
Advances in electronic materials (specifically, semiconductors and nanocomposites) enable new device technologies and improve the properties of existing technologies. In this talk, I will present efforts within my group on the growth of new materials by molecular beam epitaxy and the resulting advances in solar cells, thermoelectrics, and optoelectronics.

Specifically, I will discuss two material systems: (1) nanocomposites consisting of metallic nanoparticles (such as ErAs and TbAs) within III-V semiconductors (such as InGaAs and GaAs), and (2) dilute bismuthide semiconductors in which bismuth is incorporated into III-V materials to reduce the bandgap significantly, with unique band alignments that cannot be easily achieved in other materials. In these new materials, electronic, thermal, and optical properties can be quite different from those of conventional materials, with significant promise for applications in a variety of (opto)electronic devices.

Condensed Matter
Thursday, April 19, 2012
3:30 PM
Physics Building, Room 204

"Transport Properties of VO2 Films near the Metal-Semiconductor Transition"

Salinporn Kittiwatanakul , University of Virginia
[Host: Stu Wolf]
ABSTRACT:
Vanadium dioxide (VO2) exhibits a metal semiconductor transition (MST) at 340 K. This transition is accompanied by the abrupt change in the electrical conductivity, optical transmittance and reflectance in infrared region, which can be used in the electronic devices such as temperature sensors and electric switches. In this study, Reactive Bias Target Ion Beam Deposition was used for epitaxial VO2 growth on TiO2 (100) substrates with fixed O2 flow rate at 5.0 sccm to study transport anisotropy, and for highly textured VO2 growth on c-plane Al2O3 substrates to study the effect of different O2 flow rates (4.5-6.0 sccm). The conductivity anisotropy ratio σc/σb of VO2/TiO2 film was found to be ~41.5 at 300 K, much larger than that of single crystal VO2 and it is the largest among those previously reported. The temperature dependent anisotropy of the carrier concentration and the mobility is to be discussed. With XPS and XAS, the valence state of vanadium on different VO2/c-Al2O3 films was investigated. As the O2 flow rate increases, the XRD results show decreasing lattice parameter, hence increasing compressive strain along b-axis of monoclinic VO2; the transport measurements also show the increasing transition temperature (TMST) and the increasing change in resistivity associated with the strain. The correlation among the valence state, the strain and MST in VO2 will be discussed.
Condensed Matter
Thursday, April 12, 2012
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Radiation Effect on Materials for Nanomagnetism and Spintronics Application"

Tom Anuniwat , University of Virginia
[Host: Stu Wolf]
SLIDESHOW:
Condensed Matter
Thursday, April 5, 2012
3:30 PM
Physics Building, Room 204

"Torsion and Viscosity in Condensed Matter Physics"

Taylor Hughes , University of Illinois at Urbana-Champaign
[Host: Israel Klich]
ABSTRACT:
In this talk I will review the common appearance of torsion in solids as well as some new developments. Torsion typically appears in condensed matter physics associated to topological defects known as dislocations. Now we are beginning to uncover new aspects of the coupling of torsion to materials. Recently, a dissipationless viscosity has been studied in the quantum Hall effect. I will connect this viscosity to a2+1-d torsion Chern-Simons term and discuss possible thought experiments in which this could be measured. If time permits, I will briefly discuss a new topological defect in 3+1-d, a torsional skyrmion, which does not require a lattice deformation to exist in solids. If present, torsional skyrmions are likely to impact the propagation of electrons in materials with strong spin-orbit coupling such as topological insulators and spin-orbit coupled semiconductors.
Condensed Matter
Tuesday, April 3, 2012
3:30 PM
Physics Building, Room 204
Note special date.

"Magnetic excitation in the iron based superconductor, FeTe1-xSex: fluctuation near maximal quantum criticality"

Jooseop Lee , University of Virginia
[Host: Seunghun Lee]
Condensed Matter
Thursday, March 22, 2012
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Fractional Topological Insulators"

Claudio Chamon , Boston University
[Host: Israel Klich]
ABSTRACT:
The prediction and experimental discovery of topological band insulators and topological superconductors are recent examples of how topology can characterize phases of matter. In these examples, electronic interactions do not play a fundamental role. In this talk I shall discuss cases where interactions lead to new phases of matter of topological character. Specifically, I shall discuss fractional topological states in lattice models which occur when interacting electrons propagate on flattened Bloch bands with non-zero Chern number. Topologically ordered many-particle states can emerge when these bands are partially filled, including a possible realization of the fractional quantum Hall effect without external magnetic fields. I shall also ponder on the possible practical applications, beyond metrology, that the quantized charge Hall effect might have if it could be realized at high temperatures and without external magnetic fields in strongly correlated materials.
SLIDESHOW:
Condensed Matter
Tuesday, February 7, 2012
3:30 PM
Physics Building, Room 204
Note special date.

"The topological insulator in a Fermi sea - sink or swim?"

Doron Bergman , Caltech
[Host: Israel Klich]
ABSTRACT:
In the flurry of experiments looking for topological insulator materials, it has been found that some bulk metals very close to topological insulator electronic states support the same topological surface states that are the defining characteristic of the topological insulator. First observed in spin-polarized angle resolved photoemission spectroscopy (ARPES) in Sb [D. Hsieh et al., Science 323, 919 (2009)], the helical surface states in the metallic systems appear to be robust to at least mild disorder. We present an investigation of the nature of these "topological conductors" - bulk metals with helical surface states. We explore how the surface states can survive the insulator being turned into a metal, in both clean and disordered systems. We also explore magnetoelectric coupling phenomena in these systems, which turn out to realize an analog of the intrinsic anomalous Hall effect.
How to Get into Undergraduate Research
Thursday, December 1, 2011
5:45 PM
Physics Building, Room 204
Note special time.

"How to Get into Undergraduate Research"

Bob Jones, Despina Louca, & Blaine Norum , University of Virginia
[Host: Elton Ho]
ABSTRACT:
The purpose of this talk is to provide an opportunity for undergraduates to learn more about research in the Physics Department. Current and prospective physics majors are encouraged to participate. Topics to be discussed include: current researches in the department in different fields of interest, how research relates to the physics BS degree, how to search for research opportunities, and what professors expect from undergraduate researchers. Free pizza and drinks will be served.
Condensed Matter
Thursday, November 17, 2011
3:30 PM
Physics Building, Room 204

"The Linear and Nonlinear Magnetic Response in Strongly Correlated Metals"

Bellave Shivaram , University of Virginia
[Host: Genya Kolomeisky]
ABSTRACT:
I will describe two phenomena from the world of heavy fermion physics - one very specific and one that appears general enough to apply to all strongly correlated metals. The metal URu2Si2 exhibits a magnetic transition whose order parameter has so far been "hidden". I will describe sensitive magnetization measurements that throw light on the special nature of this hidden order in this unique metal. The more general phenomenon pertains to our observations in UPt3 that appear to establish a universal rule for the behavior of the equilibrium nonlinear susceptibility in all strongly correlated electronic systems.
Condensed Matter
Thursday, November 10, 2011
3:30 PM
Physics Building, Room 204

"Engineering Interface Magnetism via Defect Control in Complex Oxide Heterostructures"

Chris Leighton , University of Minnesota
[Host: Despina Louca]
ABSTRACT:
The remarkable functionality of complex oxides provides many opportunities for new physics and applications in oxide heterostructures. The manganite and cobaltite materials crystallizing in the perovskite structure provide excellent examples, being of interest in solid oxide fuel cells, catalysis, ferroelectric RAM, gas sensing, resistive switching memory, and oxide spintronics. However, the same delicate balance between phases that provides such diverse functionality also leads to a serious problem - the difficulty of maintaining desired properties close to the interface with other oxides. Although this problem appears universal, manifests itself in many ways, and presents a significant “roadblock” to the development of heterostructured devices for oxide electronics, there is no clear consensus as to its origin, or even whether it is driven by electronic or chemical effects. In this work, using SrTiO3/La1-xSrxCoO3 as a model system (i.e. a non-magnetic semiconductor / ferromagnetic metal interface), we have determined the fundamental origin of the deterioration in interfacial transport and magnetic properties. The effect is found to be chemically-driven, being due to a profound interaction between the strain-state and the formation of oxygen defects, which dominates the interface magnetism and transport. Most importantly, it is demonstrated that a full understanding of these effects enables design and synthesis of interfaces where the suppression in physical properties is dramatically reduced, or perhaps even eliminated. Work supported by DOE and NSF.
Condensed Matter
Thursday, October 13, 2011
3:30 PM
Physics Building, Room 204

"Resonant spin excitations in Iron Arsenide superconductors"

Stephan Rosenkranz , Argonne National Lab
[Host: Seunghun Lee]
ABSTRACT:
The iron pnictides have attracted great interest following the discovery of superconductivity up to 50K and because of their superficial similarities with the cuprate superconductors. In both systems, superconductivity emerges when longrange antiferromagnetic order is suppressed by doping or pressure. This indicates that spin correlations are intimately connected with superconductivity and may be involved in the pairing mechanism, in contrast to ordinary superconductors where the pairing is provided by phonons. Elastic and inelastic neutron scattering provides a most direct probe of both the longrange spin order as well as the strong spin fluctuations remaining when the longrange order has been suppressed, allowing detailed studies of how these spin correlations evolve as the superconducting state is approached and entered as a function of doping and temperature. Here, I will discuss what we have learned about the iron arsenides from such neutron scattering studies across the whole doping range. Particular focus will be given on the observation of a resonant spin excitation, how its sensitivity to the superconducting order parameter proves that superconductivity in these compounds is indeed unconventional, and how its doping dependence provides further clues to the origin of pairing in these compounds.
Condensed Matter
Thursday, October 6, 2011
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Surface analysis: new approaches to access, data and analyses"

Paul Pigram , Centre for Materials and Surface Science and Department of Physics, La Trobe University, Australia
[Host: Joe Poon]
ABSTRACT:
Surface characterisation is critically important in the development and validation of advanced materials, tailored molecular structures at surfaces, sensors and screening devices, electronic structures and systems. XPS and Time of flight-Secondary Ion Mass Spectroscopy (ToF-SIMS) are core laboratory-based surface analytical techniques. Major challenges remain in creating preferably automated data interpretation tools to extract molecular information from these datasets. This presentation will discuss our approach to creating a virtual research laboratory encompassing immersive remote access to surface analysis instrumentation and beam lines at the Australian Synchrotron, instrument scheduling and secure access, data repository integration, metadata capture and data visualisation. The feasibility of integrating other national and international facilities is currently being explored.
SLIDESHOW:
Condensed Matter
Monday, September 26, 2011
3:30 PM
Physics Building, Room 204
Note special date.

Slideshow (PDF)

"Fe-based superconductors at high magnetic fields"

Alexander Gurevich , Old Dominion University
[Host: Joe Poon]
ABSTRACT:
Low carrier densities and short coherence lengths in the recently discovered semi-metallic Fe-based superconductors (FBS) can result in exotic behaviors at strong magnetic fields due to the interplay of multiband superconductivity, unconventional pairing symmetry and the Zeeman and orbital pairbreaking. I this talk I will give an overview of these interesting new materials, particularly their anomalous temperature dependencies of the upper critical field Hc2(T) which often extrapolate to very high Hc2 > 100 T at low temperatures. The materials features of FBS and the multiband s pairing symmetry with the sign change of the order parameter on different sheets of the Fermi surface can significantly increase Hc2(T) and facilitate the Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) transition to the state with a spatially modulated order parameter. Small shifts of the chemical potential upon doping of FBS can produce new small pockets of the Fermi surface, triggering the FFLO instability due to the Lifshitz transition.
SLIDESHOW:
Condensed Matter
Thursday, September 8, 2011
3:30 PM
Physics Building, Room 204

"Hidden order and disorder in a spin glass state of a geometrically frustrated magnet"

Seunghun Lee , UVA
[Host: Genya Kolomeisky]
ABSTRACT:
Frustration arises in magnetic materials when pairwise interactions between the magnetic moments cannot be satisfied simultaneously, leading to exotic properties such as spin liquid and spin ice at low temperatures. Some cases, however, exhibit spin glass behaviors, and it has been a theoretical and experimental challenge to understand whether or not the spin glass state is an intrinsic ground state for the materials. Here we have studied a quasi-two dimensional triangular lattice of bi-pyramids with dominant antiferromagnetic nearest neighbor interactions. We show that the magnetic problem can be mapped into a problem of two independent degrees of freedom, tri-color and binary sign: the color represents the director of the collinear spins of each bi-pyramid, while the binary sign represents spins within each bi-pyramid that are either parallel or antiparallel to the director. At the mean-field level, the tri-color orders long range in a 3 × 3 structure, while the binary sign has numerous degenerate ground states. When combined, the two degrees of freedom yield the collinear bi-pyramid spin ground states for the hybrid lattice. Infinite number of coplanar bi-pyramid spin states can be systematically generated by collective rotations of spins from the collinear states. For long range ordered collinear and coplanar states, we identify ‘partial but extensive’ spin zero-energy modes of excitations that are qualitatively different from the ‘local’ zero-energy excitations found in the spin liquid states of the pure two-dimensional kagome and pure three-dimensional pyrochlore lattices. We argue that due to the infinite ground state degeneracy and the unique characteristics of the zero-energy excitations the ground state of the quasi-two-dimensional hybrid antiferromagnet becomes a spin glass.
Condensed Matter
Thursday, April 28, 2011
3:30 PM
Physics Building, Room 204

"Material Development for Magnetic tunnel junctions in Spin Transfer Torque Random Access Memory (STT-RAM)"

Ms. Yishen Cui , University of Virginia
[Host: Stu Wolf]
ABSTRACT:
Magnetic Tunnel Junction(MTJ) has been widely studied recently due to its potential to be applied in high density memory devices. The basic structure of MTJ is composed of two ferromagnetic layers(free layer and fixed layer) separated by an insulating barrier layer. By aligning the free layer (FL) magnetization parallel or antiparallel with the fixed layer magnetization, different resistance levels can be obtained in the junction. The most significant characteristic of STT writing technology that distinguishes it from the conventional one is that it enables the manipulation on magnetization of FL by local spin polarized current rather than an external field, leading to many advantages such as a high scalability and low energy cost. The main challenge for improving the performance of MTJs driven by STT remains in the reduction of the critical switching current density that required by the FL switching without sacrificing the thermal stability of MTJ bits. These two factors are closely related with the intrinsic properties of the material used in the ferromagnetic layers in MTJ. This talk will be focused on some ferromagnetic materials that can be potentially applied in STT-MTJs.
Condensed Matter
Thursday, April 21, 2011
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Computational Study of ultra-short-pulse laser-metal interaction"

Chengping Wu , UVa
[Host: Genya Kolomeisky]
ABSTRACT:
Rapid progress in the development of accessible sources of short (pico- and femtosecond) laser pulses opens up new opportunities for surface modification with high accuracy and spatial resolution. But the small size of the laser-modified region makes experimental characterization of laser-induced structural changes challenging and, at the same time, the fast and highly localized energy deposition in short pulse laser processing unavoidably creates the conditions of strong thermodynamic, electronic, and mechanical nonequilibrium, making the theoretical description of the structural transformations difficult. On the other hand, atomic-level computer modeling has the ability to provide detailed information on the complex structural and phase transformations induced by short pulse laser irradiation and can assist in the advancement of laser-driven applications. In this presentation, I will talk a series simulation I have done or am doing, especially focus on the simulation “laser irradiation on layered targets (Ag film-Cu substrate)”.
SLIDESHOW:
Condensed Matter
Thursday, April 7, 2011
3:30 PM
Physics Building, Room 204

"Study of amorphous magnetic thin films"

Manli Ding , UVa
[Host: Joe Poon]
ABSTRACT:
Amorphous rare-earth alloys exhibit compensation phenomena involving a low saturation magnetization and high anisotropy field near the compensation temperature. However, the anisotropy of these amorphous solids is not well understood. At the same time, some of these materials also exhibit low Gilbert damping parameter that is preferable in spin-torque-transferred devices. I will discuss the properties of the various magnetic films being investigated.
Condensed Matter
Thursday, March 31, 2011
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Improving the thermoelectric properties of Heusler compounds"

Di Wu , UVa
[Host: Joe Poon]
ABSTRACT:
Thermoelectric materials are of great interests because they can fulfill the direct conversion between thermal and electrical energy, in a environmentally clean way. Half Heusler compounds are well known electronic and magnetic materials over the last few decades. Our work concentrates on improving the thermoelectric properties of these compounds via optimization of the Seebeck coefficient and thermal conductivity.
SLIDESHOW:
Condensed Matter
Tuesday, March 29, 2011
3:30 PM
Physics Building, Room 313
Note special date.
Note special room.

Slideshow (PDF)

"Neutron scattering study on the iron-based high-Tc superconductor systems"

Wei Bao , Renmin University of China
[Host: Seunghun Lee]
ABSTRACT:
Superconductors which conduct electric current without dissipation (zero resistance) and expel magnetic field (perfect diamagnetism) were discovered 100 years ago in metal at a few Kelvin above absolute zero. Stimulated by the discovery of cuprate superconductors in doped Mott antiferromagnetic insulator, over the last two decades research has been focused on discovery of unconventional superconductors of high transition temperature (Tc) in magnetic materials. Iron-based laminar materials in several related structure families with Tc as high as 56 K have generated much excitement in the last three years, and new discovery continues to appear. Using neutron scattering technique, we have determined crystal and magnetic structure of several families of the new superconductors and the sample composition [1-5], which provide solid foundation for further investigation on electronic structure and processes. Structural and magnetic transitions have been investigated to yield phase-diagrams which show a rich variety of relationship between superconducting and magnetic orders [6,7]. Such investigation also reveals the shortcoming of widely accepted spin-density-wave scenario and provides first experimental indication of important role of the orbital order [2]. The symmetry of superconducting order parameter has strong signature in magnetic excitation spectrum. We observed with inelastic neutron scattering method the telltale spin resonance mode of the unconventional s+/- symmetry in the superconducting state of the 11 superconductor [8]. The normal state was shown to exhibit the single-lobed incommensurate excitation continuum of a typical itinerant antiferromagnet, in contrast to spin-wave cone of a localized antiferromagnet [8,9], supporting a Fermi liquid description of the normal state. [1] Y. Qiu, W. Bao, Q. Huang et al., Phys. Rev. Lett. 101, 257002 (2008). [2] Q. Huang, Y. Qiu, W. Bao et al., Phys. Rev. Lett. 101, 257003 (2008); M. Kofu, Y. Qiu, W. Bao et al., New J. Phys. 11, 055001 (2009). [3] W. Bao, Y. Qiu, Q. Huang et al., Phys. Rev. Lett. 102, 247001 (2009). [4] W. Bao et al., arXiv:1102.0830 (2010). [5] F. Ye et al., arXiv:1102.2882 (2010). [6] H. Chen et al., Europhys. Lett. 85, 17006 (2009). [7] W. Bao et al., arXiv:1102.3674 (2010). [8] Y. Qiu, W. Bao, Y. Zhao et al., Phys. Rev. Lett. 103, 067008 (2009). [9] D.N. Argyriou et al., Phys. Rev. B 81, 220503 (R) (2010).
SLIDESHOW:
Condensed Matter
Thursday, March 17, 2011
3:30 PM
Physics Building, Room 204

"Nonconventional Odd Denominator Fractional Quantum Hall States"

Gabor Csathy , Purdue University
[Host: Jongsoo Yoon]
ABSTRACT:
The fractional quantum Hall states developing in a two-dimensional electron gas in the second Landau level appear to be different from those of the lowest Landau level and continue to challenge our understanding. The even denominator state at filling factor 5/2 is currently under intense scrutiny since it is believed to arise from an exotic p-wave pairing of composite fermions described by the Pfaffian wavefunction which might support non-Abelian quasiparticles. An equally interesting and related problem is the origin of the odd denominator states of the second Landau level. While at first sight these states could be part of the composite fermion hierarchy, several recent theoretical works suggest that some might be supporting generalized Pfaffian-like correlations. Recent progress in cooling electrons allowed us to observe a new fractional quantum Hall state at the filling factor 2+6/13. We find that energy gaps of the prominent 2+1/3 and 2+2/3 states are consistent with the values predicted by the free composite fermion model. However, the weaker 2+2/5 and 2+6/13 states deviate significantly, suggesting therefore that these states are of exotic origin.
Condensed Matter
Thursday, March 3, 2011
3:30 PM
Physics Building, Room 204

"Quantum Phases of Dipolar Bosons"

Carlos A. R. Sa de Melo , Georgia Institute of Technology
[Host: Cass Sackett]
ABSTRACT:
An ensemble of heteronuclear (dipolar) molecules or Rydberg atoms can exhibit many interesting condensed-matter-like properties at ultra-low temperatures because of the long-range range interaction due to their permanent dipole moments [1]. In this talk, I discuss possible quantum phases of dipolar bosons for optical lattices [2] and continuum [3] systems in the two-dimensional regime. In the optical lattice case, several exotic low temperature phases can emerge out the dipolar superfluid. These phases include checkerboard supersolids, striped supersolids and collapsed. The emergence of a striped supersolid is particularly interesting because the anisotropy of the dipolar interaction can be controlled externally. In the case of dipolar bosons in continuum systems, I discuss the finite temperature phase diagram of purely repulsive interactions and show that for large dipolar repulsions a dipolar Wigner crystal appears at low temperatures and melts at intermediate temperatures into a dipolar hexatic fluid, before becoming a normal dipolar fluid at higher temperatures. Other exotic intermediate phases such as supersolid and hexatic superfluid (a melted supersolid) are possible. The experimental characterization of theses phases may be achieved via Bragg scattering techniques. Refs: [1] M. Iskin and C. A. R. Sa de Melo, Phys. Rev. Lett. 99, 110402 (2007). [2] I. Danshita and C. A. R. Sa de Melo, Phys. Rev. Lett. 103, 225301 (2009). [3] K. Mitra, C. J. Williams, and C. A. R. Sa de Melo, Arxiv 0903.4655v1.
Condensed Matter
Thursday, February 24, 2011
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Universal conductance in quantum multi-wire junctions"

Armin Rahmani , Boston University
[Host: Austen Lamacraft ]
ABSTRACT:
Using boundary conformal field theory, we relate the universal conductance tensors of quantum multi-wire junctions to static correlation functions in a finite system. We then propose a general method for determining the conductance. Applying the method to a Y-junction of interacting quantum wires, we verify a theoretical prediction for the conductance of the chiral fixed-point of the Y-junction and then calculate the thus far unknown conductance of its M fixed point with the time-independent density matrix renormalization group method.
SLIDESHOW:
Condensed Matter
Thursday, February 17, 2011
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Quantum Information Processing with Prethreshold Superconducting Qubits"

Michael R. Geller , University of Georgia
[Host: Eugene Kolomeisky ]
ABSTRACT:
I will discuss an alternative approach to quantum computation and simulation that is ideally suited for today's sub-threshold-fidelity qubits, especially in superconducting architectures. This approach makes use of the the n-dimensional single-excitation subspace (SES) of a system of n tunably coupled qubits. Although technically unscalable and inefficient in terms of the number of qubits required, the SES approach allows n-dimensional unitary operations to be implemented in a single shot, without the need to decompose into gates. The real power of this approach is probably in its application to quantum simulation, and I will show that for a particular class of time-dependent quantum simulation problems a practical device that would vastly outperform classical machines is within experimental reach.
SLIDESHOW:
Condensed Matter
Thursday, February 10, 2011
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Photons as phonons: from cavity QED to quantum crystallization"

Sarang Gopalakrishnan , Urbana-Champaign
[Host: Austen Lamacraft]
ABSTRACT:
Atoms in a transversely pumped optical cavity coherently scatter light between the pumping laser and the cavity mode(s) with a position-dependent intensity and phase; thus, the optical dipole potential created and experienced by an atom depends on its position in the cavity. For a single-mode cavity containing many atoms, the optical potential felt by each atom depends on the positions of all the other atoms; in this sense, the cavity mediates an infinite-range interatomic interaction with an oscillating sign. In cavities with many degenerate modes, one can realize more local interactions; these give rise to the crystallization of a Bose-Einstein condensate in a multimode cavity through a fluctuation-driven first-order quantum phase transition. This transition is described by a theory involving a nested surface of low-lying excitations, and is accompanied by a condensation of photons into one or more cavity modes. The resulting ordered state is a “supersolid,” i.e., a superfluid that spontaneously breaks a continuous translational symmetry. We discuss the nature of fluctuations near the transition and low-energy excitations in the ordered state, and prospects for their experimental detection.
SLIDESHOW:
Condensed Matter
Thursday, January 27, 2011
3:30 PM
Physics Building, Room 204

"Study of spin state transition in La 2-x Ca x CoO 4 by using neutron and soft x-ray scattering"

Kazumasa Horigane , Tohoku University
[Host: Despina Louca]
ABSTRACT:
Layered cobalt oxides have attracted much attention due to their wide variety of magnetic, electrical and structural properties. The parent confound La 2 CoO 4 is an antiferromagnetic insulator with quite high T N =275K. Upon Sr or Ca doping, a checkerboard Co 2+ -Co 3+ charge order is realized in La 1.5 Ca 0.5 CoO 4 and Sr 2 CoO 4 shows a ferromagnetic metal with T c =250K. The background of these various magnetic states is proposed the relationship of spin state transition from high spin state of Co 3+ (S=3/2) to intermediate spin state (S=1) [1]. However, the spin state of Co 3+ is now controversial because there is no direct evidence to observe the magnetic scatterings of Co 3+ by using neutron experiment. One of the big problems in this system is searching for magnetic scattering in an elastic condition. In this study, we measured global spin dynamics (inelastic condition) in La 1.5 Ca 0.5 CoO 4 using the Fermi chopper spectrometer 4SEASONS in J-PARC [2]. Figure 1 shows the magnetic excitation of La 1.5 Ca 0.5 CoO 4 below and above T N (=50K). Spin-wave due to Co 2+ spins is seen up to 16meV below T N . Another feature of the excitation spectrum is flat mode around 27meV. On the other hand, two magnetic modes seem to be merged into single mode above T N . These results can not be explained by simple spin wave theory, therefore we need to consider another parameters such as Co 3+ spin or angular moment L. In this presentation, I will discuss the origin of this novel magnetic behavior based on the Co 3+ spins and orbital angular moment L. In order to discuss whether there is a significant orbital angular moment or not in this system, I will show you the recent research of soft x-ray scattering experiment. [1] Y. Moritomo, K. Migashi, K. Matsuda and A. Nakamura, Phys. Rev. B 55, 14725 (1997) [2] M. Nakamura, R. Kajimoto, Y. Inamura, F. Mizuno, M. Fujita, T. Yokoo and M. Arai, JPSJ 78, 093002 (2009)
Condensed Matter
Thursday, November 18, 2010
4:00 PM
Physics Building, Room 204
Note special time.

"Fractional statistics and beam splitters in quantum Hall systems"

Smitha Vishveshwara , UIUC
[Host: Israel Klich ]
ABSTRACT:
One of the most fascinating aspects of the two dimensional world is the possible existence of anyons, particles which obey 'fractional' statistics different from fermionic and bosonic statistics. The quantum Hall system is predicted to host such entities. This talk will begin with an introduction to fractional particles in quantum Hall systems and anyons in magnetic fields. Correlations and signatures of these anyons will be discussed. Finally, schemes for detecting such anyons will be proposed, including those that follow the principles of beam splitters found in the entirely different setting of quantum optics.
Condensed Matter
Thursday, November 11, 2010
4:00 PM
Physics Building, Room 204
Note special time.

"Interlayer exchange couplings in Ga1-xMnxAs/GaAs diluted ferromagnetic semiconductor multilayers"

Jae-Ho Chung , Korea University
[Host: Seung-Hun Lee]
ABSTRACT:
The spintronics technology requires materials that allow simultaneous control of the charge and the spin degrees of freedom. Recent efforts dedicated in search of such materials have led us to the successful fabrication of artificial ferromagnetic semiconductor GaMnAs, where doping of magnetic Mn2+ ions into insulating GaAs results in robust ferromagnetism and semiconducting property at the same time. In order to realize useful devices from magnetic semiconductors, however, it is important to be able to control not only the magnetic properties of individual layers but also interactions between them. Previously the interactions between ferromagnetic GaMnAs layers across nonmagnetic GaAs spacers have been observed to be ferromagnetic only. Since the RKKY interaction in metallic magnetic multilayers is known to produce exchange oscillations between ferromagnetic and antiferromagnetic (AFM) as a function of layer thickness, AFM interlayer exchange coupling has been expected to be attainable also in semiconductor-based multilayers by enhancing carrier concentrations in the spacers.

To test this idea, we have fabricated GaMnAs-based multilayers with different carrier concentrations and thicknesses in the spacers. Molecular beam epitaxy was used to deposit 10 periods of Ga0.97Mn0.03As/GaAs on GaAs (001) substrates. In order to increase the carrier concentration of selected samples, Be doping at the concentration of 1.2 x 1020 cm-3 was introduced in the nonmagnetic spacer. Using polarized neutron reflectivity, we indeed observed and confirmed the occurrence of antiferromagnetic interlayer coupling in some of the samples with Be-doped spacers. Their field cycling behavior clearly indicated that the observed antiferromagnetic coupling is spontaneous and robust. All of these samples showed GMR-like transitions in magnetotransport measurements. In contrast, none of the samples without Be-doped spacers showed such behavior. These results indicate that the interlayer exchange couplings in GaMnAs-based ferromagnetic multilayers can be controlled via engineering of the spacer properties.
Condensed Matter
Thursday, September 30, 2010
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Topological Phases in One Dimension"

Lukasz Fidkowski , UCSB
[Host: Israel Klich]
ABSTRACT:
With the experimental discovery of topological insulators in 2 and 3 dimensions, there has emerged the theoretical problem of completely classifying the conditions under which such novel phases can exist. That is, given a spatial dimension and a set of generic symmetries (e.g. time reversal, particle-hole symmetry), what "topological" phases does one obtain? A complete classification has been obtained for effectively non-interacting systems. Here we present an example of a one dimensional fermionic chain in which this simple non-interacting classification breaks down when one adds quartic interactions. We generalize our methods to outline a complete classification of gapped phases of all one-dimensional systems (this is joint work with Alexei Kitaev).
SLIDESHOW:
Condensed Matter
Thursday, September 23, 2010
3:30 PM
Physics Building, Room 204

"Structural defects and the onset of flow in glassy materials"

Lisa Manning , Princeton Center for Theoretical Science
[Host: Despina Louca]
ABSTRACT:
How do glassy materials begin to flow? Experiments and simulations have confirmed that flow occurs via localized rearrangements in most jammed solids at finite temperatures and strain rates, but in the past it has been difficult to identify structural defects, or soft spots, based on the underlying structure of the packing alone. We have developed a method to systematically identify a population of soft spots by analyzing the low-energy vibrational modes of a soft-sphere packing. We show that the soft spots are structurally distinct from the remainder of the packing and that under quasi-static shear a soft spot from the population will accommodate deformation. Finally, we discuss how statistics of these soft spot populations could be used to test predictions of theoretical models for plastic flow.
Condensed Matter
Thursday, September 2, 2010
4:00 PM
Physics Building, Room 204
Note special time.

"Recent neutron scattering studies on frustrated magnets"

Masa Matsuda , ORNL
[Host: Seung-Hun Lee]
ABSTRACT:
Frustrated magnets show interesting phenomena originating from the macroscopic ground state degeneracy. Exotic states can be chosen as the ground state from many possible candidates. Usually, geometrically frustrated systems consist of edge- and corner-shared triangles (triangular and kagomé lattices, respectively) and tetrahedra (pyrochlore lattice). However, even unfrustrated lattice systems, such as square or honeycomb lattice systems, can have frustrating interactions in the presence of antiferromagnetic further-neighbor interactions. We have recently studied two interesting frustrated systems using neutron scattering technique. One is the Cr-based spinel with pyrochlore lattice and another is the honeycomb lattice system with competing interactions. Cr-based spinel compounds ACr2O4 (A=Mg, Zn, Cd, and Hg) are so far the best model systems for a network of corner-sharing tetrahedrons with isotropic nearest-neighbor antiferromagnetic interactions. The systems exhibit novel spin-Peierls phase transitions from cubic spin liquid to non-cubic Néel states at low temperatures. They also show the magnetic field-induced half-magnetization plateau states that are stable over a wide range of field. Using an elastic neutron scattering technique under magnetic field, we determined the magnetic structure in the half-magnetization plateau phase in the spinel HgCr2O4 and CdCr2O4. The magnetic structure has a common cubic P4332 symmetry. This suggests that there is a universal field induced spin-lattice coupling mechanism at work in the Cr-based spinels. This is consistent with the theoretical prediction based on the simplest Hamiltonian for the spin-lattice coupling with the nearest-neighbor exchange interaction and an elastic energy term. Bi3Mn4O12(NO3), in which the Mn4+ ions carry S=3/2, is the first honeycomb lattice system that shows no long-range magnetic order. Using neutron scattering technique, we determined that a short-range antiferromagnetic correlation develops at low temperatures. Applying magnetic field, an interesting field-induced magnetic transition occurs, in which the short-range order abruptly expands into a long-range order.
Condensed Matter
Friday, May 28, 2010
2:00 PM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"Structural Investigations of Hydrides Studied by Neutron Scattering "

Toyoto Sato , Tohoku University
[Host: Despina Louca]
ABSTRACT:
Hydrides have been expected to be high gravimetric and volumetric density hydrogen storage materials [1]. Recently the hydrides also showed some interesting materials properties which were lithium-ion conductor (ex. LiBH_4 ) [2], semi-conductor (ex. SrAlSiH) [3], and so on. In order to understand the interesting properties, the structural investigations are one important research. On the other hand, it is very difficult to detect hydrogen by x-ray because x-ray scattering occurs by interaction with the electrons. However neutrons were scattered by atomic nuclei and it is possible to identify hydrogen (deuterium) atomic positions. In addition, dynamics of hydrides studied by inelastic neutron scattering is also one interesting study because inelastic scattering cross section of hydrogen (82.02 barns) is approximately ten times as large as those of the most other elements (less than 12 barns). Therefore, neutron scattering experiments of hydrides are essential for research and developments of the hydrides. For the reason, a lot of structures and dynamics of hydrides have been elucidated by neutron scattering studies [4, 5]. In this seminar, I will show the elastic (structure) and inelastic (dynamics) neutron scattering studies on Ca-Al-H system that are an interesting hydrogen storage material [6]. [1] S. Orimo, Y. Nakamori, J.R. Eliseo, A. Züttel, C.M. Jensen, Chem. Rev. *107*, 4111, (2007). [2] M. Matsuo, Y. Nakamori, S. Orimo, H. Maekawa, H. Takamura, Appl. Phys. Lett. *91*, 224103, (2007). [3] T. Bjorling, D. Noréus, K. Jansson, M. Andersson, E. Leonova, M. Edén, U. Hålenius, U. Häussermann, Angew. Chem. Int Ed *44*, 7269 *(2005)* [4] B.C. Hauback, Z. Kristallogr. *223*, 636 (2008). [5] S.F. Parker, Coord. Chem. Rev. *254*, 215 (2010). [6] T. Sato, M.H. Sørby, K. Ikeda, S. Sato, B.C. Hauback, S. Orimo, J. Alloys Compd. *487*, 472 (2009).
Condensed Matter
Tuesday, April 27, 2010
1:30 PM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"Physical basis for the spatial organization of DNA"

Bae-Yeun Ha , University of Waterloo
[Host: Seunghun Lee ]
ABSTRACT:
DNA is not only a passive storage of life’s information but also a fascinating physical object, which actively participates in many biological processes of vital importance (e.g., DNA replication and organization). In aqueous solution, DNA is highly-negatively charged. By themselves, DNA molecules would repel each other. Also, they are molecular springs: DNA strands resist bending, twisting, stretching, and confinement. In a living cell, however, DNA is tightly packed and organized into higher-order structures. Perhaps, the most intriguing “show” DNA molecules display is their spatial organization or segregation, while maintaining a high level of compaction. How can this be accomplished? Using a simple but biology-inspired model of DNA, I will present a physical basis for DNA organization and segregation, especially in rod-shape bacteria.
Condensed Matter
Thursday, April 1, 2010
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Projection Hamiltonians for clustered quantum Hall wavefunctions"

Thomas Jackson , Yale
[Host: Israel Klich]
ABSTRACT:
One of the main measures for determining whether a trial wavefunction for the fractional quantum Hall effect is physically realistic is the properties of an associated projection Hamiltonian, an artificial few-body Hamiltonian constructed so that the given wavefunction and edge excitations lie in the zero-energy eigenspace. The results presented here address the general (and nontrivial) problem of finding a projection Hamiltonian for a quantum Hall state defined in terms of a conformal field theory. We consider lowest Landau level wavefunctions for bosons subjected to a magnetic field in the plane; "clustered" wavefunctions are those which vanish when k+1 (but not necessarily k) particles are brought to the same point. We begin by studying the zero-energy eigenstates of a projection Hamiltonian which forbids three particles to come together with relative angular momentum less than six and, in addition, forbids one of two linearly-independent states of relative angular momentum six. The counting of edge excitations of this Hamiltonian agrees with the character formula for the N=1 superconformal Kac vacuum module at generic (i.e., irrational) central charge c, despite the fact that the densest (ground) state reproduces the appropriate superconformal amplitude for all c. The irrationality of the edge theory implies that this Hamiltonian is gapless for all c. For particular c, we try to improve'' the Hamiltonian by adding additional terms (related to singular vectors in the modules), so as to obtain a rational theory. We consider specifically states whose wavefunctions are related to the M(3,p) series of Virasoro minimal models.
SLIDESHOW:
Condensed Matter
Thursday, March 25, 2010
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Symmetry breaking in smectics and surface models of their singularities"

Gareth Alexander , University of Pennsylvania
[Host: Andrew James]
ABSTRACT:
The homotopy theory of topological defects in ordered media fails to completely characterize systems with broken translational symmetry. We argue that the problem can be understood in terms of the lack of rotational Goldstone modes in such systems and provide an alternate approach that correctly accounts for the interaction between translations and rotations. Dislocations are associated, as usual, with branch points in a phase field, whereas disclinations arise as critical points and singularities in the phase field. We introduce a three-dimensional model for two-dimensional smectics that clarifies the topology of disclinations and geometrically captures known results without the need to add compatibility conditions. Our work suggests natural generalizations of the two-dimensional smectic theory to higher dimensions and to crystals.
SLIDESHOW:
Condensed Matter
Thursday, March 4, 2010
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Electron itinerancy, orbital symmetry and itinerant spin fluctuations in Fe-based superconductors as revealed by soft x-ray spectroscopies."

Norman Mannella , University of Tennessee - Knoxville
[Host: Seung-Hun Lee]
ABSTRACT:
The discovery of superconductivity with critical temperature exceeding 55 K in the iron-pnictides (FeSC) compounds has offered the community a new set of materials hosting high temperature superconductivity [1]. In this talk, I will discuss some of our most important results obtained with complementary soft x-ray spectroscopies such as core level and Angle Resolved Photoemission (ARPES) and x-ray absorption (XAS) on different families of FeSC compounds. Results concerning two main aspects particularly important for the physics of the FeSC materials will be discussed, namely 1) the bonding topology/orbital symmetry, and 2) the presence of spin fluctuations. The symmetry of the orbitals and their hybridization play a central role in the interplay between magnetism and superconductivity, and in the nature of the superconducting pairing. I will discuss the symmetry and topology of the occupied and unoccupied orbitals by presenting results of polarization-dependent ARPES and element-specific XAS measurements of the electronic structure in the normal state of BaFe 2 As 2 and BaFe 1.8 Co 0.2 As 2 single crystals. I will then focus on discussing how the presence of exchange multiplets in the Fe 3s photoemission spectra in different FeSC materials are indicative of the presence of fluctuating spin moments on the Fe sites. Due to extremely fast time scales involved, the detection of magnetic fluctuations by means of magnetic probes has so far remained elusive. Our experiment provides a strong test case for the occurring of itinerant magnetic fluctuations, whose detection has been made possible by the extremely fast time scales proper of the photoemission process. The Fermi surface topology revealed by ARPES experiments in different FeSC compounds and its possible relation to the presence of magnetic fluctuation will also be discussed. [1] Y. Kamihara, et al., J. Am. Chem. Soc. 130, 3296 (2008).
SLIDESHOW:
Condensed Matter
Thursday, February 25, 2010
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Orbital orders and orbital order driven quantum criticality"

Zohar Nussinov , Washington University at St. Luis
[Host: Israel Klich ]
ABSTRACT:
I will briefly review the rudiments of orbital order physics and then discuss several more recent results. Orbitals are described by SU(2) operators but unlike spins the orbitals live in real space and thus their interactions are highly frustrated. It will be shown that spatial orders of electronic orbital states in a crystal can be triggered by thermal fluctuations alone (contrary to earlier lore, no zero point quantum fluctuations are necessary to stabilize orders). It will be further shown how symmetry considerations alone give rise to dimensional reduction as well as topological order and selection rules that may be experimentally tested from scattering measurements. I will illustrate that in addition to charge and spin order driven quantum critical points, Orbital Order Driven Quantum Criticality, as a matter of principle, also occur in the simplest orbital Hamiltonians. New predicted orbital nematic orders will be demonstrated in some of the best known orbital dependent Hamiltonians.
SLIDESHOW:
Condensed Matter
Thursday, February 18, 2010
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Effect of order parameter fluctuations on the spectral density in d-wave superconductors"

Maxim Khodas , Brookhaven
[Host: Israel Klich ]
ABSTRACT:
My talk contains two related parts. In the first part I will discuss the spectral density in 2D d-wave superconductors in the regime of strong quantum fluctuations. I will start with the motivation based on Angular Resolved Photo- Emission Experiments. The theoretical model is then presented to capture the effects of phase fluctuations without specifying the (unknown) paring mechanism. I discuss the solution of the model and give results for the spectral function. In the second part I will discuss the Fermi Surface Reconstruction and underlying mechanism related to the anti-ferromagnetic fluctuations. I will show how the pockets obtained on the mean field level are modified by the fluctuations of the staggered magnetization. Finally, I would conclude with a short summary.
SLIDESHOW:
Condensed Matter
Thursday, January 28, 2010
4:00 PM
Physics Building, Room 204
Note special time.

"Orbital ordering in CaV 2 O 4 : A neutron scattering study"

Oliver Pieper , Helmholtz- Zentrum Berlin
[Host: Seunghun Lee]
ABSTRACT:
CaV 2 O 4 is a quasi-one dimensional spin-1 Heisenberg antiferromagnet. The magnetism arises from the partially filled t 2g- levels of the V 3+- ions, which in addition give an orbital degree of freedom to the system. In contrast to the isovalent vanadium spinel compounds, the low dimensionality in CaV 2 O 4 already arises from the crystal structure. It contains weakly coupled double-chains of edge-sharing VO 6 octahedra, where the particular octahedral staggering creates a zigzag-like arrangement of the vanadium ions. This in return gives rise to strong magnetic direct exchange interactions between nearest and next nearest neighbor vanadium ions and to geometrical frustration. However, the strength of the exchange interactions is strongly influenced by the particular occupation of the t 2g- orbitals. Depending on the type and degree of octahedral distortion, the system can be interpreted as a frustrated Haldane chain or a spin-1 ladder. We have used single crystal neutron diffraction and neutron spectroscopy to determine the spin structure as well as the complex excitation spectrum of CaV 2 O 4 . The results are analyzed theoretically and from this the leading exchange paths are deduced and discussed in terms of orbital ordering.
Condensed Matter
Thursday, December 10, 2009
10:30 AM
Physics Building, Room 313
Note special time.
Note special room.

"Magnetic phase diagram of a spin-chain system Ca2+xY2-xCu5O10-d: oxygen hole-doping effects"

Keeseong Park , Stony Brook University/ Brookhaven National Laboratory
[Host: Despina Louca]
ABSTRACT:
Oxygen hole-doping effects on a spin-chain system, Ca2+xY2-xCu5O10-d (CaYCuO) are reported. CaYCuO is a good specimen to study the magnetic properties of the CuO2 chain at the ground state because it has no complex structure other than the chain and it has hole dopability up to the formal copper valence number of +2.4. Specifically, we can dope holes into the CuO2 chain by substituting Ca2+ for Y3+ or by utilizing oxygen deficiency. After a systematic study of the two methods to dope holes, we found that oxygen doping makes a more critical change in magnetic ordering in the chain than the replacement of Ca2+. Oxygen deficiency effects of the chain on the magnetic properties were explained using a mean field theory. A new relation for the effective hole doping was found as p = x-(2/3)d.
Condensed Matter
Thursday, December 3, 2009
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Spin exchange interactions and magnetic properties"

Mike Whangbo , NC State University
[Host: Seunghun Lee]
ABSTRACT:
This talk will briefly review how the spin exchange interactions of magnetic solids are described quantitatively and qualitatively. When the choice of a spin lattice is made by inspecting the geometrical pattern of the magnetic ion arrangement or by seeking the novelty of the physics the chosen model generates, interesting but erroneous interpretations often result. The importance of choosing a spin lattice on the basis of electronic structure considerations is emphasized. The magnetic solids to be discussed in this talk include: Cs 2 CuCl 4 , Na 3 Cu 2 SbO 6 , Bi 4 Cu 3 V 2 O 14 , Cu 3 (CO 3 ) 2 (OH) 2 , AgCrO 2 , Ca 3 CoMnO 6 , MnWO 4 , [Cu(HF 2 )(pyz) 2 ]BF 4 .
SLIDESHOW:
Condensed Matter
Monday, November 23, 2009
3:30 PM
Physics Building, Room 204
Note special date.

Slideshow (PDF)

"Non-Fermi liquid fixed point for an imbalanced gas of fermions in 1+ ε dimensions"

Andrew James , University of Virginia
[Host: Austen Lamacraft]
ABSTRACT:
We consider a two species gas of fermions with population imbalance. Using the renormalisation group in d=1+ ε dimensions, we show that for spinless fermions and ε > 0 a fixed point appears at finite attractive coupling where the quasiparticle residue Ζ vanishes, and identify this fixed point with the transition to Larkin--Ovchinnikov--Fulde--Ferrell order (inhomogeneous superconductivity). When the two species of fermions also carry spin degrees of freedom we find a repulsive fixed point, indicating a transition to spin density wave order.
SLIDESHOW:
Condensed Matter
Thursday, November 19, 2009
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Counting statistics of electron transport in nanostructures"

Christian Flindt , Harvard
[Host: Israel Klich]
ABSTRACT:
Fluctuations of the electrical current running through a nano-scale conductor reveal information beyond what is contained in the conductance alone. In this talk I will give an overview of my recent works on counting statistics, the stochastic theory of charge transport in nanostructures. As an example, I will show how current fluctuations can be a useful tool to detect mechanical bistabilities and frequency shifts in nanoelectromechanical systems. While most works have focused on systems whose dynamics is Markovian, I will discuss the influence of memory effects on the counting statistics, illustrated with a model of transport through a double quantum dot in a dissipative environment. Finally, I show that high-order current-current correlation functions (cumulants) in general oscillate as functions of basically any system parameter. A prediction that has been confirmed by recent experiments, as I will describe. Recent papers: C. Flindt et al., PRL 100, 150601 (2008), PNAS 106, 10116 (2009)
SLIDESHOW:
Condensed Matter
Thursday, November 12, 2009
4:00 PM
Physics Building, Room 204
Note special time.

"New Physics in Multicomponent Ultracold Atomic Gases"

Ryan Barnett , Joint Quantum Institute
[Host: Austen Lamacraft]
ABSTRACT:
Bose-Einstein Condensation in multicomponent systems often exhibits physics which goes beyond traditional condensed matter paradigms due to the extra degrees of freedom. Examples of multicomponent condensates include condensed mixtures of atoms or atoms with internal spin degrees of freedom. In this talk I will discuss examples of such multicomponent condensates focused on during my recent research. The first example I will consider is a rotating condensate composed of two types of atoms with different masses. I will discuss the structure of the vortex configurations for such mixtures, and argue the existence of a counterintuitive phase where the two superfluids and external drive all rotate at different rates. During the remainder of the talk I will focus on spinor condensates. In optical traps the macroscopic spin configuration is determined by the spin-exchange interaction. The resulting mean-field ground states have a variety of point-group symmetries. I will discuss a geometrical representation of the mean-field states of these systems, and will also describe how this method is useful for understanding the coherent spinor dynamics, collective excitations, and topological defects. I will next explain how the phenomenon of order-by-disorder from quantum magnetism is naturally exhibited in spin-two condensates. Finally, I will discuss the rich structure of the vortex lattices which can occur when spinor condensates are rotated.
Condensed Matter
Thursday, October 29, 2009
4:00 PM
Physics Building, Room 204
Note special time.

"Local electronic properties of graphene"

Brian Leroy , Univ. of Arizona
[Host: Keith Williams]
ABSTRACT:
Combining scanning probe microscopy with electrical transport measurements is a powerful approach to probe low-dimensional systems. The local information provided by scanning probe microscopy is invaluable for studying effects such as electron-electron interactions and scattering. Using this approach, we have probed the local electronic properties of mono- and bilayer graphene with atomic resolution. We studied the effect of ripples, charged impurities and defects on the local density of states. We find that long-range scattering from ripples and impurities shifts the Dirac point leading to electron and hole puddles. Short-range scattering from lattice defects mixes the two sublattices of graphene and tends to be strongly suppressed away from the Fermi energy. In addition, in bilayer graphene we observe an opening of a band gap due to the application of a transverse electric field.
Condensed Matter
Thursday, October 22, 2009
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Refrigeration using superconducting tunnel junctions"

Sukumar Rajauria , NIST, Maryland
[Host: Bellave Shivaram]
ABSTRACT:
In the recent years, nano-refrigeration using electron tunneling in hybrid Normal metal - Insulator - Superconductor (N-I-S) junctions has gained increasing attention. Its basic principle is the energy selective tunneling due to the presence of an energy gap in the superconductor density of states. With a sub-gap voltage bias, only the most energetic electrons can tunnel out of the normal metal, leaving behind the electrons with less energy. We have measured with a high resolution the differential conductance of S-I-N-I-S junctions, whose analysis gives us an access to the normal metal electronic temperature as a function of the voltage. A quantitative model is proposed, that includes the electron-phonon coupling and the Kapitza resistance at the interface with the substrate. With this model, we have achieved a thorough description of the charge and heat currents. We have also shown that the normal metal phonon temperature drops significantly below the substrate temperature. At very low temperature (T < 200mK) and low bias, the coherent Andreev current dominates the quasi-particle current. By analyzing quantitatively the heat balance in the S-I-N-I-S junction, we demonstrate that the Andreev current does carry heat. This thermal contribution heats the normal metal electrons, overriding over a large voltage range the tunneling-based cooling. References [1] S. Rajauria, P. S. Luo, T. Fournier, F. W. J. Hekking, H. Courtois, and B. Pannetier, Phy. Rev. Lett. 99, 047004 (2007). [2] S. Rajauria, Ph. Gandit, T. Fournier, F. W. J. Hekking, B. Pannetier, and H. Courtois, Phy. Rev. Lett. 100, 047004 (2008). [3] S. Rajauria, H. Courtois and B. Pannetier, submitted (2009).
SLIDESHOW:
Condensed Matter
Thursday, October 15, 2009
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Classical-quantum mappings for geometrically frustrated systems"

Stephen Powell , Joint Quantum Institute
[Host: Andrew James]
ABSTRACT:
Several systems have recently been demonstrated to show "non-LGW" quantum phase transitions, with different orderings on two sides of a continuous transition. We present two examples of classical statistical systems --- spin ice in a [100] magnetic field and an ordering transition of close-packed dimers on a cubic lattice --- that appear to show continuous (second-order) transitions that lie outside the Landau paradigm. In both cases, strong local constraints mean that neither of the neighboring phases can be understood as thermally disordered, excluding the standard route to a continuum critical theory. Instead, we derive critical theories for both transitions by mapping from three-dimensional classical problems to two-dimensional quantum problems. For the dimer model, this mapping provides a direct connection to previous work on non-LGW transitions of lattice bosons at fractional filling factors.
SLIDESHOW:
Condensed Matter
Thursday, September 17, 2009
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Toric-boson model: Toward a topological quantum memory at finite temperature"

Alioscia Hamma , Perimeter Institute
[Host: Israel Klich]
ABSTRACT:
We discuss the existence of stable topological quantum memory at finite temperature. At stake here is the fundamental question of whether it is, in principle, possible to store quantum information for macroscopic times without the intervention from the external world, that is, without error correction. We study the toric code in two dimensions with an additional bosonic field that couples to the defects, in the presence of a generic environment at finite temperature: the toric-boson model. Although the coupling constants for the bare model are not finite in the thermodynamic limit, the model has a finite spectrum. We show that in the topological phase, there is a finite temperature below which open strings are confined and therefore the lifetime of the memory can be made arbitrarily (polynomially) long in system size. The interaction with the bosonic field yields a long-range attractive force between the end points of open strings but leaves closed strings and topological order intact.
SLIDESHOW:
Condensed Matter
Thursday, May 7, 2009
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Vertical nano-composite heteroepitaxial thin films with manganites and ferroelectrics"

Yonghang Pei , University of Virginia
[Host: Bellave Shivaram]
SLIDESHOW:
Condensed Matter
Monday, April 20, 2009
3:30 PM
Physics Building, Room 204
Note special date.

Slideshow (PDF)

"The Effect of Thermal Fluctuations on the Problem of Euler Buckling Instability"

Luke Langsjoen , University of Virginia
[Host: Genya Kolomeisky]
ABSTRACT:
Renormalization techniques are used to study the effect of thermal fluctuations on the classical Euler buckling instability of a homogenous rod in 2 spatial dimensions. It is discovered that temperature has the effect of exponentially stiffening the rod, yielding a non-zero minimum critical force as a function of the length of the rod.
SLIDESHOW:
Condensed Matter
Thursday, April 16, 2009
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"The thin film Giaever transformer - vortex drag in a superconductor thin-film bilayer "

Gil Refael , Caltech
[Host: Israel Klich ]
ABSTRACT:
The normal-field induced superconductor-insulator transition in amorphous thin films, and the possible intervening metallic state, can be qualitatively explained within two paradigms: as vortex condensation, or as a percolation transition between the competing normal and superconducting phases. An experiment that may qualitatively distinguish these two paradigms is a drag experiment on a bilayer system, consisting of two parallel films, where a voltage response in one layer to current in the other is measured. The drag due to vortices is expected to dramatically exceed the Coulomb drag that may arise from the charge carriers in the percolation scenario. In my talk, I will present recent results estimating the drag response within the vortex and percolation paradigms, in the limit of no tunneling between the layers.
SLIDESHOW:
Condensed Matter
Wednesday, April 15, 2009
3:30 PM
Physics Building, Room 204
Note special date.

Slideshow (PDF)

"Mechanical Behavior of a Ni-based Crystalline and a Zr-based Amorphous Materials Subjected to Surface Severe Plastic Deformation "

Jiawan Tian , University of Tennessee
[Host: Despina Louca]
ABSTRACT:
A surface-treatment process, surface-severe-plastic deformation (S2PD), is developed and applied on both crystalline and amorphous materials to introduce plastic deformation in the near-surface layer. A S2PD-processed crystalline component is expected to have enhanced fatigue properties because the refined grains in the near-surface layer and the coarse grains in the interior have good resistance to the crack initiation and propagation, respectively. The microstructures and mechanical properties of the processed specimens were systematically investigated. It is shown that the S2PD process has the capability of simultaneously creating (a) a work-hardened surface layer, (b) a nanocrystalline (nc) surface layer, (c) a surface region with compressive-residual stresses, and (d) a grain-size gradient with a nc surface and a coarse-grained interior for the polycrystalline superalloy. Improved yield strength and fatigue strength were found after the process. For the amorphous material, thermal properties of the processed near-surface layer were characterized by means of the differential-scanning calorimetry (DSC). Effects of the treatment on the microhardness were studied by the nanoindentation. After the treatment, the plastic-flow deformation in the unconstrained sample edge was observed. In the sub-surface layer, the impact-induced shear-band operations generate the extrusion and intrusion marks on the side surface. XRD and highenergy synchrotron diffraction techniques were used to inspect the possible crystalline phase. A nanoindentation test shows that on the side surface, the hardness increases and, then, decreases with the distance from the processed surface. Four-point-bending-fatigue behavior has been studied and related to the modified surface structure and the compressive-residual stress induced by the process.
SLIDESHOW:
Condensed Matter
Thursday, April 9, 2009
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Characterization of Doped Silicon and Endofullerenes by Raman Spectroscopy for Sensing and Transport Devices"

Brian Burke , University of Virginia
[Host: Bellave Shivaram ]
SLIDESHOW:
Condensed Matter
Tuesday, April 7, 2009
3:30 PM
Physics Building, Room 204
Note special date.

Slideshow (PDF)

"Simple Elements at high densities: En-Route to metallic hydrogen and insulating lithium"

[Host: Despina Louca]
SLIDESHOW:
Condensed Matter
Monday, April 6, 2009
3:30 PM
Physics Building, Room 204
Note special date.

Slideshow (PDF)

"Neutron and X-ray scattering studies of frustrated, quantum, and multiferroic transition metal oxides"

Junghwa Kim , University of Virginia
[Host: Seunghun Lee]
SLIDESHOW:
Condensed Matter
Thursday, April 2, 2009
4:00 PM
Physics Building, Room 204
Note special time.

"A study on the spin-state transition and complex magnetic coupling in Perovskite Cobaltite"

Juan Yu , University of Virginia
[Host: Bellave Shivaram]
Condensed Matter
Thursday, March 26, 2009
4:00 PM
Physics Building, Room 204
Note special time.

"Continuum Calogero-Sutherland Models"

Michael Stone , University of Illinois
[Host: Paul Fendley]
ABSTRACT:
The Calogero-Sutherland family of models consist of point particles moving on a line or circle and interacting with a repulsive inverse-square potential. Both the classical and quantum versions are completely integrable, and are the subject of an extensive literature. I will describe their origin, their connection with the Hall effect and the Benjamin-Ono equation, and some of the subtleties that arise from the existence of two inner products.
Condensed Matter
Monday, March 23, 2009
11:00 AM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"Understanding Magnetism in Multiferroics"

Micky Holcomb , University of California, Berkeley
[Host: Stu Wolf]
ABSTRACT:
Magnetoelectric multiferroics are materials that exhibit multiple order parameters, such as ferroelectricity and ferromagnetism. The potential for coupling between such order parameters and their manipulation through external stimuli (electric or magnetic fields) allows the exploration of novel physics and device functionalities. Among a large number of materials systems, the BiFeO3 system has emerged as an attractive model system, mainly because both the ferroelectric Curie temperature and the antiferromagnetic Neel temperature are well above room temperature. In order to understand magnetoelectric coupling, the individual order parameters must first be understood. While the ferroelectric order can be probed using conventional capacitive measurements as well as by scanned probe techniques (such as piezoforce microscopy) probing the antiferromagnetic order requires the use of optical probes, such as SHG and x-ray photoemission spectromicroscopy. Angle and temperature dependent absorption measurements using a state-of-the-art high-resolution photoemission microscope allowed imaging and direction determination of the order parameters in this multiferroic. These studies reveal the first observation of electrical control of antiferromagnetism and the extension to electrical control of ferromagnetism through exchange bias. Though this study, a generic method for separating order parameters in complex systems has been developed and applied.
Condensed Matter
Thursday, March 19, 2009
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Six-Port Reflectometer: an Alternative Network Analyzer for THz Region"

Guoguang Wu , University of Virginia
[Host: Bellave Shivaram]
SLIDESHOW:
Condensed Matter
Tuesday, March 17, 2009
3:30 PM
Physics Building, Room 204
Note special date.

Slideshow (PDF)

"Geometrical Interpretation of the Non-universal Casimir Energy of An Infinite Cylindrical Wedge"

Hussain Zaidi , University of Virginia
[Host: Genya Kolomeisky]
ABSTRACT:
The majority of calculations in the literature on the Casimir energy of curved bodies focus on the universal part of the energy, implicitly assuming that the non-universal terms subtracted from the formal expression for the energy have no physical consequence. We explicitly calculate the non-universal terms for the particular case of an infinite cylindrical wedge and show that these terms are important quantities that arise out of the dependence of the surface tension and the bending/rigidity coefficients of a body on the energy cut-off. This lends support to a recent phenomenological argument in favor of a geometrical interpretation of the non-universal parts of the Casimir energy.
SLIDESHOW:
Condensed Matter
Friday, March 13, 2009
11:00 AM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"Orbital degrees of freedom on triangle-based lattice"

Takuro Katsufuji , Waseda University
[Host: Seunghun Lee ]
ABSTRACT:
Orbital degrees of freedom arising from the degeneracy of d states give rise to intriguing behaviors in various transition-metal oxides. For example, spinel vanadates (ZnV2O4, MnV2O4, FeV2O4, AlV2O4), where two d electrons occupy the triply degenerate t2g orbitals, exhibit various types of orbital ordering. In this talk, orbital ordering of V ions on triangle-based lattice (SrVxGa12-xO19, BaV10O15) is discussed. In these compounds, V trimers in a spin-singlet state are formed, but spins on a part of the V ions survive, which are frustrated or ordered at low temperatures. Such a spontaneous segregation into spin-singlet clusters and magnetic ions could be a characteristic of the spin-orbital system on triangle-based lattice.
Condensed Matter
Thursday, March 12, 2009
4:00 PM
Physics Building, Room 204
Note special time.

"Single-molecule biophysics with a protein nanopore"

Liviu Movileanu , Syracuse University
[Host: Keith Williams]
ABSTRACT:
Advances in rational protein design and single-molecule technology allow for biochemical sampling at high temporal and spatial resolution and for the detection, manipulation, and exploration of individual molecules. We have developed a methodology for examining single biopolymer dynamics within a protein nanopore, a simple system that is highly pertinent to several more complex biological processes such as the translocation of nucleic acids and polypeptides through transmembrane pores. The ionic current through a single protein nanopore was determined by single-channel electrical recordings in lipid bilayers. The results revealed unprecedented details of biopolymer behavior at single-molecule resolution. These examples demonstrate an unusual control of single biomolecules and pore-based nanostructures by using simple principles learned from physics and modern biology.
Condensed Matter
Thursday, February 19, 2009
3:30 PM
Physics Building, Room 204

"Imaging Dirac Fermions in a Two-dimensional Sheet of Carbon"

Yuanbo Zhang , University of California, Berkeley
[Host: Despina Louca]
ABSTRACT:
It has been four years since the first successful isolation of graphene, a single atomic sheet of carbon, and enthusiasm for this material is still growing. Part of this excitement arises from the fact that electrons in graphene behave as massless "relativistic" particles (Dirac fermions) with an effective speed of light equal to c/300. Microscopic disorders in graphene, such as ripples in the carbon sheet or static "puddles" of charge, profoundly alter the behavior of these electrons. I will describe our recent experiments aimed at directly probing these perturbations and imaging their influence on Dirac fermions down to the atomic scale. Our measurements, performed using the techniques of scanning tunneling microscopy and spectroscopy, reveal unexpected electronic interference patterns that vary as a function of both electron energy and applied electric field (via a gate electrode). I will show that electron interference in graphene nanodevices arises from scattering off of static puddles of electrons, and that these puddles are caused by charged impurities underneath the graphene sheet.
Condensed Matter
Thursday, February 12, 2009
3:30 PM
Physics Building, Room 204

Slideshow (PDF)

"Quenching spin decoherence in diamond and single-molecule magnets"

Susumu Takahashi , University of California, Santa Barbara
[Host: Despina Louca]
ABSTRACT:
Overcoming spin decoherence is critical to spintronics and spin-based quantum information processing devices. For spins in the solid state, an interaction with fluctuations of the surrounding spin bath is a major source of spin decoherence. One approach to reducing spin bath fluctuations is to bring the spin bath into a well-known quantum state that exhibits little or no fluctuations. A prime example is the case of a fully-polarized spin bath. We present our recent demonstrations of significant suppression of spin decoherence measured with high-field electron paramagnetic resonance (EPR). One example is nitrogen-vacancy (NV) centers in diamond [1]. Another is S=10 Fe8 single-molecule magnets [2]. We will also present the development of UCSB free-electron laser (FEL)-based pulsed EPR spectrometer which aims for nano-second time resolution.[1] S. Takahashi et al., Phys. Rev. Lett. 101, 047601 (2008). [2] S. Takahashi et al., Phys. Rev. Lett. in-press; arXiv: 0810.1254.
SLIDESHOW:
Condensed Matter
Thursday, February 5, 2009
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Dimensional Reduction at a Quantum Critical Point"

Cristian Batista , Los Alamos National Laboratory
[Host: Seunghun Lee]
ABSTRACT:
Competition between ground states near a quantum critical point is expected to lead to unconventional behavior in low dimensional systems. New phases of matter have been predicted, and explanations proposed for unsolved problems including non-Fermi liquid behavior and high temperature superconductivity using two-dimensional (2d) theories. In this talk, I will present a theory that describes the Bose-Einstein condensate (BEC) quantum critical point (QCP) in layered systems with a frustrated inter-layer coupling. I will demonstrate that the main effect of this geometric frustration is to reduce the dimensionality of the QCP (its critical exponents are the ones expected for a 2d system). In addition, I will present the first experimental evidence of dimensional reduction at a QCP observed in the Mott insulator BaCuSi 2 O 6 (Han Purple).
SLIDESHOW:
Condensed Matter
Wednesday, February 4, 2009
3:30 PM
Physics Building, Room 204
Note special date.

Slideshow (PDF)

"Mind the Gap of Fe Superconductors"

Tinyong Chen , Johns Hopkins University
[Host: Despina Louca]
ABSTRACT:
A family of new Fe superconductors of SmFeAsO1-xFx and others have been discovered in 2008 that contain the puckered FeAs planes instead of the hallmark CuO2 planes in the cuprate superconductors. Central to any superconductor is the nature of its superconducting gap, its value, its structure if any, its temperature dependence. We used Andreev reflection spectroscopy to investigate the gap of these new (1111) Fe superconductors and its temperature dependence. Although the gap values and transition temperatures are different for the Fe superconductors with different F doping, the values of 2 Δ /kBTC are all close to 3.53 that of a BCS s-wave superconductor. The gap is nearly isotropic with temperature dependence close to that of an s-wave superconductor. We did not find evidences of pseudogaps, but the spin density wave transition can induce spurious “pseudogaps” in the system. These characteristics of the Fe superconductors are dramatically different from those of the cuprate superconductors.
SLIDESHOW:
Condensed Matter
Thursday, January 29, 2009
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Nanopores and nanofluidics for single DNA studies"

Derek Stein , Brown University
[Host: Keith Williams]
ABSTRACT:
Lab-on-a-chip fluidic technology takes inspiration from electronic integrated circuits, from which its name, its fabrication methods, and its ?smaller, cheaper, faster? paradigm are derived. For silicon-based electronics, miniaturization eventually gave rise to qualitatively different behavior, as quantum mechanical phenomena grew increasingly important. As we shrink fluidic devices down to the nanoscale to probe samples as minute as a single molecule, what physical phenomena will dominate in this new regime, and how might we take advantage of them? This talk will focus on our studies of single DNA molecules using nanofluidic devices and solid-state nanopores. We are studying how nanofluidic structures, whose critical dimensions are tens to hundreds of nanometers, can manipulate long DNA molecules by a variety of nanoscale phenomena, including electrokinetics, hydrodynamics, Coulomb interactions, and the statistical properties of polymers. Our work also focuses on solid-state nanopores, single-nanometer-scale devices that can not only manipulate single molecules, but also detect them electronically. The basic principle behind this is that when DNA is electrophoretically driven through a nanopore, it blocks a measureable fraction of the ionic current that is transmitted through the pore. Thanks to its size, the nanopore also forces each base along the DNA to pass through in sequence, suggesting intriguing possibilities for genetic analysis.
SLIDESHOW:
Condensed Matter
Monday, January 26, 2009
3:30 PM
Physics Building, Room 204
Note special date.

Slideshow (PDF)

"Combining ferroelectricity and magnetism: the low-energy electrodynamics"

Diyar Talbayev , LANL
[Host: Despina Louca]
ABSTRACT:
Multiferroics, materials with coexisting magnetic and ferroelectric orders, hold the promise of implementing multifunctional devices, for example, an electric-write magnetic-read memory element. The success of this venture depends on our fundamental understanding of magnetoelectric coupling – the interaction that mixes magnetism with ferroelectricity. Although the electrical switching of a magnetic domain has yet to be achieved, various degrees of electric control of magnetism and magnetic control of ferroelectric state have been demonstrated. Magnetic and lattice vibrations and their mixing by the magnetoelectric coupling play a central role in the properties of multiferroics. This low-energy electrodynamics can help unravel the fundamental interactions between magnetic and lattice degrees of freedom. I will present a study of long-wavelength magnetic excitations in several important classes of multiferroics and demonstrate the relationship between the magnetic excitations and the materials’ magnetoelectric functionality.
SLIDESHOW:
Condensed Matter
Thursday, January 15, 2009
4:00 PM
Physics Building, Room 204
Note special time.

"Topological Quantum Computing with Read-Rezayi States "

Layla Hormozi , NIST
[Host: Israel Klich ]
ABSTRACT:
A topological quantum computer is a hypothetical device in which intrinsic fault-tolerance is embedded in the hardware of the quantum computer. It is envisioned that in these devices quantum information will be stored in certain "topologically ordered" states of matter, and quantum computation is carried out by braiding the world-lines of quasiparticle excitations that obey non-Abelian statistics, around one another, in specific patterns. Certain fractional quantum Hall states are among the prime candidates for realizing non-Abelian quasiparticles that can be used for topological quantum computation. I will review some of the properties of these states, and describe a method for finding braiding patterns which can be used to carry out a universal set of quantum gates on encoded qubits based on non-Abelian quasiparticles that can be realized as excitations of the Read-Rezayi series of fractional quantum Hall states.
Condensed Matter
Thursday, December 4, 2008
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"A Study of the Group-IV Diluted Magnetic Semiconductor GeMn"

Melissa Commisso , University of Virginia
[Host: Bellave Shivaram]
SLIDESHOW:
Condensed Matter
Monday, November 3, 2008
3:30 PM
Physics Building, Room 204
Note special date.

Slideshow (PDF)

"Time-reversal symmetry breaking and spontaneous anomalous Hall effect in Fermi fluids"

Kai Sun , University of Illinois
[Host: Paul Fendley]
ABSTRACT:
We study the spontaneous non-magnetic time-reversal symmetry breaking in a 2D Fermi liquid without breaking either the translational symmetry or the U(1) charge symmetry. Using a Berry phase approach, we found that for a large class of models, including all one- and two-band models, the time-reversal symmetry breaking states can be classified into two classes, dubbed type I and II, depending on the accompanying spatial symmetry breaking patterns. The properties of each class are studied. In particular, we show that the states breaking both time-reversal and chiral symmetries (type II) are described by spontaneously generated Berry phases and exhibit anomalous Hall effect in the absence of magnetic fields and magnetic impurities. We also show examples of the time-reversal symmetry breaking phases in several different microscopically motivated models and calculate their associated Hall conductance within a mean-field approximation. In particularly, we found a simple lattice structure in which the time-reversal symmetry breaking phases can be stabilized by infinitesimal interactions.
SLIDESHOW:
Condensed Matter
Thursday, October 23, 2008
4:00 PM
Physics Building, Room 204
Note special time.

"Study of Silicon Devices by Inelastic Tunneling Spectroscopy"

James Kushmerick , NIST
[Host: Keith Williams]
Condensed Matter
Thursday, October 9, 2008
4:00 PM
Physics Building, Room 313
Note special time.
Note special room.

Slideshow (PDF)

"Neutron scattering and magnetization studies of the kagome lattice antiferromagnet"

Kit Matan , ISSP, U of Tokyo
[Host: Seunghun Lee]
ABSTRACT:
The collective behavior of interacting magnetic moments can be strongly inﬂuenced by the topology of the underlying lattice. In geometrically frustrated spin systems, interesting spin dynamics and chiral correlations may develop that are related to the spin arrangement on triangular plaquettes. I will talk about our studies of the spin-wave excitations and spin chirality on a two-dimensional geometrically frustrated lattice. Our new chemical synthesis methods allow us to produce large single crystal samples of KFe3(OH)6(SO4)2, an ideal kagomé lattice antiferromagnet. The spin-wave excitations have been measured using high-resolution inelastic neutron scattering. We directly observe a ﬂat mode which corresponds to a lifted "zero energy mode," verifying a fundamental prediction for the kagomé lattice. A simple Heisenberg spin Hamiltonian provides an excellent ﬁt to our spin-wave data. The antisymmetric Dzyloshinskii-Moriya interaction is the primary source of anisotropy and explain the low-temperature magnetization and spin structure. In addition, combined thermodynamic and neutron scattering measurements reveal that the phase transition to the ordered ground-state is unusual. At low temperatures, application of a magnetic ﬁeld induces a transition between states with different non-trivial spin-textures. The transition indicated by the sudden increase in the magnetization arises as the spins on alternating layers, which are previously oppositely canted due to the ferromagnetic interplane coupling, rotate 180° to align the canting moment along the c-axis. These observations are consistent with the ordering induced by the Dzyloshinskii-Moriya interaction. Elastic neutron scattering measurements in high ﬁeld verify the 180° spin rotation at the transition.
SLIDESHOW:
Condensed Matter
Thursday, October 2, 2008
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Measurement-Only Topological Quantum Computation"

Parsa Bonderson , Microsoft / UCSB
[Host: Paul Fendley]
ABSTRACT:
The topological approach to quantum computing derives intrinsic fault-tolerance by encoding qubits in the non-local state spaces of non-Abelian anyons. The original prescription required topological charge measurement for qubit readout, and used braiding exchanges of anyons to execute computational gates. We present an anyonic analog of quantum state teleportation, and use it to show how a series of topological charge measurements may replace the physical transportation of computational anyons in the implementation of computational gates.
SLIDESHOW:
Condensed Matter
Thursday, April 24, 2008
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Exploration of Novel Tunnel Barrier Materials for STT-RAM "

Wei Chen , University of Virginia
[Host: Jongsoo Yoon]
ABSTRACT:
The basic structure of magnetic tunnel junctions (MTJs) consists of two ferromagnetic(FM) layers sandwiched by an ultra thin insulating barrier, and it shows high or low resistance depending on the relative direction of the the magnetization of two FM layers. Conventional Magnetic random access memory (MRAM) using MTJs as storage units are switched using external filed, so it has the scaling problem beyond 65nm node. A new switching mechanism called Spin Torque Transfer (STT) has been proposed and experimentally confirmed. In our work, new tunnel barrier materials are being explored to enhance the performance of this new STT-RAM technology. One of the particular tunnel barrier materials VO2 has the metal-insulator transition close to room temperature, and we're trying to incorporate VO2 into MTJs stack as the smart barrier for the STT switching so that the MTJs could be switched in low resistance state and read at high resistance state.
SLIDESHOW:
Condensed Matter
Thursday, April 10, 2008
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"The Mysterious Metallic Phase in 2D Superconductors and the Resulting Phase Diagram"

Yize (Stephanie) Li , University of Virginia
[Host: Jongsoo Yoon]
ABSTRACT:
Conventional treatments of electronic transport predict that no metallic phase exists in two-dimensional (2D) superconducting materials at zero temperature (T=0). This view has been challenged by the observation of magnetic field (B) induced metallic behavior in amorphous MoGe and Ta thin films. We have demonstrated that the metallic phase in Ta thin films has an intrinsic origin and associates with nonlinear voltage-current (I-V) characteristics that are qualitatively different from those of superconducting and insulating phases. Based on transport measurement, we can map the phase diagram of Ta thin films in B-T-Disorder space and study the physics it reveals. We also investigated the nature of the B-induced insulating phase. We found that the peak structure of differential IV traces displayed a non-monotonic change as a function of B, which might be a signature for the localized Cooper pairs.
SLIDESHOW:
Condensed Matter
Thursday, April 3, 2008
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Effects of Short-Range Order on the Structure and Dynamics of Relaxor Ferroelectrics"

Peter Gehring , NIST
[Host: Seunghun Lee]
ABSTRACT:
Relaxors are disordered perovskite (ABO3) oxides, typically characterized by the presence of mixed valence B-site cations, that have found widespread use in numerous device applications because they exhibit low hysteresis and record-setting piezoelectric coefficients. Relaxors derive their name from an unusually frequency-dependent dielectric susceptibility, but they also display a rich variety of unusual physical phenomena including simultaneously soft zone-center and zone-boundary phonons, temperature dependent diffuse scattering, and an anomalous thermal expansion where a transition to a low-temperature invar-like behavior is observed. Recent neutron elastic and inelastic scattering results on the lead-based relaxors PbMg1/3Nb2/3O3 (PMN),PbZn1/3Nb2/3O3 (PZN), and their solid solutions with PbTiO3 will be discussed that indicate the development of static, short-range polar order at high temperatures is central to these phenomena. These results can be understood by analogy with random-field models in which just two temperature scales are required to describe the essential features of relaxor compounds.
SLIDESHOW:
Condensed Matter
Thursday, March 27, 2008
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Investigation of high temperature TE compounds"

Jack Simonson , University of Virginia
[Host: Jongsoo Yoon]
SLIDESHOW:
Condensed Matter
Thursday, March 20, 2008
4:00 PM
Physics Building, Room 204
Note special time.

"Fascinating Exotic Phenomena in Layered Ruthenates"

Zhiqiang Mao , Tulane University
[Host: Seunghun Lee]
ABSTRACT:
Perovskite ruthenates (Sr,Ca) n+1 Ru n O 3n+1 exhibit a rich variety of fascinating ordered ground states. Spin-triplet superconductivity, metamagnetic quantum criticality, itinerant ferromagnetism, antiferromagnetic Mott insulating, and half-metallic behavior were all found in close proximity to one another. These diverse ground states originate from the strong interplay of charge, spin, lattice, and orbital degrees of freedom. They offer a unique opportunity to tune the system and study the physics of novel quantum phases. In this talk, I will first give a brief overview on studies in this area, and then present our recent work on double layered ruthenates (Sr 1-x Ca x ) 3 Ru 2 O 7 (0 ≤ x ≤ 1). We have established a magnetic phase diagram for this system using the high quality single crystals grown by the floating-zone technique; this phase diagram exhibits significant new phenomena. We find a very unique magnetic state in close proximity to a two-dimensional ferromagnet with T c =0 K for 0.1 < x < 0.4. This state exhibits a surprisingly large Wilson ratio RW (e.g. R W ≈ 700 for x = 0.2); it freezes into a cluster glass (CG) at low temperatures. Furthermore, we observe evidence of non-Fermi liquid behavior as the frozen temperature of the CG phase approaches zero near x = 0.1. The origin of such a state will be discussed.
Condensed Matter
Thursday, February 28, 2008
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Random Telegraph Signal in Carbon Nanotube Device"

Jack Chan , University of Virginia
[Host: Jongsoo Yoon]
ABSTRACT:
Due to the low dimensionality of carbon nanotubes (CNTs), charging of a single defect site near a CNT may have a profound effect on modifying carrier transport mobility in a long CNT channel. Random Telegraph Signals (RTS) have been studied in carbon nanotube field effect transistors (CNT-FETs). If the energy of the defect center is close to the Fermi level of the CNT-FET, trapping and detrapping of carriers would modify the carrier transport in the channel, and give rise to RTS. RTS is observed as a switching between discrete current levels, representing a carrier being trapped and detrapped successively in the defect center. We speculate that RTS spectra could provide a characteristic signature of specific adsorbates or adducts on the nanotube channel. This capability is of interest not only for potential sensing technology but also provides a way to introduce controllable quantum interference resonances in the channel transport.
SLIDESHOW:
Condensed Matter
Thursday, February 14, 2008
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Frustrations in Relaxors"

Kazuma Hirota , University of Tokyo
[Host: Seung-Hun Lee]
ABSTRACT:
Relaxors have been extensively studied for a variety of applications as ideal dielectric materials because they often exhibit extremely large dielectric and piezoelectric constants while the dielectric loss and temperature dependence are small. Although it is now widely believed that heterogeneity embedded and appearing in relaxors are relevant to various phenomena specific to relaxors, there is still no established microscopic theory for relaxors. The prototypical relaxors Pb(Mg1/3Nb2/3)O3 (PMN) consists of Pb2+ on the A site of the ABO3 Perovskite structure and Mg2+ and Nb5+ on the B site. To keep the charge neutrality, Mg2+ and Nb5+ have to form a solid solution with a ratio of 1:2 so as to have an average valence of 4+. However, it is likely that a large difference between the ionic radius of Mg2+ and that of Nb5+ prefers the 1:1 solid solution resulting in an alternating arrangement of Mg2+ and Nb5+. The 1:2 state and the 1:1 state are mutually exclusive, thus the system falls in a state of frustration. The concept of frustration has been studied almost exclusively in magnetism, e.g., a geometrical frustration in an antiferromagnetic triangular lattice, though phenomena related to frustration are widely seen in nature. Since there is no unique ground state in a frustrated system, the system becomes unstable among various different states, which may cause large fluctuations leading to a extremely large susceptibility against an external field and to a novel exotic phase. We now consider that heterogeneous structures appearing in relaxors can originate from such a frustration. In this presentation, a review is given on a series of neutron and x-ray scattering experiments on spatial structures and dynamics of polar nano regions in relaxors. We then would like to discuss how such experimental results can be understood in the frame work of frustration. We also discuss what we will be able to study the microscopic mechanism of relaxors by controlling the frustration through lattice distortion, charge imbalance and dimensionality.
SLIDESHOW:
Condensed Matter
Monday, February 11, 2008
3:30 PM
Physics Building, Room 204
Note special date.

Slideshow (PDF)

"Order by distortion and chiral magnetism in CdCr2O4"

Gia-Wei Chern , Johns Hopkins University
[Host: Seunghun Lee]
SLIDESHOW:
Condensed Matter
Thursday, February 7, 2008
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Neutron scattering study on static and dynamic spin structures in quasicrystalline magnets"

Taku Sato , University of Tokyo
[Host: Seunghun Lee]
ABSTRACT:
Quasicrystals have distinct spatial symmetry characterized by highly-ordered but non-periodic (quasiperiodic) atomic structure, which differs both from the periodic and random structures. Ordering and excitations of quasiperiodically arranged magnetic moments (spins) are yet fundamental open problems, despite the intensive efforts continuously made since the discovery of the quasicrystal. In this talk I will present recent development of understanding on the static and dynamic spin structures in quasicrystalline magnets, using the extensively studied Zn-Mg-RE systems as typical examples. In magnetization measurements, the Zn-Mg-RE quasicrystals all show spin-glass-like behavior, indicating random freezing of spins at low temperatures, however, well-defined short-range order have been observed in the neutron scattering. The inelastic response of Zn-Mg-RE falls into two classes: For RE = Tb and Dy, a broad inelastic peak has been observed with very weak Q dependence, suggesting an existence of the strongly localized collective excitation modes. On the other hand, for RE = Ho, temperature-independent S(Q, h ω ) was observed in the neutron-energy-gain side (h ω >0) for an incredibly large temperature range (up to 200 K!). The anomalous spin fluctuations may be related to criticality of electron wave functions in the quasiperiodic lattice.
SLIDESHOW:
Condensed Matter
Thursday, January 31, 2008
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Frustrated and Satisfied Ground States in Pyrochlore Magnets"

Bruce Gaulin , McMaster University
[Host: Seung-Hun Lee]
ABSTRACT:
Geometrical frustration arises quite generally when pairwise interactions in magnetic materials are incompatable with their local geometry. This often involves magnetic materials made up of assemblies of triangles or tetrahedra. The frustration is manifest by disordered low temperature states for the magnetic material - some of which are described by spin liquids, spin glasses, and spin ice. I will discuss (mostly) neutron scattering work using DCS at NIST on two magnetic pyrochlores Tb2Ti2O7 and Ho2Ti2O7, which can be thought of as Ising-like moments decorating a network of corner-sharing tetrahedra. Tb2Ti2O7 displays a spin liquid, or cooperative paramagnetic ground state, but can be brought to order in an applied magnetic field. Ho2Ti2O7 displays a static disordered "spin ice" state at low temperatures.
SLIDESHOW:
Condensed Matter
Thursday, December 6, 2007
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Giant Negative Magnetization in a Class of Layered Molecular-Based Magnets"

Dr. Randy Fishman , Oak Ridge National Laboratory
[Host: Jongsoo Yoon]
ABSTRACT:
Bimetallic oxalates are a class of layered molecular-based magnets with transition metals M(II) and M'(III) coupled by oxalate molecules in an open honeycomb structure. Energy, structure, and symmetry considerations are used to construct a reduced Hamiltonian, including exchange and spin-orbit interactions, that explains the magnetic compensation and giant negative magnetization in some of the ferrimagnetic Fe(II)Fe(III) compounds. By shifting the Fe(II) ions with respect to the oxalate molecules, the organic cation between the magnetic layers alters the C_3 -symmetric crystal field and the orbital angular momentum of the ground-state doublet at the Fe(II) sites. We provide new predictions for the spin-wave gap, the effects of uni-axial strain, and the optical flipping of the negative magnetization in Fe(II)Fe(III) bimetallic oxalates.
SLIDESHOW:
Condensed Matter
Thursday, November 29, 2007
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Understanding short- and medium range order in materials using total neutron scattering"

Dr. Thomas Proffen , Los Alamos National Laboratory
[Host: Despina Louca]
ABSTRACT:
Determination of the atomic structure is mainly based on the measurement of Bragg intensities and yields the average structure of the infinite crystalline material. However, this approach ignores any defects or local structural deviations that manifest themselves as diffuse scattering. It also fails in case of disordered materials, badly crystalline such as many nano-materials, or not crystalline at all, such as glasses. In some cases crystalline and amorphous phases coexist making the traditional crystallographic structure refinement difficult or incomplete. The total scattering pattern or the derived atomic pair distribution function (PDF), however, contains structural information over all length scales [1] and can be used to obtain a complete structural picture of complex materials. One of the great advantaged of the PDF is the fact that one can limit the range on atom-atom distance over which the structural model is refined. Focusing on small distances up to a few Angstroms will illuminate the local structure where as refinements over a wide range will yield the medium and long range structure. It is interesting to consider, that instruments such as the high resolution neutron powder diffractometer NPDF located at the Lujan Neutron Scattering Center at Los Alamos National Laboratory allows the measurement of PDFs up to distances in excess of 200Å or 20nm. As a result one can obtain a ‘complete’ structural fingerprint of nanoparticles that are frequently smaller in size as demonstrated in a recent study of gold nanoparticles [2]. [1] Th. Proffen, S.J.L. Billinge, T. Egami and D. Louca, Z. Krist. 218, 132-143 (2003). [2] K.L. Page, Th. Proffen, H. Terrones, M. Terrones, L. Lee, Y. Yang, S. Stemmer, R. Seshadri and A.K. Cheetham, Chem. Phys. Lett. 393, 385-388 (2004).
SLIDESHOW:
Condensed Matter
Thursday, November 8, 2007
4:00 PM
Physics Building, Room 204
Note special time.

"Mesoscopic physics in a quantum magnet chromium"

Yeong-Ah Soh , Dartmouth College
[Host: Seunghun Lee]
ABSTRACT:
Cr is a simple metal and quintessential spin density wave antiferromagnet that has a quantum critical point. The Hall number collapses on entering the antiferromagnetic state because the Fermi surface is reduced as some of the carriers are localized to produce the magnetic order. Our studies of the Hall effect in Cr[1] were the first to measure the Hall effect in any metal near an antiferromagnetic quantum critical point and showed that it can be a very sensitive probe to study quantum phase transitions. We recently extended our studies to films to explore a two-dimensional system. By growing extremely high quality Cr films we observe quantization of spin density waves and scattering from antiferromagnetic domain walls, effects which can be easily tuned by the film thickness.
Condensed Matter
Thursday, November 1, 2007
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Classical and Quantum Frustrated Magnets "

Yong Baek Kim , University of Toronto
[Host: Seunghun Lee]
ABSTRACT:
We will discuss the properties of classical and quantum Heisenberg models on the lattices with corner sharing triangles; namely two dimensional Kagome and three-dimensional Hyper-Kagome lattices. The roles of thermal and quantum fluctuations, and the resulting ground states will be the focus of the discussion. Connection to recent experiments on Volborthite, Na4Ir3O8, and Zn-paratacamite will also be discussed.
SLIDESHOW:
Condensed Matter
Thursday, October 18, 2007
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Single Spin Logic"

[Host: Bellave Shivaram]
ABSTRACT:
Electronic devices have traditionally relied on charge to encode, store, process and transmit information. A well-known example is the celebrated Field Effect Transistor, which is the workhorse of all modern digital electronic chips. When a transistor channel is filled with charge, the transistor is “on” and encodes the binary bit 1. When the channel is depleted of charge, the transistor is “off” and encodes the binary bit 0. Switching between bits is therefore accomplished by moving charge within the transistor which causes current flow and associated power dissipation. This is a fundamental shortcoming of all charge based devices. Since charge is a scalar, and has only a magnitude, logic bits 0 and 1 must be demarcated by a difference in the magnitude of the stored charge. As a result, switching between bits always involves changing the magnitude of charge and therefore causing a current flow and I2R dissipation. This cannot be avoided. An electron’s spin, on the other hand, is a pseudo vector with a polarization. If spin polarization is made “bistable” by placing the electron in a static magnetic field, then the two allowed polarizations (parallel and anti-parallel to the field) can encode bits 0 and 1. Switching would require simply flipping the spin without moving charge in space and causing a current flow (I2R = 0). This can result in tremendous energy saving which is currently the most important goal in electronics. The Single Spin Logic (SSL) idea1 is based on this paradigm. Spins of single electrons in quantum dots encode digital bits. Logic gates are configured by placing the dots in suitable layouts to engineer the spin-spin interactions between them. Inputs are provided by aligning the spins in input dots along desired orientations using locally generated magnetic fields. The arrival of a new input string takes the system to an excited state. When the spins ultimately relax to the many body ground state, the spin polarizations in “output” dots represent the result of a computation in response to the input bits. I will show how the universal NAND gate is configured in this way. With the NAND gate, any arbitrary circuit can be built. Detailed quantum mechanical calculations show that switching in these circuits dissipate the minimum energy allowed by thermodynamics (the Landauer-Shannon limit), which is kTln(1/p) where p is the bit error probability. With p = 10-9, the energy dissipated is ~ 21 kT, whereas modern transistors dissipate 40,000 – 50,000 kT. The SSL is incomparably superior to spin based transistors (Spin Field Effect Transistors, Spin Junction Transistors, etc.) which do not compare well with traditional transistors. The reason is that they still rely on charge for encoding information and do not exploit the advantage of spin. I will conclude by showing that SSL type constructs are best realized with organic nanostructures where the spin relaxation time can be extremely long. We have measured a spin relaxation time of 1 second at 100 K in a nanostructure of the -conjugated organic tris(8-hydroxyquinolinolato aluminum) popularly known as Alq3 2.
SLIDESHOW:
Condensed Matter
Thursday, October 11, 2007
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Narrow Gap Semiconductors: spin splitting with no magnetic fields and more,….."

Giti Khodaparast , Virginia Tech
[Host: Keith Williams]
ABSTRACT:
In light of the growing interest in spin-related phenomena and devices, there is now renewed interest in the science and engineering of narrow gap semiconductors (NGS). NGS offer several unique features such as small effective masses, large effective g-factors, high intrinsic mobilities, large spin-orbit coupling effects and zero-field spin splitting. For example, In InSb quantum wells (QWs), spin-resolved cyclotron resonance (CR) caused by the non-parabolicity of the conduction band and electron spin resonance have been observed. In order to increase our understanding of the dynamics of carriers and spins in NGS, we performed several time-resolved measurements such as magneto-optical Kerr effect. In this talk, magneto-optical studies on InSb QWs and ferromagnetic semiconductors such as InMnSb will be discussed. Our results are important for understanding the electronic and magnetic states in NGS.
SLIDESHOW:
Condensed Matter
Thursday, October 4, 2007
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Superconductivity in graphene-based structures"

Igor Mazin , Naval Research Laboratory
[Host: Seung-Hun Lee]
ABSTRACT:
MgB2 and CaC6 are some of the most interesting new stars on the superconducting skies. The former, with Tc=39K is the most high-temperature conventional superconductor, and it is by far superior technologically to the cuprate-base high-Tc materials. The latter has Tc >13K, nearly an order of magnitude higher than that of old intercalated graphites. Theory says that despite both materials being, essentially, doped graphenes, superconductivity comes from two different bands, one existing in MgB2 but not in CaC6 and the other in CaC6 but not in MgB2. In this talk I will explain what are this bands and why they are responsible for superconductivity in the respective compounds, and will discuss whether or not it is possible to invent a new material that would combine superconducting advantages of both.
SLIDESHOW:
Condensed Matter
Thursday, September 27, 2007
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Recent Results on CMR and Multiferroic Manganese Oxides"

Jeff Lynn , National Institute of Standards and Technology
[Host: Seunghun Lee]
ABSTRACT:
A few results from recent neutron and x-ray scattering results will be presented on both CMR and multiferroic systems. For the multiferroics, we have been investigating the magnetic structure of hexagonal HoMnO 3 as a function of temperature and field [1], which is a commensurate antiferromagnetic (T N =72 K) ferroelectric (T C =875 K). Three different chiral symmetries describe the zero field magnetic phases, with strong dielectric anomalies associated with the phase transitions. The spin dynamics are well described by a Heisenberg model in two dimensions. Orthorhombic TbMnO 3 develops a longitudinally polarized spin density wave state below 41 K, with a change in magnetic structure at 28 K that permits the development of ferroelectricity, while the magnetic structure remains incommensurate [2]. The magnetism is particularly sensitive to Na doping [3]. For the RMn 2 O 5 system (R=Tb, Dy, Ho) strong anomalies in the specific heat, thermal expansion, and dielectric constant are a manifestation of the magnetic coupling to the ferroelectricity [4]. Strong magnetoelastic coupling is also found in the triangular antiferromagnetic multiferroic CuFeO 2 [5]. For the Kagome staircase system Co 3 V 2 O 8 the rich variety of magnetic phases and lock-in transitions is a signature of competing interactions [6], and is quite different from ferroelectric Ni 3 V 2 O 8 . For the CMR systems, we will review recent results for the polaron dynamics in optimally doped La-BaMnO 3 and La-SrMnO 3 , and compare these results with La-CaMnO 3 [7] and the bilayer system [8]. The overall behavior observed in the CMR regime of the manganites is quite similar to that observed in the relaxor ferroelectrics as well as the spin and charge stripes found cuprate oxides, demonstrating a commonality of many of the underlying physical concepts of these perovskite oxides.
SLIDESHOW:
Condensed Matter
Thursday, September 6, 2007
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Resonant X-ray Scattering Study of Quadrupole-Strain Interactions in Rare-Earth Tetraborides "

Sungdae Ji , UVA/NIST
[Host: Seunghum Lee]
SLIDESHOW:
Condensed Matter
Thursday, April 26, 2007
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Coherence and optical spin rotations in quantum dots"

Sophia Economou , Naval Research Lab
[Host: Eddy Barnes/Paul Fendley]
ABSTRACT:
Optically controlled quantum dots, sometimes referred to as artificial atoms, have been the subject of intense research in recent years. This is due both to their potential role as qubits for quantum computing, and to novel emerging physics, as a result of the interplay of confinement and the semiconductor environment. In the first part of the talk I will present such an effect, Spontaneously Generated Coherence (SGC), in which spontaneous emission of an excited level results in a coherent superposition of two lower levels. This phenomenon, predicted in the context of atomic physics, is so far unobserved in atoms. I will sketch our theory of SGC in quantum dots and present the experimental results of the first observation of this effect. In the second part of my talk the focus will be on the design of arbitrary optical spin rotations, which are necessary operations for quantum computing. Simulations show that our approach yields high quality gates, up to two orders of magnitude faster that existing proposals.
SLIDESHOW:
Condensed Matter
Thursday, April 5, 2007
4:00 PM
Physics Building, Room 204
Note special time.

"From fundamental understanding to predicting new nanomaterials for high capacity hydrogen storage and fuel cell technologies"

Taner Yildirim , NIST
[Host: Bellave Shivaram]
ABSTRACT:
Developing safe, cost-effective, and practical means of storing hydrogen is crucial for the advancement of hydrogen and fuel-cell technologies. The current state-of-the-art is at an impasse in providing any materials that meet a storage capacity of 6wt% or more required for practical applications. The main obstacles in hydrogen storage are slow kinetics, poor reversibility and high dehydrogenation temperatures for the chemical hydrides, and very low desorption temperatures/energies for the physisorption materials such as metal-organic frameworks (MOF). Recently we have proposed a novel concept to overcome these obstacles. From accurate quantum mechanical calculations, we show that light transition metals (TM) such as a Ti-atom affixed to several nanostructures such as nanotubes/C60 and small organic molecules (C2 H4 ) strongly bind up to five hydrogen molecules. The first H 2 adsorption is dissociative with ~0.25 eV energy barrier while other adsorptions are molecular with significantly elongated H-H bonds. The metal-hydrogen interaction is found to be very unique, lying between chemi and physisorption, with a binding energy of 0.4 eV compatible with room temperature desorption and absorption. Simulations at high temperature indicate that such hybrid systems of transition metals affixed to nanostructures are quite stable and exhibit associative desorption upon heating, a requirement for reversible storage. These results not only advance our fundamental understanding of dissociative adsorption of hydrogen on transition metals in nano-structures but also suggest new routes to better storage and catalyst materials. Finally, time permitted, we will discuss the possibility of dimerization, polymerization, and incorporation of the predicted TM-nanostuctures in nanoporous materials such as MOF to improve the life-cycle and kinetics of the predicted storage materials.
Condensed Matter
Thursday, March 29, 2007
4:00 PM
Physics Building, Room 204
Note special time.

Slideshow (PDF)

"Search for stripes in lightly hole doped antiferromagnetic YBCO"

Andras Janossy , Budapest University of Technology and Economics
[Host: Stu Wolf]
ABSTRACT:
Stripes, a periodic spin and charge modulation in superconducting and antiferromagnetic cuprates are a manifestation of an instability of the two dimensional hole system. A multifrequency electron spin resonance study of the antiferromagnetic domain structure in undoped and lightly Ca doped antiferromagnetic Y(Gd)Ba2Cu3O6 single crystals will be discussed. Gd substituting for Y serves as a non-perturbing localised ESR probe of the antiferromagnetic CuO2 layers. Hole doping, variation of temperature and magnetic fields change the antiferromagnetic structure. ESR and infrared transmission experiments in high magnetic fields show that the stripe structure is not an array of hole rich lines in YBCO.
SLIDESHOW:
Hoxton Lecture
Thursday, March 22, 2007
7:30 PM
Chemistry Building, Room 402
Note special time.
Note special room.

"From the Big Bang to the Nobel Prize"

John Mather , Goddard Space Center
ABSTRACT:
The history of the universe in a nutshell, from the Big Bang to now, and on to the future – John Mather will tell the story of how we got here, how the Universe began with a Big Bang, how it could have produced an Earth where sentient beings can live, and how those beings are discovering their history. Mather was Project Scientist for NASA’s Cosmic Background Explorer (COBE) satellite, which measured the spectrum (the color) of the heat radiation from the Big Bang, discovered hot and cold spots in that radiation, and hunted for the first objects that formed after the great explosion. He will explain Einstein’s biggest mistake, show how Edwin Hubble discovered the expansion of the universe, how the COBE mission was built, and how the COBE data support the Big Bang theory. He will also show NASA’s plans for the next great telescope in space, the James Webb Space Telescope. It will look even farther back in time than the Hubble Space Telescope, and will look inside the dusty cocoons where stars and planets are being born today. Planned for launch in 2013, it may lead to another Nobel prize for some lucky observer.
Condensed Matter
Monday, March 12, 2007
3:30 PM
Physics Building, Room 204
Note special date.

"Neutron Scattering Investigation of Excitations Spectrum in Doped and Undoped Spin-Ladders"

Bella Lake , Hahn-Meitner Institut
[Host: Seunghun Lee]
ABSTRACT:
The ‘telephone number compound’ family, (La,Sr,Ca) 14 Cu 24 O 41 , is composed of copper oxide planes which form alternating layers of ladders and chains. In the absence of Sr these materials are intrinsically hole doped with a valence of +2.25 per Cu.In contrast, for La 4 Sr 10 Cu 24 O 41 the hole doping is much reduced and magnon heat conductivity measurements show that the ladders are free of holes [1]. A third member of the family, Sr 2.5 Ca 11.5 Cu 24 O 41 , is hole-doped with ~2 holes per seven rungs of the ladder (the exact doping is debated). This compound is of particular interest because it shows a number of phenomena in common with high-Tc cuprates, including linear temperature dependence of resistivity above 130K, charge ordering below 60K and superconductivity under applied pressure [2]. Our neutron scattering measurements on the undoped ladder La 4 Sr 10 Cu 24 O 41 reveal gapped one-magnon and multi-magnon excitations. The results have been modelled to find accurate values of the exchange constants [3]. The doped ladder Sr 2.5 Ca 11.5 Cu 24 O 41 has also been measured. The holes give rise to a number of changes in the excitation spectrum which will be described and discussed.
Condensed Matter
Monday, February 19, 2007
3:30 PM
Physics Building, Room 204
Note special date.

"A unifying perspective on charge fractionalization"

Alexander Seidel , NHMFL/Florida State
[Host: Paul Fendley]
ABSTRACT:
Condensed matter physicists are generally faced with the task of solving problems involving some 1024 particles that interact strongly. Amazingly, in many cases this task of seemingly hopeless complexity is amenable to the following simple strategy: Try to find a way to (almost) switch of the interactions in a manner that preserves all the fundamental properties of the system. If this is possible, one says that the system is "adiabatically connected" to a non-interacting system. In the past 20 years, however, much focus has been on problems where the traditional approach does not seem feasible. In particular, a new paradigm has surfaced which applies to certain novel incompressible quantum liquids that are said to have "topological order". This new paradigm encompasses the fractional quantum Hall liquids, as well as some theoretically proposed scenarios for novel magnetism in materials similar to the parent compounds of high transition temperature superconductors. The phenomenology of topologically ordered states is very exotic, including fractionally charged excitations and fractional braiding statistics. This fact seems to preclude the possibility that these states have simple non-interacting limits. In spite of this, it will be shown in this talk that such a trivial limit does exist for fractional quantum Hall systems. In fact, by studying quantum Hall states on cylinders with varying circumference, these states can be adiabatically evolved into simple one-dimensional charge-density-wave systems. This point of view provides simple, intuitive pictures for some of the exotic properties of fractional quantum Hall systems. In particular, the principles of charge fractionalization in two spatial dimensions and in one spatial dimension are completely unified by this approach. The potential usefulness of this adiabatic continuity for some unresolved problems will also be discussed.
Condensed Matter
Thursday, February 15, 2007
4:00 PM
Physics Building, Room 204
Note special time.

"Perfect Pulses"

Israel Klich , Caltech
[Host: Paul Fendley]
ABSTRACT:
Generic perturbations of a Fermi liquid create noise in the form of numerous particle-hole pairs. In the talk I will show that it is possible to excite particles from a Fermi sea in a noise-free fashion by electromagnetic pulses of a special form. The resulting many-body states are characterized by a finite number of particles excited above the Fermi surface accompanied by no disturbance below it. The excitations can be regarded as realization of vertex operators. I will also discuss potential applications in creating entangled electron hole pairs, and effects of interactions.
Condensed Matter
Wednesday, February 7, 2007
3:30 PM
Physics Building, Room 204
Note special date.

"Phase ordering in atomic gases"

Austen Lamacraft , Oxford
[Host: Paul Fendley]
ABSTRACT:
The ordering of matter into different phases is a central preoccupation of many areas of physics, from condensed matter to cosmology. Hand in hand with the existence of different phases goes the question of which dynamical processes are responsible for = their formation, which may be equally important in determining what is observed in a given situation. Recent experimental advances in the creation of degenerate atomic gases have begun to realize the prospect of a rich variety of new phases in atomic matter, involving the hyperfine degrees of freedom, mixtures of different species, or spatial order on optical lattices. With each new phase comes the issue of how that phase will appear under laboratory conditions.=C2=A0 In this talk I'll discuss the theoretical treatment of phase ordering = in=C2=A0 several recent experiments, and the possibility of observing new dynamical phenomena in the future.
Condensed Matter
Monday, February 5, 2007
3:30 PM
Physics Building, Room 204
Note special date.

"PATTERNS, STABILITY AND COLLAPSE FOR TWO-DIMENSIONAL BIOLOGICAL SWARMS"

Maria D'Orsogna , UCLA
[Host: Paul Fendley]
ABSTRACT:
One of the most fascinating biological phenomena is the self-organization of individual members of a species moving in unison with one another, forming elegant and coherent aggregation patterns. Schools of fish, flocks of birds and swarms of insects arise in response to external stimuli or by direct interaction, and are able to fulfill tasks much more efficiently than single agents. How do these patterns arise? What are their properties? How are individual characteristics linked to collective behaviors? In this talk we discuss various aspects of biological swarming by investigating a non-linear system of self propelled agents that interact via pairwise attractive and repulsive potentials. We are able to predict distinct aggregation morphologies, such as flocks and vortices, and by utilizing statistical mechanics tools, to relate the interaction potential to the collapsing or dispersing behavior of aggregates as the number of constituents increases. We also discuss passage to the continuum and possible applications of this work to the development of artificial swarming teams.
Condensed Matter
Thursday, February 1, 2007
4:00 PM
Physics Building, Room 204
Note special time.

" Mutual influence of vortices and quasiparticles in d-wave superconductors "

Predrag Nikolic , Harvard
[Host: Paul Fendley]
ABSTRACT:
Vortices in clean d-wave superconductors at low temperatures can behave as quantum particles. Their quantum dynamics is made possible by the smallness of their cores, due to short coherence length in typical cuprates, and by the presence of massless fermionic quasiparticles, which give rise to certain universal effects. We calculate a small finite renormalization of vortex mass by the nodal quasiparticles, and demonstrate the absence of Bardeen-Stephen damping of vortex motion in the limits of zero temperature, no disorder and vanishing core size. Being liberated from strong friction, light vortices can experience significant quantum fluctuations that can explain several phenomena observed in cuprates, including the Nernst effect and density waves. We also show that quantum fluctuations of localized vortices can significantly affect quasiparticle spectra. The local electronic density of states (LDOS) near a quantum fluctuating vortex shows no zero-energy peak, but has satellite features at energies set by the vortex trapping potential. These are proposed to be the origin of the sub-gap LDOS peaks observed in recent STM experiments near the vortex cores.
Condensed Matter
Thursday, November 30, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Phase Transitions in Nano-dimensional Materials"

Syed Qadri , U. S. Naval Research Laboratory
[Host: Stu Wolf]
ABSTRACT:
Research interest in the field of nanodimensional materials is growing rapidly due to their interesting size-dependent electronic, magnetic, optical and mechanical properties that have many potential industrial applications. Metals and semiconductors with dimensions in the nanometer realm exhibit novel optical, electrical, and chemical properties which are closely associated with their structural characteristics. In addition, metastable phases and size-dependent phase transitions are observed when the particle sizes are reduced to nanodimensions. Several examples from our previous work will be presented to illustrate the existence of these unusual structural characteristics. X-ray diffraction is a non-destructive characterization tool that plays an important role in the synthesis and in understanding the physical properties of the nanoparticles.
Condensed Matter
Thursday, November 23, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"N/A"

Thankgiving Recess , N/A
[Host: N/A]
Condensed Matter
Thursday, November 16, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Charge ordering instead of Jahn-Teller distortion"

Daniel Khomskii , Universitaet zu Koeln, Germany
[Host: Seunghun Lee]
ABSTRACT:
Due to Jahn-Teller effect the systems with orbital degeneracy usually have an orbital ordering with corresponding lattice distortion. However this distortion weakens and finally disappears when the electrons become itinerant. We show that in the intermediate regime between localized and itinerant electrons there exist a new possibility: orbital degeneracy may be lifted not by Jahn-Teller distortion, but by charge ordering, or charge differentiation. We demonstrate, by theoretical calculations and experimentally, that this happens in particular in rare earths nickelates such as YNiO3 or LuNiO3. The same physics apparently operates also in several other systems, such as ferrates (e.g. CaFeO3) and some others. I will also discuss a possible role of small charge transfer gap and oxygen holes in this phenomenon.
Condensed Matter
Wednesday, November 15, 2006
3:30 PM
Physics Building, Room 204
Note special date.

"Unusual features in magnetism and magnetoresistance of double perovskite oxides"

Prof. D.D. Sarma , Indian Institute of Science
[Host: Bellave Shivaram]
ABSTRACT:
Sr2FeMoO6 is a magnetic metal with an unusually high Curie temperature (~ 420 K) and belongs to the double perovskite family of compounds. It shot in to fame a few years ago due to its remarkable magnetoresistive properties. [1] We discuss the origin of ferromagnetism in this and related compounds based on a mechanism driven by the kinetic energy, [2-4] establishing these as members of a new class of magnetic materials; this mechanism also explains [4,5] the occurrence of ferromagnetism in dilute magnetic semiconductors, such as Mn-doped GaAs and in pyrochlore, Tl2Mn2O7. [6] We also show [7] that the magnetoresistance in Sr2FeMoO6 arises from a magnetically triggered nearly-resonant tunnelling condition in contrast to other mechanisms discussed so far in the context of CMR and GMR materials.
Condensed Matter
Thursday, November 2, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"To tunnel or not to tunnel - or why clathrates are so fascinating"

Veerle Keppens , University of Tennessee
[Host: Joe Poon]
ABSTRACT:
Motivated by the search for improved thermoelectric materials, several compounds have attracted attention that combine the high electron mobilities found in crystals with a low thermal conductivity approaching values typical for glasses. The common structural feature of these “Electron Crystal Phonon Glasses” (ECPG) is that they contain loosely bound atoms that reside in a large crystalline “cage”. These “rattlers” scatter phonons and greatly reduce the thermal conductivity of the material. One family of ECPGs is formed by the Ge-clathrates Sr 8 Ga 16 Ge 30 and Eu 8 Ga 16 Ge 30 . The presence of the rattler significantly softens the elastic behavior. Combined with results from low-temperature ultrasonic attenuation, neutron-scattering, thermal conductivity and microwave absorption measurements, it provides clear evidence for the existence of a new type of four-well tunneling states.
Condensed Matter
Thursday, October 26, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Quantum phase transitions and magnon stability in gapped spin chains and ladders"

Andrey Zheludev , Oak Ridge National Laboratory
[Host: Seung-Hun Lee]
ABSTRACT:
The lowest energy excitations in quantum disordered spin chains and ladders are a triplet of massive magnons. The stability of these quasiparticles depends critically on the symmetries of the actual spin Hamiltonian. When the spin system undergoes a quantum phase transition, for example one induced by a strong external magnetic field, the symmetry of the ground state changes, and so do the excitations. I will present a comparative inelastic neutron scattering study of three distinct one-dimensional disordered spin systems: the S=1 quasi-1D bond-alternating antiferromagnet NTENP, the uniform anisotropic S=1-chain Haldane-gap compound NDMAP and the uniform isotropic composite'' Haldane spin chain IPA-CuCl3. For each material I will discuss the field-induced condensation of magnons, and analyze the spectra measured below, at and above the transition point. The high-field phase will be characterized either as a Bose-Einstein magnon condensate, with a cartelistic gapless Goldstone mode, or as a gapped "quantum spin solid". The issue of magnon stability in each phase will be addressed in detail.
Condensed Matter
Thursday, October 19, 2006
2:00 PM
Physics Building, Room 203
Note special time.
Note special room.

"Graphene: symmetries, phase transitions, Hall effect"

Igor Herbut , Simon Fraser University
[Host: Seunghun Lee]
ABSTRACT:
A single sheet or graphite, or graphene, in its natural state is a semimetal with two Fermi points in its Brillouin zone. The low-energy excitations near those points are equivalent to massless relativistic fermions, with the Fermi velocity assuming the role of the speed of light. I will discuss the effects of Coulomb interaction between electrons in such a system at zero and finite magnetic field. The integer Hall effect in graphene will be explained as the combination of the Landau level quantization and spontaneous breaking of the emerging "chiral" symmetry in such a quasi-relativistic system.
Condensed Matter
Wednesday, October 18, 2006
4:00 PM
Physics Building, Room 204
Note special date.
Note special time.

"Low Energy Vibrational Excitations in Metallic Glasses"

Ricardo Schwarz , Los Alamos National Laboratory
[Host: Vittorio Celli]
ABSTRACT:
The specific heat and elastic constants of metallic glasses show anomalies not seen in crystals. These anomalies result from low-energy vibrational excitations in the glass. We have measured the low-temperature heat capacity, the elastic constants, and the phonon density of states of both glassy and single-phase crystalline Pd40Cu40P20. The specific heats of both alloys, plotted as CP/T3 vs. T, show different humps (commonly known as "Boson Peaks"). The elastic constants of the crystal and glass have different T-dependence: the shear modulus of the glass varies as C'(T) = C'(0)[1 - AT], whereas that of the crystal varies as C'(T) = C'(0)[1 - BT2 - DT4]. This suite of low-temperature measurements enabled us to identify the low-energy vibrational excitations responsible for these anomalies
Condensed Matter
Thursday, October 12, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Magnetic ordering in a half--polarized magnetization plateau of the pyrochlore antiferromagnet"

Doron Bergman , UC Santa Barbara
[Host: Seunghun Lee]
ABSTRACT:
The classical pyrochlore antiferromagnet (AFM) is considered the most'' geometrically frustrated system. Classically, this leads to the absence of any ordering transition at non-zero temperature, even in an applied magnetic field. Recent experiments on the spinel chromites, ACr2o4(A=Cd,Hg) show the existance of a very robust magnetization plateau in a strong magnetic field, and a simultaneous magnetic ordering. We describe a model of spin-lattice coupling that explains both the plateau formation and the observed ordering on the plateau. The predictions are confirmed by recent neutron scattering and x-ray scattering experiments (S. H. Lee et al.). The same model applied to zero magnetic field predicts a reduced but still large ground state degeneracy, including the states observed in both the Cd and Hg materials. This is consistent with the dominance of spin-lattice interactions, with weak additional effects determining the low field magnetic ordering.
Condensed Matter
Thursday, October 5, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Ferro-electricity in Frustrated Magnets"

Collin Broholm , Johns Hopkins
[Host: Seunghun Lee]
ABSTRACT:
While electrostatics and magnetostatics are disparate phenomena in a vacuum, no symmetry forbids materials from responding magnetically to an electric field or vise versa. Materials with a strong magneto-electric response are of interest for applications and challenge our understanding of magnetic dielectrics. I discuss specific examples of magneto-electricity in metal oxides with triangular or kagomé lattices of spins with competing antiferromagnetic interactions [1-3]. It is shown that inversion symmetry breaking magnetic order can act as an effective electric field through magneto-elastic distortions that relieve frustration. The results presented in this talk are based on magnetic neutron scattering experiments.
Condensed Matter
Thursday, September 28, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Quantum Criticality"

Qimiao Si , Rice University
[Host: Jongsoo Yoon]
ABSTRACT:
Quantum criticality describes the strong fluctuations of a second order phase transition at zero temperature. There is growing evidence that it is relevant to a large part of the phase diagram of a variety of strongly correlated materials. In this talk, I will describe some of the basic issues that challenge the theoretical description of quantum criticality. I will go on to make the case for a new class of quantum critical point, with critical fluctuations that are genuinely quantum-mechanical. In the case of heavy fermion metals, the theory of local quantum criticality characterizes such quantum fluctuations in terms of the collapse of a Kondo entanglement scale. Finally, I will summarize some of the experimental evidences for this theory, particularly those concerning the evolution of the Fermi surface.
Condensed Matter
Thursday, September 14, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Applications of several MeV CW Superconducting Radio Frequency Accelerators"

Ganapathi Myneni and Geoff Kraft , University of Virginia
[Host: Bellave Shivaram]
Condensed Matter
Thursday, August 31, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Rapid Motion in the Plant Kingdom: Nature's Weapons of Mass Reproduction."

Dwight Whitaker , Williams College
[Host: Jongsoo Yoon]
ABSTRACT:
Plants and fungi have developed a number of remarkable methods to bring about rapid motion. The most rapid movements rely on stored elastic energy and take place in a timescale that is shorter than a single enzymatic reaction (~1 ms). Because these processes are entirely mechanical we can describe the motion with a straightforward biomechanical model based on classical mechanics. High-speed video lets us test the predictions of our models, which enable us to isolate the key features required for each type of movement and to assess how effectively the system performs. This information, when combined with field observations, helps us to understand the adaptive significance of the motion and put it into evolutionary context with similar species. In this talk we will present a number common plants that exhibit a variety of uncommon methods to disperse seeds and spores.
Condensed Matter
Thursday, April 27, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Desynchronization and Spatial Effects in Multistrain Diseases"

Leah Chock , Naval Research Laboratories
[Host: Jongsoo Yoon]
ABSTRACT:
Dengue fever, a multi-strain disease, has four distinct co-existing serotypes (strains). The serotypes interact by antibody-dependent enhancement (ADE), in which infection with a single serotype is asymptomatic, but contact with a second serotype leads to serious illness accompanied by greater infectivity. It has been observed from serotype data that outbreaks of the four serotypes occur asynchronously (Nisalak et al., Am. J. Trop. Med. Hyg. 68: 192). We present a compartmental model for multiple serotypes with ADE, and consider autonomous, seasonally driven, and stochastic versions of the model. For sufficiently small ADE, we find that the number of infectives of each serotype synchronizes, with outbreaks occurring in phase. However, when the ADE increases past a threshold, the system becomes chaotic, and infectives of each serotype desynchronize. Spatial effects are included in a multipatch model. We observe desynchronization between spatially distinct regions.
12th Annual National Physics Day Show
Two shows: 6:00 pm and 7:15 pm.

Thursday, April 20, 2006
6:00 PM
Physics Building - McCormick Road, Room 203
Note special time.
Note special room.

"National Physics Day Show"

Steve Thornton, Michael Timmins, Rob Watkins , UVA
[Host: University of Virginia Physics Department]
ABSTRACT:
This is a family-oriented event. Come see an hour of exciting and intriguing demonstrations! For more information on this free public event, call 434 924 3781. Free public parking is available in nearby Scott Stadium lots.
Condensed Matter
Thursday, April 13, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Quasiparticle Breakdown in a Quantum Spin Liquid"

Igor Zaliznyak , Brookhaven National Laboratory
[Host: Seunghun Lee]
ABSTRACT:
Quasiparticles are the elementary excitations carrying energy and momentum quanta in consensed matter, much like photons and elementary particles carry energy and momentum in the Universe surrounding us. Recent neutron scattering experiments demonstrate how quasiparticle description of energy spectra fails in magnetic crystals with non-magnetic ground states that can be identified as "quantum spin liquids" (QSL). The elementary excitations in such systems can be identified with massive (i.e. having non-zero rest energy) quasiparticles, called magnons, which obey Bose statistics, forming a Bose liquid. In a Bose quantum liquid, however, the single-particle dispersion can terminate at an energy where quasiparticle break-ups into two excitations become allowed, i.e. where the single-particle dispersion enters a continuum of two-particle states. Such spectrum endpoint was originally predicted by L. Landau for the superfluid helium-4, where it was subsequently extensively studied both theoretically and experimentally. A manifestation of this peculiar phenomenon in the case of a quantum spin liquid was found in the spin dynamics of the Haldane-chain S=1 antiferromagnet CsNiCl3, where it was initially identified as a crossover from the single quasiparticle to a spin-continuum response. More recently, signatures of the quasiparticle spectrum termination were observed in the excitation spectrum of the two-dimensional (2D) quantum spin liquid existing in the organo-metallic material piperazinium hexachlorodicuprate (PHCC), indicating a failure of the Bose-quasiparticle description of the QSL state of the 2D S=1/2 Heisenberg antiferromagnet in an extended region of its phase space. These findings are of great current interest as they might have important implications for the type of high-temperature superconductivity found in cuprates.
Condensed Matter
Thursday, April 6, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"SUPERCONDUCTIVITY - An Emerging Technology for Power Systems"

Don Gubser , Naval Research Laboratory
[Host: Stu Wolf]
ABSTRACT:
As the world becomes more “electrified”, efficient distribution and use of electrical power becomes increasingly important. The use of superconducting materials significantly reduces electrical energy loss in the distribution and use of electrical power as well as producing significant reductions in size and weight of power components and machinery. Although superconductivity was first discovered in 1911 the requirement of an extreme “cryogenic” environment (near absolute zero temperature) limited its utility. With the discovery in 1986 of a new class of “high temperature superconductors (HTS)” that operate at substantially higher temperatures (although still cryogenic), remarkable progress has been made in advancing a broader use for superconducting technology. Full scale demonstrations are now being built to develop engineering skills required for systems implementation of this new HTS technology and to better quantify system benefits. This talk will briefly review some of the fundamental attributes of superconductivity before turning to the main focus of the talk describing ongoing power demonstration projects (transmission lines, transformers, motors/generators, etc.). I will end with thoughts on what it will take to realize the full potential of these emerging superconducting technologies.
Condensed Matter
Thursday, March 30, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Fate of the Josephson Effect in Thin-film Superconductors"

Gil Refael , Caltech
[Host: Victor Galitski]
ABSTRACT:
The dc Josephson effect is a probe of the fundamental nature of the superconducting state. In this talk, I will analyze the case of two superconducting thin films connected by a point contact. Remarkably, the Josephson effect is absent at nonzero temperature, even below the K-T transition of the films, and the resistance across the contact is nonzero. Moreover, the point contact resistance is found to vary with temperature in a nearly activated fashion, with a universal energy barrier determined only by the superfluid stiffness characterizing the films, an angle characterizing the geometry, and whether or not the Coulomb interaction between Cooper pairs is screened. As will be shown, this should be testable even in finite systems at a proper range of length-scales and temperatures.
Condensed Matter
Thursday, March 23, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Ultrafast Phase Transformation in Metals Induced by Laser Irradiation: a Molecular Dynamic Study"

Zhibin Lin , UVA
[Host: Jongsoo Yoon]
ABSTRACT:
Short (pico- and femtosecond) laser irradiation has the ability to bring materials into a highly non-equilibrium state where the electron temperature is high while the lattice is still cold. By using Molecular Dynamic (MD) simulation, we can study the kinetics and mechanisms of laser melting and ablation phenomena at the atomic level. In this talk, I will talk about our simulation results on ultrafast melting induced by strong laser irradiation in metals and coherent acoustic phonon generation by laser excitation. In addition, I will relate to recent experimental observation in time-resolved diffraction experiments in probing the structure dynamic under extreme conditions.
Condensed Matter
Thursday, March 16, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Field Theory Studies of a Unitary Fermi Gas"

Matthew Wingate , University of Washington
[Host: Hank Thacker]
ABSTRACT:
Fermion pairing leads to interesting phenomena in many physical systems, whether the constituents are atoms, nucleons, electrons, or quarks. The newfound ability to trap and cool fermionic atoms has provided a versatile avenue for experimental exploration of fermion pairing. I have been working on using field theory techniques to describe and calculate properties of these gases. In this talk, I focus on a dilute gas of 2-component, nonrelativistic fermions whose scattering length is much larger than the average interparticle spacing. This is a setup which describes atomic gases tuned to a Feshbach resonance, as well as neutron matter. I discuss how Monte Carlo methods constitute an ab initio method for theoretical study of this gas. As an example, I show an exploratory calculation of the critical temperature separating the normal and superfluid phases. Next, I outline how effective field theory enables one to study the low temperature properties of the gas beyond superfluid hydrodynamics. It turns out that this system possesses a great deal of symmetry which tightly constrains the form of next-to-leading order behavior.
Condensed Matter
Thursday, March 2, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Adsorption of Gases on Carbon Nanotubes"

Aldo Migone , Southern Illinois University
[Host: George Hess]
Condensed Matter
Thursday, February 23, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Full Counting Statistics From Interference Between Interacting Bose Liquids in One Dimension"

[Host: Paul Fendley]
ABSTRACT:
In my talk first I will give a general introduction in physics of one-dimensional Bose condensates. Because of the absence of phase coherence, the interference between one dimensional condensates gives rise to quantum noise in the form of fluctuating fringe patterns. I describe the possible experiments for measuring the probability distribution function of interference fringe amplitudes. This probability distribution can be related to the boundary sine-Gordon model which is also known in mesoscopic physics to describe an impurity in a Luttinger liquid. The probability distribution function demonstrates an interesting crossover from broad Poissonian distribution for the case of impenetrable bosons to universal narrow asymmetric distributions at weak interactions. Finally we argue that by measuring such distributions experimentally, one can study the properties of conformal field theories with negative central charge, that appear in a variety of contexts ranging from stochastic growth models to two dimensional quantum gravity.
Condensed Matter
Thursday, February 2, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Emergence of Magnetism from a Non-magnetic Mott Insulator"

Daniel Phelan , UVA
[Host: Despina Louca]
ABSTRACT:
The parent compound of La1-xSrxCoO3 has a non-magnetic, Mott insulating ground state. As temperature is increased, the parent undergoes a non-magnetic to paramagnetic (PM) transition, and with hole-doping, static ferromagnetic (FM) correlations develop. The mechanisms by which the PM and FM states emerge from the non-magnetic state will be discussed with reference to our neutron scattering measurements. In the PM state, dynamic FM and antiferromagnetic (AFM) correlations likely result from the super-exchange interactions in a dynamically orbitally ordered state. With doping, static isotropic FM droplets are formed due to double exchange, and the FM correlation length increases with hole concentration.
Condensed Matter
Thursday, January 26, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"The Low Temperature Phase Diagram of the RKKY Model: Competing Interactions and Magnetic Percolation"

Donald Priour , University of Maryland
[Host: Victor Galitski]
ABSTRACT:
We examine magnetic moments (e.g. Mn impuritiesin GaAs) coupled via the indirect exchange RKKY interaction. We obtain via Monte Carlo the T=0 phase diagram as a function of the Mn density n_{i} and the carrier concentration n_{c}. As evidenced by a diverging correlation length and the magnetic susceptibility, the boundary between the ferromagnetic (FM) and the paramagnetic (PM) phases represents a line of zero temperature critical points with behavior very similar to a percolation transition. In particular, the ferromagnetic clusters increase in size and ultimately coalesce to span the system as the phase boundary is approached from the PM side. We have found this "ferromagnetic percolation" behavior in a variety of disordered magnetic systems with competing interactions, including the Edwards-Anderson model. In the RKKY system, we find that in the dilute limit, bulk ferromagnetism vanishes for n_{c}/n_{i} > 0.1. We also examine the impact of a strong local antiferromagnetic superexchange coupling between magnetic impurities, and we discuss the impact of a finite mean free path, which we include as a damping factor in the RKKY range function.
Condensed Matter
Thursday, January 19, 2006
4:00 PM
Physics Building, Room 204
Note special time.

"Spin-dependent Properties of Silicon-based Epitaxial Structures"

Frank Tsui , University of North Carolina, Chapel Hill
[Host: Joe Poon]
Condensed Matter
Thursday, December 1, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Bosons on the Triangular Lattice: Mott Transition in the Presence of Geometrical Frustration"

Anton Burkov , Harvard University
[Host: Victor Galitski]
ABSTRACT:
Interest in the behavior of interacting bosons on two-dimensional lattices has recently been revived by the experimental realization of superfluid-insulator transition of cold atoms in an optical lattice. Such models are also interesting from a broader prospective since they provide a simpler context to explore general aspects of conducting-insulating transitions of electrons. I will talk about our recent study of the interplay between Mott localization and geometrical frustration in a system of interacting bosons on the triangular lattice. Frustration prevents localization into simple ordered states, that can be determined by minimizing only the interaction energy. A variety of unconventional states thus become possible, notably supersolids and valence bond ordered insulators. I will describe a general phenomenological theory of these states, based on a duality transformation from bosons to vortices and compare this theory with numerical simulations.
Condensed Matter
Thursday, November 17, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Quantum Phases of the Extended Bose-Hubbard Hamiltonian: Possibility of a Supersolid State of Cold Atoms in Optical Lattices"

Vito Scarola , University of Maryland
[Host: Victor Galitski]
ABSTRACT:
Cold atoms confined to optical lattices offer the unique opportunity for direct simulation of strongly correlated models in a pristine environment. Recent experimental progress in simulating the zero-range Bose-Hubbard model with bosonic atoms in an optical lattice provides a direct experimental probe of the superfluid to Mott phase transition. Another lattice model, the extended Bose-Hubbard model, yields a rich but contested phase diagram which includes the possibility of supersolid and density wave order. I will discuss a specific proposal designed to extend the range of interaction in these otherwise zero-range systems to effectively simulate an extended Bose-Hubbard model in a cold atom optical lattice. I will also discuss potential issues affecting experimental detection of supersolid and density wave order.
Condensed Matter
Thursday, November 10, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"New Results in Quantum Hall Effects "

Wei Pan , Sandia National Laboratories
[Host: Jongsoo Yoon]
ABSTRACT:
After a brief introduction to the quantum Hall effects and the composite fermion model, I will discuss recent results in the second Landau level (LL). The data were obtained at very low temperatures (sample temperatures as low as 9 mK) in a very high mobility 2D electron system. We observe well-quantized FQHE states at the LL filling nu= 2+1/2, 2+1/3, and nu= 2+2/3 in coexistence with the reentrant integer quantum Hall states, and a new FQHE state at nu= 2+2/5. The origin of the 2+2/5 state is not clear and the numerical results strongly suggest that it is not a conventional FQHE state but a parafermionic state. There is also evidence for a second even-denominator FQHE state at nu= 2+3/8. Most importantly, we discovered an unexpected quantization of the diagonal resistance, Rxx, at the edges of several quantum Hall states. Each quantized Rxx value is close to the difference between the two adjacent Hall plateaus in the off-diagonal resistance, Rxy. Surprisingly, we can trace this observation back to a small density gradient, about 1%/cm, in our sample. Under this scenario almost all Rxx features can be explained quantitatively by an electron density gradient. These findings have very important implications for any Rxx data taken on two-dimensional electron systems, since Rxx seems now to be solely determined by Rxy, while its relationship to the diagonal resistivity, rxx, is unclear. These findings are further corroborated by data from a different sample at a temperature of 1.2K. There, Rxx shows a strictly linear dependence on the magnetic field, except for sharp spikes at B-fields where the IQHE develops. Interestingly, this linear magnetoresistance can also be explained by the density gradient model.
Condensed Matter
Thursday, November 3, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Overcoming Degeneracy in Highly Frustrated Antiferromagnets"

Chris Henley , Cornell University
[Host: Paul Fendley]
Condensed Matter
Thursday, October 27, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Observation of Superflow in Solid He-4"

Eunseong Kim , Penn State University
[Host: Jongsoo Yoon]
ABSTRACT:
We have observed superflow in both bulk solid He-4 and solid He-4 confined in porous media by torsional oscillator technique.[1] Below 0.25K non-classical rotational inertia appears as an abrupt drop in the resonant period of the torsional oscillator. The temperature dependence of the supersolid fraction is different from those found in liquid helium and weakly interacting alkali gases. The non-classical rotational inertia fraction is independent of the oscillation speed below an extremely small critical value of 10 um/s, above which strongly attenuated. The pressure dependence of the supersolid fraction in the low temperature limit reveals an intriguing maxium around 55 bars, whereafter the supersolid fraction monotonically decreases at least up to 136 bars and can be linearly extrapolated to zero at 170 bars. [1] E. Kim and M. H. W. Chan, Nature 427, 225 (2004); Science 305, 1941 (2004); J. Low Temp. Phys. 138, 859 (2005).
Condensed Matter
Thursday, October 20, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Exploring a New Route to a Two Dimensional Metal"

Jim Valles , Brown University
[Host: Joe Poon]
ABSTRACT:
According to the scaling theory of localization (1979), simple metallic phases should not exist in two dimensional electronic systems. Experiments showing the divergence of the low temperature resistance in ultrathin films of metallic elements and 2d electron gases in semiconductor heterostructures tended to support this prediction for about 15 years. More recently, however, evidence of metallic transport has begun to emerge, popping up in thin normally superconducting films and in very low density, high mobility 2D electron gas systems. The physics behind these metallic behaviors is not known although it is generally agreed that explanations must go beyond the scaling theory paradigm and include electron-electron interactions. In an effort to uncover an understandable metallic phase in two dimensions, we are studying ultrathin films composed of superconducting (S) and normal metal (N) elements. Interactions are essential to their superconducting state and such "SN" systems have been predicted to undergo a quantum superconductor to metal transition (SMT) as N is increased. I will describe how our transport and tunneling experiments on SN (Pb/Ag) bilayer films exhibit deviations from standard superconductor proximity effect theories that are consistent with an impending SMT. For example, the quasiparticle density of states of superconducting bilayers acquires a hybrid superconductor-metal appearance. This characteristic suggests that coexisting but separate superconducting and metal quasiparticle populations develop in the approach to the metallic phase.
Condensed Matter
Thursday, October 13, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Spin Polarized Superfluid He-3"

Harry Kojima , Rutgers
[Host: Jongsoo Yoon]
ABSTRACT:
In high magnetic field and at low temperatures, liquid He-3 condenses into a unique superfluid phase (A1) in which the condensate is spin-polarized. Owing to the broken relative symmetry in gauge and spin spaces, A1 phase hydrodynamics has unusual properties. To demonstrate the unusual hydrodynamics, two experiments on (1) magnetic fountain effect and (2) spin-entropy wave (SEW) propagation will be discussed. (1)Owing to the ability to respond to gradients in pressure, temperature as well as magnetic field, A1 phase should develop a steady pressure gradient across two chambers when a steady magnetic field gradient is applied across a superleak connecting the two chambers. Such magnetic fountain effect has been observed but with shorter relaxation time than expected. The observed relaxation is likely related to the important but not yet well-understood spin relaxation phenomenon occurring at the wall boundaries. (2)The superfluid-normal fluid counter-flow propagating mode in A1 phase is accompanied by oscillations in both entropy and spin densities. The large “spin stiffness” makes the velocity of this SEW is much greater that it would be for “entropy stiffness” alone. The measured SEW velocity gives the superfluid density directly. Anisotropy in the superfluid density may be studied with SEW. The liquid-crystal-like property of anisotropy “texture” may be probed and its kinetic anisotropy “texture phase transition” has been observed.
Condensed Matter
Thursday, September 29, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Spin Separation in Cyclotron Motion"

Leonid Rokhinson , Purdue
[Host: Yongsoo Yoon]
ABSTRACT:
We demonstrate spatial separation of carriers with opposite spin orientations in a non-magnetic semiconductor. An ability to manipulate spin of charge carries in a controllable fashion is central to the rapidly developing field of spintronics, as well as for the development of spin-based devices for quantum information processing. However, creation of spin-polarized currents is proven to be a formidable challenge and, previously, required either injection from magnetic materials or application of strong Zeeman magnetic field. We show that in a non-magnetic semiconductor with spin-orbit interactions spins can be spatially separated in a “spin spectrometer”, utilizing difference in momenta and, thus, cyclotron radii, for two spin polarizations. For holes in GaAs almost 100% bipolar spin filtering has been achieved in magnetic focusing geometry with spatial separation of polarized beams by 0.2 microns. We confirmed spin polarization of the injected currents by applying strong Zeeman field and using point contacts as spin filters. Spin-orbit interaction constant has been measured directly in these experiments. The new technique of spin injection/detection opens a possibility to investigate density and electric field dependence of spin-orbit interactions, spin dynamics at a few tenths of picoseconds without RF fields, and shed light on such outstanding problems as “0.7 anomaly” in quantum point contacts by measuring spin polarization of charge carriers.
Condensed Matter
Thursday, September 22, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Current-Induced Spin Polarization of 2DEG in Perpendicular Magnetic Field"

Maxim Vavilov , Yale University
[Host: Jongsoo Yoon]
ABSTRACT:
An electric field applied to a two-dimensional electron gas with spin-orbit coupling generates spin magnetization. Based on a quantum kinetic equation, we study the dependence of the magnitude and direction of the spin magnetization on the strength of a perpendicular magnetic field. We show that as magnetic field increases, the magnitude of the magnetization decreases. However, in sufficiently strong magnetic fields, the magnetization acquires a giant oscillating component; the amplitude of this component may exceed the magnetization at zero magnetic field. The large amplitude of magnetization oscillations is related to the enhanced electron-hole asymmetry in quantizing magnetic fields.
Condensed Matter
Thursday, August 25, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Novel, Collective Insulating Phase in 2D Superconductors in High Magnetic Fields"

Sambandamurhy Ganapathy , Magnet Lab, Tallahassee and Princeton University
[Host: Jongsoo Yoon]
ABSTRACT:
Superconductor-insulator transition (SIT) in 2D films has been an active area in condensed matter physics research. The reason is the continuing theoretical focus to understand zero temperature quantum phase transitions (QPT) and the relative experimental ease with which QPT can be tuned successfully in 2D films. We present results from our study of the magnetic field (B)-tuned SIT in amorphous indium oxide films. We present three experimental evidences that suggest that a novel, collective insulating state emerges in the B-induced insulating phase. We argue that our results lend support to the central assumption of the QPT approach.
Condensed Matter
Thursday, August 18, 2005
3:30 PM
Physics Building, Room 204

"Novel Optical Studies of Ion-erosion, Growth, and Diffusion on Metal Surfaces**"

Petros Thomas , University of California at Davis
[Host: George Hess]
ABSTRACT:
The novel optical techniques of oblique-incidence reflectivity difference (OI-RD) and linear optical diffraction (LOD) are applied to several distinct surface physics problems. Studies of Ar and Ne-ion sputtering and thermal annealing of Nb(110) and Cu(111) using OI-RD will be discussed. The evolution of the average slope of the surface morphology of a moderately stepped Ni(111) surface under ion erosion will be presented. OI-RD studies of the adsorption and desorption of atomic hydrogen and deuterium on Nb(110) and Cu(111) will also be reported." **If time allows, the following topic will also be discussed: "The adsorption, growth and diffusion studies of Xe on Nb(110)".
Condensed Matter
Thursday, July 7, 2005
11:00 AM
Physics Building, Room 204
Note special time.

"Unusual Magnetic State in MnSi under Hydrostatic Pressure"

Dmitry Reznik , Institute of Solid State Physics, Forschungszentrum Karlsruhe
[Host: Seunghun Lee]
Informal CM Seminar
Thursday, May 26, 2005
11:00 AM
Physics Building, Room 313
Note special time.
Note special room.

"Topological Defects and Fractionally Quantized Vortices in Nanomagnets"

Oleg Tchernyshyov , John Hopkins
[Host: Paul Fendley]
Condensed Matter
Thursday, May 5, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Control of Electric Polarization by Using Magnetic Field in Magnetic Oxides with Long-Wavelength Magnetic Structures"

Tsuyoshi Kimura , Los Alamos National Laboratory
[Host: Seung-Hun Lee]
ABSTRACT:
Recent observations of gigantic magnetoelectric and magnetocapacitive effects in rare-earth manganites, TbMnO3 and DyMnO3 [1,2], provide a novel approach to the mutual control of magnetization and electric polarization in magnetic ferroelectrics. We can control the magnitude and/or direction of the electric polarization vector by the application of magnetic field in these manganites. In comparing the results from the both manganites, we noticed that a characteristic common to the both materials is that they possess modulated magnetic structures with long wavelengths (as compared to the chemical unit cell) which arise from competing magnetic interactions. Ferroelectricity in these materials appears to originate from the competing magnetic interactions which cause lattice modulations through magnetoelastic coupling. We extend our study to other modulated magnets with competing magnetic interactions. In this talk, I show magnetic control of electric polarization in several modulated magnets [3], which may provide new route to design magnetoelectrics. [1] T. Kimura, T.Goto, H. Shintani, K. Ishizaka, T. Arima, and Y. Tokura, Nature 426, 55 (2003). [2] T. Goto, T. Kimura, G. Lawes, A. P. Ramirez, and Y. Tokura, Phys. Rev. Lett. 92, 257201 (2004). [3] T. Kimura, G. Lawes, and A. P. Ramirez, Phys. Rev. Lett. (in press).
Condensed Matter
Thursday, April 28, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Spin Order in Geometrically Frustrated System CdxZn1-xV2O4"

Zhe Zhang , UVA
Condensed Matter
Thursday, April 14, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Optimization of Complex MNiSn for High-Temperature Thermoelectric Power Generation"

Joint CM/Atomic Seminar
Thursday, April 7, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Theory of Rydberg Gases"

Vittorio Celli , University of Virginia
Condensed Matter
Thursday, March 31, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Antiferromagnetic Order as a Competing Ground State in Electron-doped High -Tc Superconductors"

Pengcheng Dai , University of Tennessee/Oak Ridge National Laboratory
[Host: Seung-Hun Lee]
ABSTRACT:
In this talk, I will summarize recent progress on using neutron scattering to study electron-doped high-Tc superconductors. As a function of increasing doping, the as-grown antiferromagnetic (AF) insulator Pr0.88LaCe0.12CuO4 (PLCCO) can be transformed into a Tc=25 K superconductor without AF order by annealing at different temperatures. We find that underdoped PLCCO's are electronically phase separated with a three dimensional AF phase, a quasi-two-dimensional (2D) spin density wave, and a superconducting phase. An applied external field that suppresses superconductivity also enhances the quasi-2D spin density wave order, thus suggesting AF order is a competing phase for superconductivity in these materials.
Condensed Matter
Thursday, March 17, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"How to Relax Frustrated Systems: Spin and Orbital Physics in V and Cr Spinels"

Yuki Motome , RIKEN
[Host: Seung-Hun Lee ]
ABSTRACT:
Geometrical frustration gives rise to a large number of degenerate ground states and suppresses a simple-minded long-range ordering in general. Here we discuss theoretically how the degeneracy is lifted in three-dimensionally frustrated pyrochlore systems whose lattice structure consists of corner-sharing tetrahedra. The pyrochlore systems are most typically found in the so-called B spinel oxides. Focusing on the spinels with B=V and Cr, we explore the low-temperature physics by the mean-field argument and Monte Carlo simulation for effective spin-orbital-lattice coupled models. For V spinels, we clarify the strong interplay between spin and orbital degrees of freedom. Orbital ordering reduces the magnetic frustration partially and realizes weakly coupled one-dimensional chains in the three-dimensional pyrochlore lattice. For Cr spinels, we show that the system exhibits novel phase transitions under the external magnetic field due to the spin-Peierls type mechanism. An emergence of a half-magnetization plateau is discussed in detail.
Condensed Matter
Thursday, March 3, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Dipole-Dipole Excitation and Ionization in an Initially Frozen Rydberg Gas"

Wenhui Li , UVA
[Host: Tom Gallagher]
ABSTRACT:
In cold dense Rydberg atom samples, the dipole-dipole interaction strength is effectively resonant at the typical interatomic spacing in the sample, and the interaction has a 1/R3 dependence on interatomic spacing R. The dipole-dipole attraction leads to ionizing collisions of initially stationary atoms, which produces hot atoms and ions and initiates the evolution of initially cold samples of neutral Rydberg atoms into plasmas. More generally, the strong dipole-dipole forces lead to motion, which must be considered in proposed applications.
Condensed Matter
Thursday, February 24, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"TBA"

RESERVED FOR PHD CANDIDATES
Condensed Matter
Thursday, February 17, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Non-abelian Statistics"

Paul Fendley , UVA
[Host: Jongsoo Yoon]
ABSTRACT:
In quantum physics in 2+1 dimensions, particles need not behave as a boson or fermion. With non-abelian statistics, the wave function depends not only on the particles being exchanged, but also on the order in which they are exchanged. Such behavior probably occurs in the fractional quantum Hall effect. I discuss ways of building models with this behavior, and note the connection to quantum computers.
Condensed Matter
Thursday, February 10, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Vortex Dynamics and Fluctuations Near the Magnetic Field Tuned Superconductor-Insulator Transition"

Victor Galitski , Santa Barbara
[Host: Paul Fendley]
ABSTRACT:
Superconductivity in two dimensions provides a unique area in which a fascinating variety of novel and fundamental phenomena occur. In this talk, I will review recent theoretical and experimental results on disordered films, which undergo a magnetic field tuned superconducting-insulator transition at low temperatures. I will focus on the unusual phases and fluctuation phenomena evident in the experimental studies of the field-tuned transition. First, I will explain how rare disorder fluctuations can enhance global superconductivity and increase the critical magnetic field at which samples become superconducting. Next, I will briefly summarize the recently developed theory of quantum superconducting fluctuations, which explains transport properties above the transition. At the end of my talk, I will focus on the low-temperature metallic phase observed in certain materials. This metallic state is truly mysterious and can not be explained by any conventional theory (involving bosonic vortices as basic excitations). I will argue that under certain circumstances the statistics of the vortices can change from bosonic to fermionic. Such a statistical transmutation may explain the nature of the metallic state. I will discuss possible experimental signatures of the resulting vortex Fermi liquid.
Condensed Matter
Thursday, February 3, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"TBA"

RESERVED FOR PHD CANDIDATES
Condensed Matter
Thursday, January 27, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Superfluid-Insulator Transition in a Moving System of Interacting Bosons"

Anatoli Polkovnikov , Harvard University
[Host: Paul Fendley]
ABSTRACT:
Cold atomic systems with their high tunability and nearly perfect isolation from environment became very attractive for studying strongly correlated systems. Particularly exciting is the possibility to address problems of quantum dynamics far from equilibrium, which are beyond the reach of conventional condensed matter systems. Most of theoretical studies of dynamics in these systems employ essentially classical (Gross-Pitaevskii) equations of motion. At the same time in quilibrium a lot is known about various quantum regimes, where the classical description does not work. In this talk I will discuss a problem where both quantum and dynamic effects are important. In particular, I will describe a moving system of interacting bosons in a periodic optical lattice potential and generalize the conventional superfluid-Mott insulator transition to this highly non-equilibrium situation. I will discuss implications of our results to recent and future experiments.
Condensed Matter
Thursday, January 20, 2005
4:00 PM
Physics Building, Room 204
Note special time.

"Non-equilibrium Transport Through a Phonon Coupled Molecule"

[Host: Paul Fendley]
ABSTRACT:
The theory of non-equilibrium many-body systems is only partially understood, and developing the theory further is of relevance now as nanostructures (such as single molecule and NEMS devices) are routinely operated in the non-equilibrium regime. In this talk I will present a comprehensive theory for non-linear transport through a single molecule device where the tunneling electrons are coupled to a vibrational mode of the molecule. I will present results for the current and the noise through the device and show their sensitivity to phonon equilibration rates and hence to the steady state phonon distribution function. I will also outline a method to tackle the regime of low temperatures and strong electron-phonon coupling where non-equilibrium conditions do not allow for a variational treatment. Instead the key issue is the formulation of rate equations for the reduced density matrix that can be solved in the semiclassical limit. The steady-state solutions lead to a multi-peaked density matrix, implying an absence of bistability in the current but structure in the noise. I will also demonstrate how departures from equilibrium produce decoherence that prevents the formation of characteristically quantum features such as the polaron peak in the spectral function, and present generalizations of this method to other strongly correlated systems such as the non-equilibrium Kondo model.
Condensed Matter
Thursday, December 9, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"Continuous Tuning of 2D-Superconductivity in Strontium Titanate Field Effect Transistors"

Feng Pan , University of Colorado
[Host: Jongsoo Yoon]
ABSTRACT:
Understanding the doping mechanisms in metal-oxide superconductors is an important part of probing the superconductor theory. Modulation of the carrier concentration using electric fields to tune superconductivity is a good method for this approach for its ability to minimize changes in both chemical impurities and structure. This talk explores the electric field modulation of normal state properties and superconductivity in thin-film field-effect transistors based on lanthanum-doped strontium titanate channels and undoped strontium titanate gate insulation. Electric field tuning of normal-state sheet resistance and channel critical current are observed in enhancement (positive gate potential) and depletion (negative gate potential) modes, consistent with an electric field induced variation of channel depletion width. Detailed analysis of the non-linear channel current-voltage characteristics is consistent with electric field modulation of a 2-dimensional Kosterlitz-Thouless transition and shows the scaling of T_KT (Kosterlitz-Thouless transition temperature) with conduction channel thickness and normal conductance at different gate fields. Also, small signal modulation experiments were performed on these devices. Modulation of critical current, sheet resistance, and T_KT is found for frequencies out to at least 1 kHz, which indicates possible exciting superconductor electronics.
Condensed Matter
Thursday, November 18, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"Rydberg Atoms are Forever"

L. D. Noordam , AMOLF - Amsterdam
[Host: Tom Gallagher]
Condensed Matter
Tuesday, November 16, 2004
3:30 PM
Physics Building, Room 204
Note special date.

"Spin Transport in One-Dimensional Systems"

Cristina Bena , UCLA
[Host: Eugene Kolomeisky]
Condensed Matter
Thursday, November 11, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"Magnetic, Transport, and Thermodynamic Studies of Weakly Magnetic Systems of Transition Metal Oxides"

Zhixian Zhou , FSU
[Host: Jongsoo Yoon]
ABSTRACT:
LnBaCo2O5.5 (Ln=Gd, Eu) and Sr3Ru2O7 are examples of weakly magnetic systems of 3d and 4d transition metal oxides, respectively. The former is nonmetallic and exhibits magnetic properties of two sublattice magnetic systems with an in-plane ferromagnetic interaction and a relatively weak temperature dependent inter-plane magnetic coupling. The latter is a paramagnetic metal with strongly correlated electrons near a magnetic instability. The magnetization, resistivity and magnetoresistance (MR) of single crystals of GdBaCo2O5.5 and EuBaCo2O5.5 are measured over a wide range of dc magnetic fields (up to 30 T) and temperature. The data suggest an equal ratio of low spin (S=0) and intermediate spin (S=1) Co3+ ions below TMI, with no indication of additional spin state transitions. The low field magnetization shows a transition to a highly anisotropic ferromagnetic phase, followed by another magnetic transition to an antiferromagnetic phase at a slightly lower temperature. Significant anisotropy between the a-b plane and c axis was observed in magnetic and magnetotransport properties for both compounds. The specific heat and electrical resistivity of Sr3Ru2O7 single crystals are measured in several magnetic fields applied along the c-axis for temperatures below 2 K and at fields up to 17 T. Near the critical metamagnetic field at B1 *~7.8 T, the electronic specific heat divided by temperature increases logarithmically as the temperature decreases, over a large range of T, before saturating below a certain T* (which is sample dependent). This crossover from a non-Fermi Liquid to a Fermi Liquid state is also observed in the resistivity data near the critical metamagnetic field for I || c and B || c. At the lowest temperatures, a Schottky-like upturn with decreasing temperature is observed.
Joint Materials Science and Condensed Matter Physics
Thursday, September 30, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"The Development and Analysis of Texture in Thin Metallic Films from Electrodeposition and Sputtering"

Erik Svedberg , Seagate
[Host: Bellave Shivaram]
Condensed Matter
Thursday, July 29, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"Center for Spins in Nanotechnology -CeSpIN -a proposal for a new interdisciplinary center at UVa"

Stuart Wolf , DARPA
[Host: William A. Jesser]
ABSTRACT:
In this talk I will discuss the details of a proposed new center at UVa that will explore how electron spin can be utilized in electronics that will transcend Moore's law. There are two parallel paths that this center will explore. The first is the development of spin devices that will be nanoscale extensions of the spin transport electronics that will be introduced in the very near term as non-volatile random access memory and magnetic sensors. The second path is the exploration of spin coherent effects that will directly impact the development of a quantum information processor. There are challenges in both paths but the key to a new paradigm of electronics will be the development of a single technology that can provide both conventional logic and memory as well as quantum logic and memory and spins seem to fit the bill.
Condensed Matter
Thursday, July 22, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"Future Nanoelectronics: Materials Science and Physics"

John R. Tucker , University of Illinois
[Host: William A. Jesser]
ABSTRACT:
A new nanoelectronics must be developed over the next two decades to sustain present rates of progress. The general outlines for what that technology should do are beginning to come into focus, but as yet there is no consensus on how to proceed toward these goals. In this talk, I will summarize research on nanometer MOS transistors and atom-scale silicon devices by my colleagues and myself over the last ten years. Our long-term strategy envisions increasingly higher levels of synergy between materials science and quantum physics in developing this new technology.
Condensed Matter
Wednesday, May 19, 2004
4:00 PM
Physics Building, Room 204
Note special date.
Note special time.

"TBA"

Charles Molhoek , UVA
[Host: M. Reed]
Condensed Matter
Thursday, April 22, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"Building a Magnetic Force Microscope"

Junzong Yu , UVA
[Host: Jongsoo Yoon]
Condensed Matter
Thursday, April 15, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"TBA"

Yoonseok Lee , UFL
[Host: Jongsoon Yoon]
Condensed Matter
Thursday, April 8, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"TBA"

Jian Li , UVa
[Host: Jongsoo Yoon]
Condensed Matter
Thursday, March 4, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"Spintronics: Fundamentals and Applications"

Igor Zutic , University of Maryland
[Host: Olivier Pfister]
ABSTRACT:
Spintronics is an interdisciplinary field in which the central idea is the manipulation of spin degrees of freedom in solid state systems. The motivation to examine spintronics ranges from fundamental studies, where the changes of the spin degrees of freedom can be a sensitive probe for basic physical phenomena, to applications that are neither feasible nor effective with conventional electronics. This talk will focus on two examples: (1) spin-polarized transport in hybrid structures containing superconductors and (2) a proposal for magnetic p-n junctions. Our prediction that a superconducting response can be used to probe a novel class of ferromagnetic semiconductors has recently led to the first direct measurement of the spin polarization in these materials. In the second example, we develop a theory of inhomogeneously doped semiconductors. We predict the spin-voltaic effect, a spin-analogue of the photo-voltaic effect. We show that the direction of the charge current (which can even flow at no applied bias) can be switched by the reversal of an equilibrium magnetization or of a polarization of the injected spin. The same spin-voltaic effect can be used to develop a novel class of tunable magnetic transistors.
Condensed Matter
Thursday, February 26, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"Nanoscopic Transport in the Ultrathin Metal Films and Molecular Organic Crystals"

[Host: Jongsoo Yoon]
ABSTRACT:
A description of the properties of low dimensional electron gases remains one of the most important and challenging problems in condensed matter physics. I will present electron transport and tunneling measurements on ultrathin, metal films. I will show that in the strong disorder limit a gap emerges in the density of states that is solely attributable to fundamental many body electron-electron interaction effects, i.e. the Coulomb gap. Interestingly, a quantum metal state can be realized, in otherwise highly insulating films, by suppressing the Coulomb gap via a magnetic field. In the second part of my talk I will describe ongoing research into the properties of the two dimensional electron gas of Field Effect Transistors (FETs) fabricated on high quality organic molecular single crystals. Practical applications of the molecular organic materials, such as flexible, large-area electronic devices will also be discussed.
Special Condensed Matter Seminar.
Monday, February 23, 2004
4:00 PM
Physics Building, Room 204
Note special date.
Note special time.

"Electron transport in granular arrays"

Julia Meyer , Univ. Minnesota
[Host: Paul Fendley]
ABSTRACT:
Electron-electron interactions in low-dimensional systems have attracted a great deal of attention in recent years. We show that arrays of large, strongly coupled quantum dots present an analytically tractable, yet non-trivial model of such systems. A single dot strongly coupled to leads exhibits almost no Coulomb blockade (save for corrections that are exponentially small in the dot-lead conductance). In the array geometry with large inter-grain conductance g>>1, however, the interactions drive the system into an insulating state with a charge gap proportional to exp(-g). The latter reflects the energy cost to create a large-size, unit-charge soliton -- the only charged excitation the system supports. In 2d, such solitons bring about a Berezinskii-Kosterlitz-Thouless crossover at a certain (g-dependent) critical temperature. Upon changing the charge imbalance (e.g. by a gate voltage), the array undergoes a phase transition into the pinned Wigner crystal state. The model, thus, allows one to follow the system from a good metal (at high temperature) all the way to the Wigner crystal insulator (at low temperature) within a single framework.
Condensed Matter
Thursday, February 19, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"Fluctuations in electronic, spintronic and photonic systems"

Eugene Mishchenko , Harvard University
[Host: Paul Fendley]
ABSTRACT:
I will present both the experimental results and the theory for the non-equilibrium fluctuations of electric current in mesoscopic systems (diffusive wires and chaotic quantum dots). Non-interacting electrons exhibit Poissonian statistics while correlations in mesoscopic systems lead to the universality of current fluctuations. I will consider the effects of spin-polarized transport on the noise and magnetoresistance of spintronic systems. Finally, the electron-optical analogies will be discussed in relation to the fluctuations of radiation from chaotic absorbing ('grey body') and amplifying ('random laser') optical media.
Condensed Matter
Thursday, February 12, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"Multi-walled Carbon Nanotube-based electromechanical oscillators"

[Host: Yongsoon Yoon]
ABSTRACT:
I will describe our fabrication and characterization of torsional oscillators which use multi-walled carbon nanotubes (MWNTs) as the torsion springs. We have, through direct force measurements with a scanning probe microscope, measured the torsional properties of individual MWNTs. We discovered a surprising increase in the torsional stiffness of the MWNTs with repeated deflections. We have also actuated the devices electrostatically, demonstrating a self-contained electromechanical device. We use optical interferometry to measure the on-resonance behavior of the electrostatically-driven devices.
Special Condensed Matter Seminar. Please note special time and day
Wednesday, February 11, 2004
4:00 PM
Physics Building, Room 204
Note special date.
Note special time.

"Quantum Computation with Josephson Devices"

Dr. Yang Yu , The Research Laboratory of Electronics at MIT
[Host: Joseph Poon]
ABSTRACT:
Quantum computers offer exponential speedup over classical computers in solving certain tasks. Superconducting Josephson devices are promising candidates in realizing qubits for quantum computer due to the ease of scaling up. The main challenge of superconducting qubits is decoherence, arising from the coupling between the superconducting qubits and the environment. Here we directly measure the intrawell energy relaxation time au_d between macroscopic quantum levels in the double well potential of a Nb superconducting qubit. The qubit's decoherence time, estimated from au_d, is longer than 20 micro s, indicating a strong potential for quantum computing employing Nb-based superconducting qubits.
Special Condensed Matter Seminar. Please note special day and time.
Tuesday, February 10, 2004
4:00 PM
Physics Building, Room 204
Note special date.
Note special time.

"Non-Abelian Anyons and Topological Order in Solids"

Kirill Shtengel , Microsoft
[Host: Paul Fendley]
ABSTRACT:
A concept of topological order originally introduced by Wen in the context of Fractional Quantum Hall Effect has drawn much attention from people working in the fields of HTSC, frustrated magnetism and quantum computation. Topological order is manifested by a non-trivial degeneracy of the ground state on 2D surfaces of non-zero genus (such as a torus) and non-trivial mutual statistics of excitations. E.g., a Z_2 topological order in a magnetic system should lead to spin-charge separation -- one of the interesting (yet unlikely) possible mechanisms for HTSC. From the point of view of quantum computation, one of the biggest challenges is making it fault-tolerant. We hope to use topological properties to encode quantum information in a way that is highly resistant to decoherence. So far one (and only) type of systems where topological order is known to exist are systems with Fractional Quantum Hall Effect. After reviewing the current state of search for topological phases in condensed matter, I will discuss models with non-Abelian topological order. In particular, I will present a version of extended Hubbard model whose low-energy physics can lead to a phase with such non-Abelian order. General arguments for stability of these exotic phases as well as some new ideas about physical implementation of such systems will be presented. If found experimentally, these systems will provide a basis for building a truly fault-tolerant quantum computer.
Condensed Matter
Thursday, February 5, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"Nernst effect: from simple to correlated metals"

[Host: Paul Fendley]
ABSTRACT:
Since the discovery of large Nernst effect in the pseudogap of hole-doped cuprates this transport coefficient is becoming a new tool with which to probe correlated electronic behavior. I will first survey the recent experimental data on materials as varied as superconductors, charge and spin density waves, heavy Fermi liquids and ferromagnets. In some cases I will suggest possible common origins of such behavior and support them with calculations.
Condensed Matter
Wednesday, February 4, 2004
3:30 PM
Physics Building, Room 204
Note special date.

"Anisotropic Quantum Correlations in Nanostructures"

Igor Altfeder , Harvard University
[Host: Joe Poon]
ABSTRACT:
Recent advances in scanning tunneling microscopy (STM) have shown that a variety of exotic related phenomena, involving spins and anisotropic interactions, can be realized in atomically flat (MBE grown) thin metal films due to a nontrivial interplay of structural and electronic self-organization. The two of these phenomena will be discussed in my talk: anisotropic Kondo-like "confinement" in metallic quantum wells, and unidirectional charge ordering (CDW stripes) on metal surfaces. In all studied cases, we observe the formation of wire-like objects, where the long-range electronic coherence is spontaneously confined to a single degree of freedom.
Condensed Matter
Thursday, January 29, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"Playing with Superconductivity and Magnetism in Nanoscale"

Jiyeong Gu , Argonne National Laboratory
[Host: Jongsoo Yoon]
ABSTRACT:
Recent technological advances made it possible to create hybrid nano-structures with high quality ferromagnet/superconductor (F/S) interfaces. The F/S systems have been in focus of intensive experimental and theoretical studies because of both exciting fundamental problems and potential device applications. The physical properties of both F and S films will be strongly modified near the interface due to mutual proximity effect, for example, superconducting correlations penetrate into F and the spin polarization can extend into S. Main questions to be addressed in this talk will be the type and length scale of the induced superconducting correlations and spin relaxation length at the interface. Also, first experimental observation of superconducting switching effect depending on the mutual magnetization directions of F-layers in F/S/F trilayer system will be discussed.
Condensed Matter
Monday, January 26, 2004
3:30 PM
Physics Building, Room 204
Note special date.

"Radiation-Induced Magnetoresistance Oscillations in a 2D Electron Gas"

[Host: Paul Fendley]
ABSTRACT:
Recent measurements of a 2D electron gas subjected to microwave radiation reveal a magnetoresistance with an oscillatory dependence on the ratio of radiation frequency to cyclotron frequency. Oscillations grow with radiation intensity, with the minima saturating at zero resistance. We have performed a diagrammatic calculation which yields radiation-induced resistivity oscillations with the correct period and phase. Results are understood via a simple picture of photoexcited disorder-scattered electrons contributing to the dc conductivity. Sufficient intensity drives the calculated minima to negative resistivity, a situation shown by Andreev, Aleiner, and Millis to be unstable to the development of an inhomogeneous current distribution with zero resistivity. Hence, our result, taken together with theirs, provides an explanation for the experiments.
Condensed Matter
Monday, January 19, 2004
4:00 PM
Physics Building, Room 204
Note special date.
Note special time.

"Magnetic Structures and Magnetovolume Anomalies in R2Fe17 intermetallic compounds"

Oleksandr Prokhnenko , Institute of Physics, Academy of Sciences of the Czech Republic
[Host: Despina Louca]
ABSTRACT:
Effects of high pressure and Mn substitution on the creation and evolution of different magnetic structures in R2Fe17 compounds with non-magnetic rare earths R = Y, Ce, Lu were studied by both microscopic (neutron diffraction) and macroscopic (magnetization) techniques. Main results show instability of the ferromagnetic ground state in all compounds whereas range of stability of incommensurate antiferromagnetic phase expands down to the lowest temperatures. This effect is described in terms of distance dependent interlayer exchange interaction giving the value of the lattice c-parameter as a critical parameter for the appearance of the ferromagnetic – antiferromagnetic transition in studied compounds.
Condensed Matter
Thursday, January 15, 2004
4:00 PM
Physics Building, Room 204
Note special time.

"Opportunities for new Physics in O-D/1-D Hybrids"

Dr. Keith Williams , Delft Universities
[Host: Joseph Poon]
ABSTRACT:
Over the last decade, nanocrystals, fullerenes and nanotubes have provided many new opportunities for studies of low-dimensional physical phenomena once accessible only in lithographic heterostructures. These phenomena include spectacular optical properties, ballistic transport, Coulomb blockade, Kondo resonance, and numerous other effects related to electron, exciton, and phonon confinement. This talk will begin by reviewing observations of these phenomena in the nanomaterials; I will then discuss proposals for hybrid 0-D/1-D systems which illustrate what novel physics lies ahead in this promising field.
Joint Condensed Matter and Chemical Physics Seminar
Thursday, December 18, 2003
4:00 PM
Physics Building, Room 204
Note special time.

"Scanning Tunneling Spectroscopy of Nanostructures: Mirages in Quantum Corrals"

[Host: Joseph Poon and Ian Harrison]
ABSTRACT:
Scanning tunneling microscopy/spectroscopy on so-called Kondo systems consisting of magnetic atoms adsorbed on non-magnetic surfaces has shown that suitable two-dimensional nanostructures can influence the surface electron transport that is a consequential part of the observable STM process. Almost everyone has seen Eigler's stunning STM pictures in which individual atoms were assembled to form a chosen two-dimensional configuration, call it a nanostructure, on the metal substrate.1 Some of these shapes, when closed, are referred to as quantum corrals.2 A particularly intriguing example is an elliptical corral (major axis <15 nm) composed of up to 70 individually-placed atoms or molecules on a surface-state-supporting Cu(111) surface.3 It has been observed with the STM that both the pictorial image and the Fano-related spectroscopic signature of a single Kondo atom4 such as Co placed at one of the foci showed a mirage when STM measurements were made at the opposing unoccupied focus. The generic physics of the resonance electron transfer and transport occuring in a wide variety of surface dynamics processes including those responsible for the quantum mirages will be outlined. The consequent Fano-like spectra depend upon both the position of the STM tip and also on the size and shape, hence 2-D quantum states of the nanostructure confinement. For the 10's of nm corrals of experimental interest, the level spacings are comparable with the Kondo resonance width. This results in non-trivial spectra showing size-dependent oscillatory structure in both the energy-dependent amplitude and in the lineshape asymmetry. Calculated mirage spectra illustrate the useful inter-dependence upon the contrasting nm-scale confinement size and shape and the atomic-scale resonance-defining properties which depend upon the species.5 This is a nice example of a timely problem in high-visibility contemporary science and technology which has usefully and synergistically been addressed by complimentary experimental observation, analytic theory, and computationally-more-intensive modeling.
Condensed Matter
Thursday, October 30, 2003
4:00 PM
Physics Building, Room 204
Note special time.

"Statistical mechanics methods for genome-wide modeling of translation"

Dr. Leah Shaw , Cornell University
[Host: E. Kolomeisky]
ABSTRACT:
In living cells, DNA serves as the template from which mRNA is synthesized. mRNA is then "read," or translated, by ribosomes to produce proteins. Previous studies have shown a nonlinear relationship between mRNA and protein levels, due to the complexity of the translation process. A model is under development to help explain the quantitative relationship between mRNA and protein levels for all genes in Escherichia coli. Statistical physics methods enable a detailed understanding of a single mRNA with a uniform sequence. Realistic, nonuniform sequences are a far more complex case, but mean field equations provide a good approximation for protein production rates. Details of the model will be discussed, and preliminary results comparing the model to experimental data will be presented.
Condensed Matter
Thursday, October 23, 2003
4:00 PM
Physics Building, Room 204
Note special time.

"Novel Radiation-induced zero-resistance states in high mobility two-dimensional electron systems"

Dr. Ramesh Mani , Harvard University
[Host: Paul Fendley]
ABSTRACT:
We report the experimental detection of novel zero-resistance states [1], which are induced by electromagnetic wave excitation in ultra high mobility GaAs/AlGaAs heterostructure devices including a two-dimensional electron system. Radiation-induced vanishing-resistance states, which do not exhibit concomitant Hall resistance quantization, are demonstrated in the large filling factor, low magnetic field limit, at liquid helium temperatures. It is shown that the observed resistance minima follow the series B = [4/(4j+1)] Bf with j=1,2,…, where Bf = 2fm*/e, m* is an effective mass, e is electron charge, and f is the radiation frequency. These resistance-minima exhibit an activated resistance as a function of the temperature that leads into zero-resistance states at the lowest temperatures. The dependence of the effect is reported as a function of experimental parameters such as the electromagnetic wave frequency, incident power, temperature, and the current. [1] R. G. Mani, J. H. Smet, K. von Klitzing, V. Narayanamurti, W. B. Johnson, and V. Umansky, Nature 420, 646 (2002).
Condensed Matter
Thursday, October 16, 2003
4:00 PM
Physics Building, Room 204
Note special time.

"Atomic Scale Structure of Giant Magnetoresistance and Spin Tunnel Junction Multilayers"

Dr. Xiaowang Zhou , UVA- Engineering
[Host: J. Ruvalds]
Condensed Matter
Thursday, October 2, 2003
4:00 PM
Physics Building, Room 204
Note special time.

"Weak Coupling Instabilities of Diffusive Fermi Liquids"

Dr. Xiao Yang , UVA
[Host: P. Fendley]
Condensed Matter
Thursday, September 25, 2003
4:00 PM
Physics Building, Room 204
Note special time.

"Playing with dimensionality - magnetism and transport in hydrides"

Dr. Bjorvin Hjovarson , Uppsala, Sweden
[Host: B. Shivaram]
ABSTRACT:
The use of hydrogen to modify the electronic structure in magnetic thin films and heterostructures has opened new routes to tailor magnetic interactions in materials. Hydrogen can, for example, be used to control the strength and character of magnetic interactions. For example, the switching from antiferromagnetic to ferromagnetic order, and vice versa, has been demonstrated for exchange coupled magnetic superlattices. As the sign of the interlayer exchange coupling (J) can be switched by the insertion of hydrogen, J´ arbitrarily close to zero must be accessible. When the exchange interaction between adjacent magnetic layers is completely suppressed, the heterostructure can then be taken to consist of a collection of quasi two-dimensional magnetic sheets, when the ferromagnetic layers are very thin. Consequently, the introduction of hydrogen can be viewed as a route to tune the dimensionality of these structures.
Condensed Matter
Thursday, September 18, 2003
4:00 PM
Physics Building, Room 204
Note special time.

"Weak Coupling Instabilities of Diffusive Fermi Liquid"

Xiao Yang , UVA
[Host: Paul Fendley]
SPECIAL NUCLEAR/PARTICLE PHYSICS SEMINAR
Thursday, September 11, 2003
4:00 PM
Physics Building, Room 204
Note special time.

"Nuclear Modification of Jet Fragmentation"

Dr. Xin-Nian Wang , Lawrence Berkeley Lab
[Host: Blaine Norum]
Condensed Matter
Thursday, August 28, 2003
4:00 PM
Physics Building, Room 204
Note special time.

"The physics of magnetoresistive random access memory (MRAM) based on magnetic tunnel junctions"

Nick Rizzo , Motorola Labs
[Host: Bellave Shivaram]
ABSTRACT:
Resistive Random Access Memory (MRAM) is a new nonvolatile solid state memory that has the potential to be fast, low power, high density, and have unlimited read/write cycles. These combined characteristics make MRAM superior to other memory technologies. For a fully functioning MRAM, the understanding and control of a rich variety of physical phenomena is required. The bit cell is a magnetic tunnel junction, which consists of two thin magnetic layers separated by an ultrathin layer (1 nm) of oxidized Al. The tunneling process itself is an example of a macroscopic quantum effect. The magnetoresistance changes as much as 50% for parallel to antiparallel layer magnetizations. The magnetoresistance is a result of s-d exchange which polarizes conduction electrons to be parallel with the layer magnetizations. The uniformity of the tunnel barrier is critical for well-defined resistance values and to minimize tunneling hotspots. We show a resistance uniformity of less than 1% within a die, which implies an average thickness uniformity of a fraction of an angstrom. The bit shape is typically elliptical, with a magnetic free layer of thickness 4-6 nm. The bit shape defines a shape anisotropy along with a switching field. Variations in lithography and material properties cause a distribution in switching fields - a distribution that must be minimized for error free programming. The state of the bit (0 or 1) is programmed using current pulses that are nanoseconds in duration. The pulses are sent down conductive lines that are surrounded by a thin permeable magnetic film to enhance the generated field. A major challenge to error free programming is to minimize bit switching that is thermally activated. We have characterized thermal activation in our bits and showed that it obeys classic Arrhenius-Neel activation theory for single energy barriers. One way that we have developed to minimize the effects of thermal activation is to use multilayer films to increase the energy barrier to magnetization reversal. In the first part of this talk I will discuss the development of the MRAM read and write process at Motorola, covering the topics described above. I will then review the performance of Motorola's 1 Mb MRAM test vehicle. Finally, I will conclude by listing the major unsolved challenges that MRAM needs solved to become a dominant memory technology.
Special Condensed Matter Seminar
Friday, July 18, 2003
3:30 PM
Physics Building, Room 313
Note special date.
Note special room.

"X-ray Microscopy using synchrotron radiation -- a tool of nanotechnology"

Dr. Hwa Shik Youn , Beamline Division Head, Pohang Accelerator Laboratory, Pohang, Korea
[Host: George Hess]
Condensed Matter
Thursday, May 1, 2003
3:30 PM
Physics Building, Room 204

"Hydrogen in vacuum systems and SRF cavities"

Dr. G. Myneni , Thomas Jefferson National Accelerator Lab
[Host: Belave Shivaram]
Condensed Matter
Thursday, April 24, 2003
3:30 PM
Physics Building, Room 204

"Regulated Self-assembly of Silicon-Germanium Quantum Dots"

Tom Vandervelde , UVA
[Host: Jongsoo Yoon]
Condensed Matter
Thursday, April 17, 2003
3:30 PM
Physics Building, Room 204

"Measurement of Thermal Properties at Mesoscopic Scales"

Pillip Kim , Columbia University
[Host: Jongsoo Yoon]
ABSTRACT:
The thermal properties of nanoscale materials are of fundamental interest and also play a critical role in controlling the performance and stability of nanodevices made of these materials. However, the measurements of thermal properties of nanomaterials at a mesoscopic scale have been technically challenging problems. We have fabricated submicron scale devices hybrided with nanoscale materials using state-of-art microfabrication thechniques. The thermal conductivity and thermoelectric power of carbon nanotubes and other nanowires have been measured at mesoscopic levels and exhibit distinticvely different behaviors from bulk material measurement.
Condensed Matter
Thursday, April 10, 2003
3:30 PM
Physics Building, Room 204

"Anomalous magnetoresistance effect in ultrathin manganite films"

Qi Li , Penn State Univ.
[Host: J. Yoon]
ABSTRACT:
Recently, doped manganites have been studied extensively due to various unusual phenomena observed in the system, such as the colossal magnetoresistance effect which is associated with a metal-insulator transition. Although a fundamental understanding of the system is not yet available, it is well known experimentally that spin, charge, and lattice degree of freedom are strongly coupled. We have studied epitaxial ultrathin manganite films in which bi-axial lattice distortion is imposed due to the lattice strain. Two anomalous effects are found which are not present in single crystals. One is unusually large low field magnetoresistance effect occurring only in compressively strained thin films and the other is giant anisotropic magnetoresistance. The details of the results will be presented and these results are not yet explained using existing models.
Condensed Matter
Thursday, April 3, 2003
3:30 PM
Physics Building, Room 204

"Emergent excitations and novel phase transitions in geometrically frustrated magnets"

Seunghun Lee , National Institute of Standards and Technology
[Host: Despina Louca]
ABSTRACT:
Strongly correlated systems where the degrees of freedom cannot order despite their strong interactions have constituted an important issue in modern condensed matter physics. Such systems usually have many competing states as ground states that can lead to qualitatively new states of matter. An example is geometric frustration, a magnetic phenomenon in which the topology of the lattice induces a macroscopic ground state degeneracy and prohibits the spin system from ordering. The important issues in this field are: (1) what the nature of the spin liquid phase is and (2) how the system responds to the ground state degeneracy. In this talk, I will address these issues by discussing a spinel antiferromagnet ZnCr2O4. In ZnCr2O4, the magnetic Cr3+ ions form a lattice of corner-sharing tetrahedra with uniform nearest neighbor antiferromagnetic couplings and makes the system as the most frustrated magnet so far. Recently we found by inelastic neutron scattering that a composite spin degree of freedom emerges in the cubic spinel. In the gapless spin liquid phase, spins self-organize into weakly interacting antiferromagnetic hexagonal loops rather than fluctuating individually. The emergence of the composite spin degree of freedom suggests an organizing principle for frustrated systems such that if macroscopic condensation is not possible, interacting degrees of freedom combine to form rigid clusters. We have also shown that the system can lift the degeneracy via a spin-Peierls-like phase transition from the cubic spin liquid to a tetragonal Neel state. Finally, if time allows, I will also discuss how the spin liquid state and the phase transition change in the presence of bond/site disorders or further neighbor interactions.
Condensed Matter
Thursday, March 20, 2003
3:30 PM
Physics Building, Room 204

"ELECTROACTIVE POLYMERIC AND ORGANIC MATERIALS FOR THIN-FILM-TRANSISTOR APPLICATIONS"

Andrew J. Lovinger , Bell Laboratories, Lucent Technologies
[Host: Bellave S. Shivaram]
ABSTRACT:
The last few years have seen enormous growth in the area commonly referred to as "plastic electronics". The active elements of all such circuits are field-effect transistors. I will summarize our work on organic and polymeric transistors, which recently resulted in the first prototype of a flexible plastic display. The emphasis of the talk will be on the materials-science aspects, specifically on structure-morphology-processing-property correlations. We have synthesized and characterized a large number of organic and polymeric semiconductors, both p- and n-type, that are processable by vapor- or solution techniques (including printing). I will discuss the requirements that must be satisfied at the various levels of structure (chemical, molecular, unit-cell, crystalline, and bulk) for optimal charge transport. One centrally important requirement in this regard is the orientation of the semiconductor molecules on the substrate. For one organic compound we have also been able to visualize the conductive pathways themselves at a submolecular level of resolution. Lastly, a few applications of these materials will also be briefly discussed.
Condensed Matter
Thursday, March 13, 2003
3:30 PM
Physics Building, Room 204

"Thermal Measurements for Thin Films"

[Host: Jongsoo Yoon]
Condensed Matter
Thursday, February 27, 2003
3:30 PM
Physics Building, Room 204

"Ground-state properties of artificial bosonic atoms, the Bose interaction blockade, and the single-atomic pipette"

Ryan Kalas , UVA
ABSTRACT:
We analyze the ground-state properties of an artificial atom made out of repulsive bosons attracted to a center for the case when all the interactions are short-ranged. The properties of this artificial bosonic atom, which can be created by optically trapping ultracold particles of alkali vapors, can be varied by adjusting both the strength of "nuclear" attraction and the interparticale repulsion. The dependence of the ground-state energy of the atom on the number of particles has a minimum whose position is experimentally tuneable. This implies that the number of bound bosons has a staircase dependence on external parameters which may be used to create a single-atomic pipette -- a set-up allowing the transport of atoms into and out of a reservoir one at a time.
Condensed Matter
Thursday, February 20, 2003
3:30 PM
Physics Building, Room 204

"Hybrid Nanostructures and Devices"

Seunghun Hong , FSU
[Host: Jongsoo Yoon]
ABSTRACT:
Physics, Institute of Molecular Biophysics, and Center for Materials Research and Technology, Florida State University, Tallahassee, FL 32306 Recent dramatic progress of nanotechnology and biological science allows us to combine organic molecules (e.g. biomolecules, self-assembled monolayer etc.) with solid state nanostructures (e.g. metal nanoparticles, carbon nanotubes, microfabricated circuits etc.) to build a generation of new hybrid nanoscale devices. These include nanoscale biological sensors and protein motor-based nanomechanical systems. One promising nano-manufacturing method for these hybrid devices is the surface-templated assembly process. In this strategy, direct deposition methods such as dip-pen nanolithography are utilized to functionalize the desired solid substrate area with organic molecules, and nanostructures (e.g. carbon nanotubes, nanoparticles, proteins etc.) in the solution are specifically assembled onto the functionalized area via molecular recognition mechanism. In this presentation, we will discuss about 1) new properties of hybrid nanostructures and 2) important scientific issues related with the surface-templated assembly process.
Condensed Matter
Thursday, February 13, 2003
2:30 PM
Physics Building, Room 313
Note special time.
Note special room.

"The Effect of Quenched Impurities on First-Order Transitions"

John Cardy , IAS and Oxford University
[Host: Paul Fendley]
ABSTRACT:
In two dimensions, it is known that the presence of any finite concentration of quenched impurities drives a first-order phase transition into a continuous one (or destroys it completely). In three dimensions, this is believed also to occur, above some critical concentration. However, the nature of the continuous transition which may result is very poorly understood. This talk will review the physics behind these results, discuss some of the numerical efforts at understanding the problem, and finally describe one model for which a more complete picture is available.
Condensed Matter
Thursday, January 30, 2003
3:30 PM
Physics Building, Room 204

"Novel Copperless Superconductors"

Dr. Mazin , Naval Research Lab.
[Host: Joseph Poon]
ABSTRACT:
I will review some of the recent discoveries in superconductivity outside of the high Tc cuprates domain. I will speak in particular about two-band superconductivity in MgB2, competition of superconductivity and magnetism in MgCNi3, possible p-wave superconductivity vs. Fulde-Ferrel state in ZrZn2 and superconductivity in hexagonal Fe under pressure.
Condensed Matter
Thursday, January 23, 2003
3:30 PM
Physics Building, Room 204

"Valence bond liquids and solids in geometrically frustrated magnets"

Oleg Tchernyshyov , Johns Hopkins
[Host: Paul Fendley]
ABSTRACT:
Frustrated magnets are models of strongly interacting systems. Frustration reveals itself through a vast degeneracy of the classical ground state of a magnet and is responsible for a large number of soft modes that destroy magnetic order. Possible alternative phases that could emerge in place of the Neel state are the valence-bond solid, which breaks some lattice symmetry, and the valence-bond liquid, which has no broken symmetries yet differs from a paramagnet by the existence of a hidden topological order. We have recently found examples of valence-bond liquids and solids in a study of almost classical frustrated antiferromagnets on the pyrochlore lattice and its planar analogs.
Condensed Matter
Thursday, November 14, 2002
3:30 PM
Physics Building, Room 204

"Ground-state properties of one-dimensional matter and quantum dissociation of a Luttinger liquid "

Eugene Kolomeisky , University of Virginia
[Host: John Ruvalds]
Condensed Matter
Thursday, November 7, 2002
4:00 PM
Physics Building, Room 204
Note special time.

"Destructive regime and superconductor-normal metal transition in ultrathin cylinders"

Prof. Ying Lu , Penn State
[Host: Y. Yoon]
ABSTRACT:
We have carried out experiments on ultrathin superconducting cylinders, which revealed the existence of a destructive regime, the loss of superconductivity around half-integer flux quanta even in the zero temperature limit, predicted by de Gennes for doubly connected samples with a diameter smaller than zero-temperature superconducting coherence length [Y. Liu, et al., Science 294, 2332 (2001)]. More recently we have done measurements on the superconductor-normal metal (S-N) transition tuned by magnetic flux in these cylinders, and found evidence for the formation of normal bands near the S-N transition.
Condensed Matter
Thursday, October 31, 2002
4:00 PM
Physics Building, Room 204
Note special time.

"Atomically Engineered Active Sites and Environments for Supported Metal Catalysts"

Prof. Matthew Neurock , UVA - Chemical Engineering
[Host: J. Poon]
ABSTRACT:
Heterogeneous catalysis is responsible for over \$400 billion in annual income each year which results from the sale of pharmaceutical and chemical intermediates made via catalytic transformations. Knowledge of the atomic structure at and near the active catalytic site and how it influences reactivity could revolutionize our ability to design more active and selective catalysts. Significant advances in both theory and simulation have occurred over the past decade thus making theoretical chemistry an invaluable partner to experiment in this endeavor. In this talk, we use both ab initio Molecular Dynamics and ab initio-based dynamic Monte Carlo methods to simulate the dynamics and catalytic kinetics. We show that it is now possible to track the nature of the active surface site along with the local “molecular” environment about the active surface ensemble. We specifically probe the influence of surface coverage, bimetallic alloys, and the molecular networks that form at a liquid/metal interface on catalytic activity. This talk will focus on the application of these tools to the selective hydrogenation of olefins, the synthesis of oxygenates, and methanol oxidation for the direct methanol fuel cell.
Condensed Matter
Thursday, October 24, 2002
4:00 PM
Physics Building, Room 204
Note special time.

"Superconductor Divices"

Prof. A. Lichtenberg , UVA-Superconducting Materials, Device and Circuit Research for THz Receivers
[Host: Bascom Deaver]
Condensed Matter
Thursday, October 17, 2002
4:00 PM
Physics Building, Room 204
Note special time.

"Quantum Trajectories and Particle Diffraction""

[Host: V. Celli]
Condensed Matter
Thursday, October 10, 2002
4:00 PM
Physics Building, Room 204
Note special time.

"Photomission Spectroscopy of High Temperature Superconductors"

Prof. D. Dessau , Univ. of Colorado
[Host: J. Ruvalds]
Condensed Matter
Thursday, October 3, 2002
4:00 PM
Physics Building, Room 204
Note special time.

"The Materials Research Science and Engineering Center at UVA: From Materials to Quantum Cellular Automata"

Prof. Robert Hull , UVA Materials Sciece and Engineering, MRSEC Director
[Host: J. Ruvalds]
Condensed Matter
Thursday, September 12, 2002
4:00 PM
Physics Building, Room 204
Note special time.

"Many Body effects in atomic Bose Condensates"

Prof. Cass Sackett , UVA-Physics
[Host: J. Ruvalds]
Condensed Matter
Thursday, September 5, 2002
4:00 PM
Physics Building, Room 204
Note special time.

"Dimers on a Triangular Lattice"

Paul Fendley , University of Virginia
[Host: J. Ruvalds]
Condensed Matter
Thursday, April 11, 2002
4:00 PM
Physics Building, Room 204
Note special time.

"Quantum fluctuations of charge and phase transitions of alarge Coulomb-blockaded quantum dot"

Xiaoya Qi , University of Virginia
[Host: D. Louca]
ABSTRACT:
We analyze ground-state properties of a large gated quantum dot coupled via a quantum point contact to a reservoir of one-dimensional interacting spinless electrons. We find that the classical step-like dependence of the dot population on the gate voltage is preserved under certain conditions. We point out that the problem is related to the classical one-dimensional Ising model with inverse-square interactions. This Ising universality class further subdivides into (i) the Kondo/Ising class and (ii) the tricritical class. For systems of the Kondo/Ising class, and repulsive electrons, the gate voltage dependence of the dot population is continuous for sufficiently open dots, while taking the form of a modified staircase for dots sufficiently isolated from the reservoir. At the phase transition between the two regimes the magnitude of the dot population jump is universal. For systems in the tricritical class we find in addition an intermediate regime where the dot population jumps from near integer value to a region of stable population centered about a half-integer value. In particular, this tricritical behaviour (together with the two dependencies already seen in the Kondo/Ising class) is realized for non-interacting electrons.
Condensed Matter
Thursday, April 4, 2002
4:00 PM
Physics Building, Room 204
Note special time.

"Probing Superconductors with Infrared - Electron-boson coupling in high temperature superconductors"

Jiufeng Tu , Brookhaven National laboratory
[Host: D. Louca]
ABSTRACT:
High-temperature superconductivity in an oxide containing quasi-two-dimensional copper-oxygen planes was observed by Bednorz and Muller in 1986. Recently, several high-temperature superconductors without copper or oxygen have been discovered including MgB2 with a Tc of 39K and electric field doped C60 with a Tc as high as 117K. Infrared spectroscopy has emerged as one of the most powerful experimental tools for the study of correlated electron systems and for high-Tc superconductors in particular. This talk will be focused on the infrared studies of two representative high-Tc superconductors: MgB2 (Tc = 39.6 K) and optimally doped Bi2Sr2CaCu2O8+¦Ä (Tc = 91.5 K). Effects of electron-boson coupling are observed in optical conductivities for both systems and their significance with respect to superconductivity will be discussed. In general, having a small free carrier plasma frequency (< 3 eV) seems to be an universal characteristic shared by almost all high-temperature superconductors with a Tc > 30 K, which means that the issue of reduced screening should be treated carefully in all of these systems.
Joint Condensed Matter-Chemical Physics Seminar
Thursday, February 7, 2002
4:00 PM
Physics Building, Room 204
Note special time.

"Combinatorial approach to materials discovery "

Professor I. Takeuchi , University of Maryland
[Host: Joseph Poon and Ian Harrison]
ABSTRACT:
Throughout the history of mankind, scientists and engineers have relied on the slow and random trial-and-error process for materials discovery. The combinatorial approach to materials is an emerging new method of materials research aimed at drastically increasing the rate and efficiency at which new materials are discovered and improved. In order to rapidly survey a large compositional landscape, up to thousands of compositionally varying samples are synthesized, processed, and screened in a single experiment. We have developed thin film combinatorial techniques for exploring new materials phases in a number of key technology areas including electronic, magnetic, and smart materials. Our synthesis tools include combinatorial pulsed laser deposition systems and a UHV combinatorial co-sputtering system. Our rapid characterization tools include scanning SQUID microscopes for mapping properties of magnetic materials libraries and scanning near field microwave microscopes for screening dielectric and conducting materials. Recent developments in investigation of ferromagnetic shape memory alloys and ferroelectric materials will be discussed.
Joint Condensed-Matter/High Energy Seminar
Wednesday, February 6, 2002
3:00 PM
Physics Building, Room 204
Note special date.
Note special time.

"On the Bethe Ansatz Solution of the Two-Channel Anderson Impurity Model"

Carlos Bolech-Gret , Rutgers
[Host: P. Fendley]
Condensed Matter
Wednesday, January 30, 2002
3:00 PM
Physics Building, Room 204
Note special date.
Note special time.

"Non-conventional metals: odd-frequency density waves and d-density wave"

Eugene Pivovarov , Caltech
[Host: P. Fendley]
ABSTRACT:
We discuss the metallic states which are ordered and therefore are not convetional Fermi liquids. In odd-frequency density waves the order parameter vanishes at zero frequency and there is a conventional Fermi surface. However, the frequency dependence of the gap leads to an unusual temperature dependence for various thermodynamic and transport properties. The d-density wave (DDW) is a candidate for the explanation of the pseudogap phase in superconducting cuprates. The gap vanishes at the nodal points, which causes the pure DDW state to be metallic. However, DDW can coexist with either antiferromagnetic or superconducting phases. We consider a microscopic model which favors DDW and do the mean-field-theoretical derivation of the phase diagram (this work is in progress).
Note: Special Condensed Matter Seminar
Monday, January 28, 2002
4:00 PM
Physics Building, Room 204
Note special date.
Note special time.

"Advection of Particles by Burger's Turbulence"

[Host: Alan McKane]
Condensed Matter
Thursday, January 24, 2002
4:00 PM
Physics Building, Room 204
Note special time.

"Photonic Bandgap Crystal, Flattop Narrow-Band Filter, Modulators, and Switches"

Phuc Tran , China Lake/Phillip Morris
[Host: V. Celli]
ABSTRACT:
Since the concept of Photonic Bandgap Crystal (PBC) was introduced in 1989 as a means to make low lasing threshold lasers, many many more applications of PBC have been suggested, particularly in the telecommunication area. In this talk, we will discuss the R-matrix technique to calculate the transmission through a PBC and the dispersion of surface waves on a PBC. On the application side, we will discuss the design of a flattop, narrow-band, tunable filter for use in Dense Wavelength Division Multiplexing (DWDM) systems. By incorporating non-linearity, we can make an all-optical modulator or switch. A nonlinear Finite Domain Time Dependent (FDTD) technique to simulate such a device will also be presented.
Condensed Matter
Thursday, January 17, 2002
4:00 PM
Physics Building, Room 204
Note special time.

"Carbon Nanotubes: a playground for Luttinger liquid physics"

Smitha Vishveshwara , UCSB
[Host: E. Kolomeisky]
ABSTRACT:
The electronic properties of single-walled metallic carbon nanotubes can be effectively captured by a system of interacting electrons in one dimension. As shown by both theory and experiment, and as discussed here, the nanotube thus exhibits signatures of Luttinger liquid behaviour. Furthermore, as a consequence of Luttinger liquid physics, we find that nanotubes are expected to display bizarre phenomena such as charge fractionalization; anamolous suppression of Andreev current when contacted to superconductors; hosting of entangled quantum states from a superconducting source.
Condensed Matter
Thursday, November 8, 2001
4:00 PM
Physics Building, Room 204
Note special time.

"Polymer-like viscosity near the critical point of xenon"

Robert F. Berg , NIST
[Host: G. Hess]
ABSTRACT:
This talk will describe two measurements of viscosity very close to xenon's critical point. The first experiment measured the viscosity increase caused by near-critical conditions. It revealed that, close to the critical point, xenon is partly elastic: It can stretch as well as flow. The second experiment, planned for 2002, will look for the shear-rate-induced viscosity decrease predicted by theory. Such "viscoelasticity" and "shear-thinning" are ordinarily seen only in much more complicated fluids such as polymer solutions. Both experiments are designed for the Space Shuttle. We required the Space Shuttle's microgravity because Earth's gravity compresses any fluid near its critical point. A layer of fluid even as thin as a dime collapses under its own weight, increasing the density at the bottom of the sample and distorting the data. Conducting the experiments on the Space Shuttle reduces these density differences by a factor of 100.
Condensed Matter
Thursday, October 18, 2001
4:00 PM
Physics Building, Room 204
Note special time.

"Electron-nuclear double resonance-mediated dynamic nuclear polarization"

Arthur Brill , University of Virginia
[Host: D. Louca]
ABSTRACT:
Because electronic magnetic moments are orders of magnitude greater than nuclear magnetic moments, for a given magnetic field strength and temperature, electronic paramagnetism far exceeds nuclear paramagnetism. The sensitivities of certain measurements in many branches of physics and chemistry depend upon the extent of nuclear spin polarization. Dynamic nuclear polarization (DNP) processes enable experimenters to transfer electron spin polarization to nuclear spin systems. In this seminar a newly-proposed method, with the potential for achieving as great as 50% polarization of protons and deuterons in much less time and with much lower microwave power than the DNP methods in current use, will be described.
31st Annual Llewellyn G. Hoxton Lecture

Thursday, April 26, 2001
7:30 PM
Chemistry Building, Room Chemistry Auditorium
Note special time.
Note special room.

"Electronics in the Internet Age"

Justin Rattner , Intel Fellow and Director of Intel's Microprocessor Research Lab
Condensed Matter
Thursday, April 19, 2001
4:00 PM
Physics Building, Room 204
Note special time.

"Molecular impurities in He clusters: microsolvation shells and superfluidity"

F. A. Gianturco , University of Rome
[Host: V. Celli]
ABSTRACT:
The study of unstable chemical species and of fast chemical reactions has profited greatly, in the last few years, from the development improvements of a novel experimental approach, which has been called the helium cluster beam isolation spectroscopy. In this method a beam of clusters, containing from about 103 to about 105 He atoms, picks up in flight one or more atoms or molecules (neutral or ionized) which can then be studied downstream with laser spectroscopy. The corresponding microscopic understanding of the possible structures of the weakly-interacting 4He atoms around or "outside" the molecular impurities requires the treatment of quantum effects on nuclear motions and the selection of the main, dominant structures within each cluster as a function of its size. In the present work we present new results for the quantum structures and the vibrational and rotational shifts for an OCS molecule in 4He clusters using the Diffusion Monte Carlo (DMC) method. A comparison with the recent results [S. Grebenev et al., J. Chem. Phys. 112, 4485 (2000)] from the experimental group in Göttingen will also be reported.
Condensed Matter
Thursday, April 12, 2001
4:00 PM
Physics Building, Room 204
Note special time.

"X-ray Diffraction and Spectroscopic Investigations of Nanophased Iron"

Rama Balasubramanian , James Madison University
[Host: D. Louca]
ABSTRACT:
Nano-phased iron oxides often found in corrosion coatings have properties that are scientifically interesting and industrially important. Naturally weathered iron oxides have particle size in the nanometer regime. Characterizing these oxides therefore require highly specialized spectroscopic tool to accurately quantify the relative % composition of the oxides usually present in a complex mixture. I will be presenting the use of Mossbauer, Micro-Raman and x-ray diffraction tools to accurately characterize each of the oxides in the mixture. Spectroscopic investigations on the influence of chromium in forming nano-phased goethite suggests that with increasing chromium concentration superparamagnetic behavior is enhanced. Micro-structural properties of mechanically alloyed Fe-Al alloys will be presented.
Condensed Matter
Thursday, April 5, 2001
4:00 PM
Physics Building, Room 204
Note special time.

"Giant Magnetoresistance in Spin Valve Multilayers"

Derek Stewart , UVA
[Host: J. Ruvalds]
Joint Physics & Chemistry Colloquium - Alan T. Gwathmey Seminar Series
Thursday, April 5, 2001
4:00 PM
Chemistry Building, Room 304
Note special time.
Note special room.

"Recent Advances in Single Molecular Manipulation in Biophysics"

Prof. Carlos Bustamante , University California Berkeley
[Host: Ian Harrison]
Condensed Matter
Thursday, March 29, 2001
4:00 PM
Physics Building, Room 204
Note special time.

"Growth and Optical Properties of III-Nitride Wide Bandgap Semiconductors"

Professor Jingyu Lin , Kansas State University
[Host: E. Kolomeisky and J. Poon]
ABSTRACT:
III-nitride wide band gap semiconductors (GaN, AlGaN and InGaN) have been a subject of intense recent investigation and have emerged as an important materials system for applications in areas of optoelectronic devices which are active in the blue and ultraviolet (UV) wavelength regions and electronic devices capable of operation at high temperatures, high power levels, and in harsh environments. In this talk, the epitaxial growth and fundamental optical and transport studies of III-nitrides, including GaN epilayers, InGaN and AlGaN alloys, and multiple quantum wells comprising alternating layers of InxGa1-xN/GaN and GaN/AlxGa1-xN will be discussed. In particular, how the picosecond time-resolved photoluminescence studies in providing input for the development of suitable materials quality and device design for specific applications as well as for the understanding of fundamental optical transitions and lasing mechanisms will be presented.
Special Condensed Matter
Monday, March 26, 2001
4:00 PM
Physics Building, Room 204
Note special date.
Note special time.

"Carbon nanotube quantum wires and quantum dots"

David Cobden , Univ. of Warwick
[Host: Despina Louca]
ABSTRACT:
Recent experiments on transport through individual single-wall carbon nanotubes with electrical contacts have shown that a broad range of physical phenonema can occur in such molecular-electronic devices [1,2]. I will review our present understanding of metallic nanotubes as interacting quantum wires and dots, with more emphasis on the quantum dot regime. As a result of the one-dimensional (1D) band structure, the spectral characteristics of nanotube dots are in informative contrast with those of dots formed in other systems having 2D or 3D bands. Also, these 1D quantum dots are preserved, effectively floating in vacuum, when the substrate beneath the tubes is etched away. In the resulting tightrope geometry we can begin to study the interplay of the molecule's electronic properties with a wide range of environmental factors. Finally, having achieved surprisingly good electrical contacts between the metal leads and tubes, we find that nanotube dots prove to be an excellent system for studying new aspects of Kondo physics. [1] Cees Dekker, "Carbon Nanotubes as Molecular Quantum Wires". Physics Today 52, 22-28 (1999). [2] Paul McEuen and others, Special reports in Physics World vol 13 issue 6 (June 2000).
Condensed Matter
Thursday, March 1, 2001
4:00 PM
Physics Building, Room 204
Note special time.

"GaAs Varactor Based Frequency Tripler Technology"

Gerhard Schoenthal , University of Virginia
[Host: Eugene Kolomeisky]
Condensed Matter
Thursday, February 22, 2001
4:00 PM
Physics Building, Room 204
Note special time.

"The Density of States in the Quantum Critical Regime of the Metal-Insulator Transition"

Dr. Winfried Teizer , University of California at San Diego
[Host: J. Poon]
ABSTRACT:
In the quantum critical regime, Coulomb interactions modify the density of states at the Fermi level and ultimately open a soft Coulomb gap since the decreasing electron mobility inhibits efficient screening. I will present tunneling spectroscopy data on amorphous GdxSi1-x, a material that can be continuously and reversibly tuned through the metal-insulator transition. On the metallic side, we find a signature of strong Coulomb interactions in the density of states, as theoretically expected. As the metal-insulator transition is approached from the metallic side, the tunneling spectrum shows a precursor of a soft Coulomb gap prior to reaching the insulating regime. I will extract the quantitative relationship of the density of states at the Fermi energy and the transport conductivity in the quantum critical regime.
Condensed Matter
Thursday, February 8, 2001
4:00 PM
Physics Building, Room 204
Note special time.

"Resonant Inelastic X-ray Scattering from Insulating Cuprates"

Peter Abbamonte , University of Groningen
[Host: D Louca]
ABSTRACT:
In this talk I will give a general introduction to the technique of inelastic x-ray scattering (IXS) and its use for measuring elementary excitations in condensed matter. I will describe how resonance techniques can be used to achieve (1) enhancement of the inelastic cross section and (2) sensitivity to wave function symmetry. These points will be illustrated with polarization-dependent spectra from the High Tc parent insulators La2CuO4 and Sr2CuO2Cl2, and by their interpretation in terms of symmetry-selection rules. I will conclude with some general statements about the utility of IXS - and where to go from here.
Condensed Matter
Thursday, February 1, 2001
4:00 PM
Physics Building, Room 204
Note special time.

"Quantum Wigner crystal and apparent metal-insulator transition of dilute 2D holes in GaAs at B=0"

Dr. Jongsoo Yoon , University of California at Berkeley
[Host: B. Shivaram]
ABSTRACT:
The scaling theory of localization for non-interacting electrons predicts that all states in 2D are localized by any amount of disorder in the zero temperature limit. In contrast, a "metallic" behavior characterized by a decreasing resistivity with decreasing temperaure, and thus an apparent metal-insulator transition as the charge-carrier density is reduced, is observed on many different low disorder dilute 2D systems such as Si MOSFET, GaAs, SiGe, and AlAs at low temperatures. It is found that the charge-carrier density where a metal-insulator transition occurs monotonically decreases with decreasing disorder. In an exceptionally high mobility 2D hole system in a GaAs heterostructure, a metal-insulator transition is observed at a density where the clean quantum Wigner crystal is expected, suggesting that the metal-insulator transition is related to the melting of the Wigner crystal. Details of transport characteristics in this GaAs system at both zero applied magnetic field (B=0) and with magnetic fields applied parallel to the 2D plane will be presented.
Condensed Matter
Thursday, November 30, 2000
4:00 PM
Physics Building, Room 204
Note special time.

"Thermoelectric Properties in Sb doped TiNiSn Half-Heusler Alloys"

Yu Xia , University of Virginia
[Host: Joseph Poon]
Condensed Matter
Thursday, November 16, 2000
4:00 PM
Physics Building, Room 204
Note special time.

"Smoke and Mirrors: The MgO saga"

John Larese , Brookhaven National Laboratory
[Host: George Hess]
Condensed Matter
Thursday, October 19, 2000
4:00 PM
Physics Building, Room 204
Note special time.

"Quantum Dots: A Tunable Kondo Effect"

Robert Konik , University of Virginia
[Host: E. Kolomeisky]
Condensed Matter
Thursday, October 12, 2000
4:00 PM
Physics Building, Room 204
Note special time.

"Nucleation When Diffusion Becomes Important: A Tale of Two Fluxes"

Professor Ken Kelton , Department of Physics - Washington University
[Host: Joseph Poon]
ABSTRACT:
The classical theory of nucleation does not properly describe nucleation processes when long-range diffusion becomes competitive with the interfacial processes. A new model is presented for time-dependent homogeneous nucleation in condensed phases, which takes account of the coupled fluxes of interfacial attachment and long-range diffusion. Numerical solutions from this new model show that the time-dependent nucleation rates scale with the dominant mobility and that the steady-state rates and induction times frequently differ significantly from values predicted by the classical theory. Surprisingly, the composition of the region of the parent phase near sub-critical crystal clusters is shifted toward that of the new phase. Consequences are discussed for solid state precipitation and the crystallization of Al-rare earth metallic glasses.
Condensed Matter
Thursday, September 28, 2000
4:00 PM
Physics Building, Room 204
Note special time.

"Turbulent Shear Flow: Analytic Result for a Universal Amplitude"

[Host: Eugene Kolomeisky]
ABSTRACT:
In the turbulent boundary layer above a flat plate, the velocity profile is known to be proportional to the logarithm of the plate separation distance. We will arrive at this experimental observation from theoretical arguments and will give an estimate of the proportionality constant, which happens to be a universal number. The final calculations will be compared with experimental data."
Condensed Matter
Thursday, September 21, 2000
4:00 PM
Physics Building, Room 204
Note special time.

"The hidden world of the diffusion equation"

Timothy Newman/P0STP0NED , University of Virginia, Physics and Biology
[Host: Eugene Kolomeisky]
ABSTRACT:
The study of persistent structures in dynamical processes is one of the most interesting recent developments in statistical mechanics. In this talk I shall give a brief review of persistence and its applications. This will be followed by the presentation of some very recent results concerning persistence in the diffusion equation, the foremost of which concerns the discovery of two (astonishingly high) critical dimensions.
SPECIAL SEMINAR
Thursday, July 20, 2000
10:00 PM
Material Science, Room 125
Note special time.
Note special room.

"Thermoacoustic Refrigeration"

Steven Garrett , United Technologies Corporation Professor of Acoustics - Penn State University
[Host: Bellave S. Shivaram]
ABSTRACT:
The interaction of heat and sound has been a subject of interest to scientists and engineers since 1816 when Laplace corrected Newton's attempt to derive the speed of sound in air from first principles. Glassblowers observed the generation of sound in the presence of temperature gradients even earlier. It was less than twenty years ago that the reverse process - the use of high-amplitude sound to produce refrigeration - was first demonstrated. Due to the discovery of the "hole-in-the-ozone" and the ratification of the Montreal Protocols, research in "thermoacoustics" has accelerated during the past decade. In 1992, an electrically-powered thermoacoustic refrigerator was placed in orbit on the Space Shuttle and a larger thermoacoustic chiller for shipboard electronics was demonstrated for a week on board a US Navy destroyer in 1995. More recently, a heat-driven thermoacoustic device was used to liquefy in excess of 100 gal/day of natural gas by burning part of the gas stream. This presentation will include a simple description of the thermoacoustic heat pumping process and will describe some of the novel devices that have exploited this process.
SPECIAL HIGH ENERGY PHYSICS SEMINAR
Tuesday, May 30, 2000
3:00 PM
Physics Building, Room 313
Note special date.
Note special time.
Note special room.

"The Measurement of Lambda Polarization at Nomad Experiment"

Kamal Benslama , University of Regina
[Host: Craig Dukes]
Condensed Matter
Thursday, April 20, 2000
4:00 PM
Physics Building, Room 204
Note special time.

"Soft Matter in a Tight Spot: The Structural Transition and the Molecular Origin"

Prof. Shenting Cui , Dept. of Chemical Engineering at Univ. of Tenn. and Chemical Technology Div. at Oak Ridge Lab.
[Host: R. E. Johnson]
ABSTRACT:
Phenomena occurring on nanometer scale are attracting increasing scientific interest due to the expected advances in nanotechnology. This talk will describe recent studies on the fluid behavior occurring at a few nanometers when one of the dimensions, the spacing between two solid surfaces, is reduced to a few molecular diameters. Such conditions are encountered in the operation of the computer magnetic disks, preparation of nano-structured materials, colloidal dispersions, and automobile lubrication, etc. When a fluid is confined within spacing comparable to the molecular dimension, interfacial interactions become predominant and this can dramatically alter the structural, thermodynamic and rheological properties of the fluid film. Experiments have found that solid-like behaviors develop for fluids confined between molecularly smooth mica surfaces. This is manifested by the ability of the film to sustain a finite stress, a phenomenon typically associated with solids, and orders of magnitude increase in viscosity. The microscopic mechanism for the phenomenon is still not well understood theoretically. We describe a comprehensive molecular simulation study that demonstrates the solid-like behavior for alkane films narrowly confined in molecularly smooth strong-adsorbing surfaces at ambient temperature and pressure. The results of the study correlate a broad range of experimental observations and point to a mechanism different from that has been previously suggested.
Special AMO/Chemical Physics Seminar
Thursday, March 30, 2000
4:00 PM
Physics Building, Room 204
Note special time.

"Characterization and Control of Ultracold Collisions"

Steve Gensemer , University of Connecticut
[Host: Robert Jones]
ABSTRACT:
We have developed several new techniques for observing and altering ultracold collisions in laser-cooled Rb. We have found that the trapping laser in a typical magneto-optical trap (MOT) can increase inelastic collision rates by more than an order of magnitude. We have also observed ultracold collision dynamics in the temporal domain for the first time, by using a series of laser pulses that interact with a colliding pair of atoms at different internuclear distances. Together with other experiments that use repulsive or attractive molecular potentials to reduce or increase inelastic or elastic collision rates, we are developing the tools to understand and utilize the extremely long-range molecular potentials involved (R > 300 bohr radii), which cannot be studied using conventional spectroscopy.
Condensed Matter
Thursday, March 23, 2000
4:00 PM
Physics Building, Room 204
Note special time.

"Progress with SQUID Gradiometers and their Application to Biomagnetism and Non-destructive Testing"

Professor Gordon Donaldson , University of Strathclyde
[Host: Bascom Deaver]
SPECIAL COLLOQUIUM
Thursday, March 2, 2000
4:00 PM
Physics Building, Room 204
Note special time.

"Holography and the Cosmological Constant Problem"

George Minic , University of Southern California - (USC)
ABSTRACT:
Holography implies that the most probable value for the cosmological constant is zero.
Joint Condensed Matter and Chemical Physics Seminar
Thursday, February 24, 2000
4:00 PM
Physics Building, Room 204
Note special time.

"Materials Research at Jefferson Lab"

Dr. Edward Gilman , Jefferson Laboratory and Norfolk State University
[Host: Joseph Poon]
ABSTRACT:
The Center for Materials Research at Norfolk State University supports a number of efforts in materials science and technology. In addition to the surface science and micro-characterization at the Applied Research Center at JLab, there are programs in free electron laser materials processing, laser spectroscopy, NMR, ESR, polymers and organic thin films. An overview of some of these research efforts will be given. In more detail, work on the Colossal Magnetoresistance (CMR) manganites will be presented. These are materials that exhibit both ferromagnetic and metallic behavior below a critical temperature, but are paramagnetic insulators above. The mechanisms of conduction and ferromagnetism in thin films will be presented and related to the structure of these materials at the atomic level. Proposed uses for these materials include read-write heads and as bolometric detectors
Condensed Matter
Thursday, January 27, 2000
4:00 PM
Physics Building, Room 204
Note special time.

"Crossover Critical Phenomena in Fluids"

Anna Wyczalkowska , University of Maryland
[Host: Despina Louca]
Condensed Matter
Thursday, December 2, 1999
4:00 PM
Physics Building, Room 204
Note special time.

"Directed Polymers and Oppressive Population Control"

Timothy Newman , University of Virginia - Physics
[Host: E. Kolomeisky]
ABSTRACT:
Abstract: In this blackboard talk, I will discuss new results in the field of directed polymers (these are topological objects found in many condensed matter systems, e.g. flux lines in superconductors, domain walls in ferromagnets, and atomic steps on vicinal surfaces). After a detailed introduction, I will describe the physics of "self-localization" of directed polymers, a state induced by elastic interactions between the polymer and its environment. In the latter part of the talk, I will show how the model of self-localization may be mapped to population dynamics, in which, along with local birth and death rules, one imposes a global constraint which fixes the total population. The dynamics of populations under such oppressive control turns out to be very rich, with phases such as "spreading", "collapse", and a novel "pseudo-traveling wave".
Condensed Matter
Thursday, November 18, 1999
4:00 PM
Physics Building, Room 204
Note special time.

"Through a Glass Darkly"

Jackie Johnson , Argonne National Lab
[Host: Despina Louca]
SPECIAL CONDENSED MATTER SEMINAR
Tuesday, November 16, 1999
4:00 PM
Physics Building, Room 204
Note special date.
Note special time.

"NSOM Studies of SiN Membranes and Photonic Bandgap Structures"

Anthony Campillo , University of Virginia - Physics
Joint Condensed Matter and Chemical Physics Seminar
Wednesday, February 24, 1999
4:00 PM
Physics Building, Room 204
Note special date.
Note special time.