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 Condensed Matter Thursday, May 30, 2019 11:00 AM Physics Building, Room 313 Adam Wei Tsen [Host: Seunghun Lee] University of Waterloo "Two-dimensional magnetism and spintronics" 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 Joseph Avron [Host: Israel Klich ] Technion "Flexible Sganac interferometers for the neophytes" 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 Professor Ying-Jer Kao [Host: Gia-Wei Chern] National Taiwan University "Tunneling-induced restoration of classical degeneracy in quantum kagome ice" 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 Professor Carlos Silva [Host: Seunghun Lee] Georgia Tech "Exciton polarons in two-dimensional organic-inorganic hybrid perovskites" 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 Yuchen Du [Host: Diana Vaman] University of Virginia - Physics "Worldline approach" 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 Xixiao Hu [Host: Joe Poon] University of Virginia - Physics "Thermoelectric transport properties of topological Bi-Sb cryogenic materials" 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 Meng Hua [Host: Jeffrey Teo] University of Virginia - Physics "Topological phases with two-fold spatial antiunitary symmetries" 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 Milovan Suvakov [Host: Marija Vucelja ] Institute of Physics Belgrade "The three-body problem: periodic solutions, topological classification" 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 Hamed Vakili [Host: Avik Ghosh] University of Virginia - Physics "Magnetic Skyrmions on a racetrack" 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 Preetha Saha [Host: Gia-Wei Chern] University of Virginia - Physics "Spin dynamics in two distinct types of classical spin liquids " 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 UVa's Advanced Research Computing Services [Host: Bryan Wright] University of Virginia "Introduction to Rivanna" 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 Peter Schauss [Host: Dmytro Pesin] University of Virginia - Physics "Quantum gas microscopy of many-body dynamics in Fermi-Hubbard and Ising systems" 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 Zhenyang Xu [Host: Despina Louca] University of Virginia - Physics "Metal-Insulator Transition in Phase Change Material Ge2Sb2Se5xTe5-5x" 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 Rafael Alexander [Host: Israel Klich] UNM "Walks, tiles, and zippers: exact holographic tensor networks for Motzkin spin chains" 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 Maxim Dzero [Host: Israel Klich] Kent State "Thermodynamic properties of disordered unconventional superconductors with competing interactions" 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 Arnab Banerjee [Host: Bellave Shivaram] Oak Ridge National Laboratory "Fractionalized excitations towards a non-Abelian phase in a Kitaev honeycomb magnet" 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 Erhai Zhao [Host: Bellave Shivaram] George Mason University "Competing orders in a quantum spin model with long-range interactions" 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 Alex Levchenko [Host: Dmytro Pesin] University of Wisconsin-Madison "Transport in Strongly Correlated 2D Electron Fluids" 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 Ed Barnes [Host: Israel Klich] Virginia Tech "Toward the next quantum revolution: controlling physical systems and taming decoherence" 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 Nirmal Ghimire [Host: Bellave Shivaram] George Mason University ""A materials-driven approach to the novel topological states of matter"" 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 Hitesh J. Changlani [Host: Bellave Shivaram] Florida State University "The mother of all states of the kagome quantum antiferromagnet" 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 Amr Ahmadain [Host: Israel Klich ] UVA-Department of Physics "Gravitational Weyl Anomalies in 1+1-Dimensional Lifshitz Field Theories" 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 Kyungwha Park [Host: Jeffrey Teo] Virginia Tech "Magnetic-Field Induced Weyl Semimetal from Wannier-Function-based Tight-Binding Model" 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 Timothy Halpin-Healy [Host: Genya Kolomeisky] "Within & Beyond the Realm of KPZ" 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 Jing Luo [Host: Gia-Wei Chern] UVA-Department of Physics "Frustrated Kondo chains and glassy magnetic phases on the pyrochlore lattice" 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 Depei Zhang [Host: Seunghun Lee] UVA - Department of Physics "Crystal Structures and Photoluminescence of a Two-Dimensional Perovskite" 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 Clarina Dela Cruz [Host: Despina Louca] Oak Ridge National Laboratory "Opportunities in Quantum Materials Research using Neutrons" 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 Adriana Moreo [Host: Despina Louca] University of Tennessee "New Directions in Theoretical Studies of High Tc Superconductors" 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.