Condensed Matter Seminars This Term

ics Condensed Matter
Thursday, August 24, 2017
11:00 AM
Physics Building, Room 313

"Available"
ics Condensed Matter
Thursday, August 31, 2017
11:00 AM
Physics Building, Room 313

"Available"
ics Condensed Matter
Thursday, September 7, 2017
11:00 AM
Physics Building, Room 313
Maiko Kofu [Host: Despina Louca]
JPARC, Japan
"Structure and dynamics of hydrogen in nanocrystalline palladium"
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.

ics Condensed Matter
Thursday, September 14, 2017
11:00 AM
Physics Building, Room 313
RESERVED
ics Condensed Matter
Thursday, September 21, 2017
11:00 AM
Physics Building, Room 313

"Available"
ics Condensed Matter
Thursday, September 28, 2017
11:00 AM
Physics Building, Room 313
Dmytro Pesin [Host: Gia-Wei Chern]
University of Utah
"Geometric theory of nonlocal transport in metals"
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.


 

ics Condensed Matter
Thursday, October 5, 2017
11:00 AM
Physics Building, Room 313
Bryan Chen [Host: Jeffrey Teo]
University of Pennsylvania
"Topological mechanics and hidden symmetry in rigid origami"
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).

ics Condensed Matter
Thursday, October 12, 2017
11:00 AM
Physics Building, Room 313
RESERVED
ics Condensed Matter
Thursday, October 19, 2017
11:00 AM
Physics Building, Room 313
Charles Reichhardt [Host: Gia-Wei Chern]
Los Alamos National Lab
"Skyrmion Lattices in Random and Ordered Potential Landscapes"
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).

ics Condensed Matter
Thursday, October 26, 2017
11:00 AM
Physics Building, Room 313
Jordan Horowitz [Host: Marija Vucelja]
MIT
"Dissipation bounds on far-from-equilibrium fluctuations"
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/

ics Condensed Matter
Thursday, November 2, 2017
11:00 AM
Physics Building, Room 313
Ori Hirschberg [Host: Israel Klich ]
"Real-space condensation in mass transport models: statics, dynamics, and large deviations"
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.

ics Condensed Matter
Thursday, November 9, 2017
11:00 AM
Physics Building, Room 313
Puru Jena [Host: Utpal Chatterjee]
VCU
"Many Faces of Carbon"
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).
ics Condensed Matter
Thursday, November 16, 2017
11:00 AM
Physics Building, Room 313
Ilya Vekhter [Host: Utpal Chatterjee]
Louisiana State University
"Interface symmetry and non-helical states in topological insulator-semiconductor heterostructures"
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. 

ics Condensed Matter
Thursday, November 30, 2017
11:00 AM
Physics Building, Room 313
Available

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