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Condensed Matter Seminar

Pouyan Ghaemi, City College of the City University of New York
Speaker: Pouyan Ghaemi, City College of the City University of New York Title: Superconductivity in topological insulators: From topological phase transition to Neutrino Oscillations. Abstract: The presence of helical surface states in topological insulators (TIs) motivated many theoretical and experimental studies on the transport properties of such two dimensional helical metals. Unfortunately the bulk conducting bands have been shown to greatly challenge experimental observation of unique transport properties of the surface states. In this talk I will show that in the presence of superconductivity, the signature of topological properties is apparent in the features of supercurrent carried through both surface and in the bulk bands of the TIs. The distinct effect of impurities on supercurrent, when compared with their effect on normal current, makes the superconducting phase an interesting platform to study transport in helical metals. I will show that the effect of impurities on supercurrent in helical metals, which resembles the interaction of Neutrinos with electrons, can explain some of puzzling experimental results on Josephson junctions on the edge of TIs. Host: Ar. Abanov

20 Jan 2017, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Nayana Shah, University of Cincinnati
Speaker: Nayana Shah, University of Cincinnati Title: Manifestations of spin-orbit coupling and topology in out-of-equilibrium hybrid superconducting systems Abstract: Recently there has been a lot of excitement generated by the possibility of realizing and detecting Majorana fermions within the arena of condensed matter physics and its potential implication for topological quantum computing. Although already at the end of twentieth century emergent Majorana end-states were shown to exist in a theoretical model of spinless p-wave superconductor (Kitaev) chain, it was only a decade later that proposals to experimentally realize such a model emerged. These were motivated by the discovery of topological insulators that ushered a new era of so-called symmetry-protected topological phases but also stemmed from existent studies of hybrid superconductor-ferromagnet systems that form the basis of another highly active area of superconducting spintronics. At the heart of these proposals are ways to judiciously use spin-orbit coupling in artificially designed hybrid settings containing conventional s-wave superconductors to effectively create topological superconductors. Making sense of the emerging physical picture and experiments requires employing and developing complementary theoretical frameworks to fundamentally comprehend the out-of-equilibrium transport behavior in these systems. I will outline the steps we have taken in that direction and the questions that remain to be answered. Refs: K. Sun, N. Shah, Phys. Rev. B 91, 144508 (2015); D. Roy, C. J. Bolech, N. Shah, Phys. Rev. B 86, 094503 (2012); D. Roy, C. J. Bolech, N. Shah, arXiv:1303.7036; N. Shah, C. J. Bolech, Phys. Rev. B 93, 085440 (2016) Host: Ar. Abanov

27 Jan 2017, 4:00PM | MIST M102
Hosted By: Ar. Abanov

A. Chubukov, University of Minnesota
Speaker: A. Chubukov, University of Minnesota Title: Superconductivity near a quantum critical point Abstract: I discuss the interplay between non-Fermi liquid behaviour and superconductivity near a quantum-critical point (QCP) in a metal. It was thought by many researchers that in D=2, non-Fermi liquid behaviour near a QCP extends to energies well above superconducting Tc, and that superconductivity involves non-Fermi-liquid quasiparticles and emerges due to peculiar interplay between strong attraction and strong pair-breaking effects from self-energy. I argue that this is not necessary always the case. I show that in a situation, when critical bosons are slow compared to electrons, fermionic self-energy plays little role for superconductivity in 2D, despite that it is strong and destroys fermionic coherence. I discuss the special role of the “first Matsubara frequency” in this regard. I present explicit results for Tc for the set of models with frequency-dependent effective interaction, including the strong coupling limit of electron-phonon interaction. Host: Ar. Abanov

1 Feb 2017, 3:00PM | MIST M102
Hosted By: Ar. Abanov

Kater Murch, Washington University
Speaker: Kater Murch, Washington University Title: Quantum measurement in superconducting circuits: mapping quantum trajectories from spontaneous emission. Abstract: Spontaneous emission of light by atoms is one of the most basic light-matter interactions and is responsible for the majority of the visible light that we see. The process of spontaneous emission can also be viewed in the context of quantum measurement, the light-matter interaction entangles the atom with the electromagnetic field and subsequent measurements of the field convey information about the state of the atom. For example, if the emitted light is detected in the form of energy quanta, the detection of an individual photon results in an instantaneous jump of the atom to a lower energy state. However, if the emission is instead measured with a detector that is not sensitive to quanta, but rather the amplitude of the field, the atom’s state will undergo different dynamics over finite timescales. In this talk, I will review how recent progress in the fabrication and control of quantum coherent superconducting circuits has enabled experiments that probe the fundamental physics of quantum measurement. These range from the observation of non-classical weak values to the generation of entanglement through measurement and the tracking of individual quantum trajectories. I will then describe our recent experiments that focus on the special case of spontaneous emission, revealing rich dynamics associated with measurement, and how we are harnessing these dynamics to extend thermodynamics into the regime of single quantum systems. Host: Ar. Abanov

10 Feb 2017, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Ilya Vekhter, LSU
Speaker: Ilya Vekhter, LSU Title: 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 briefly discuss how symmetry breaking at the interface influences the behavior of the thin films of topological insulators and proximity-induced superconductivity. Host: Ar. Abanov

17 Feb 2017, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Ivar Martin, Argonne
Speaker: Ivar Martin, Argonne Title: Quasicrystals in space and time. Abstract: Crystallization is one of the most familiar but least understood phase transitions. Why some crystal structures are more common than others? What selects between different close-packed orderings? Why do atoms sometimes prefer to arrange themselves into quasi-crystals, which correspond to projections from 5 or 6 dimensional lattices? I will show that in metallic alloys, the energetics is strongly influenced by sharp electronic Fermi surface, which favors rhombohedral, FCC, and icosahedral quasicrystal lattices for simple geometric reasons. In the second – “time” –– part of the talk, I will apply the cut-projection ideas from quasicrystals to driven systems, allowing creating multiple time dimensions, and topological band structures even starting from zero-dimensional physical systems. Refs: Ivar Martin, Sarang Gopalakrishnan, Eugene Demler, “Weak crystallization theory of metallic alloys”, Phys. Rev. B 93, 235140. Ivar Martin, Gil Refael, and Bertrand Halperin, “Topological frequency conversion in strongly driven quantum systems,” arXiv:1612.02143 Host: Ar. Abanov

24 Feb 2017, 4:00PM | MIST M102
Hosted By: Ar. Abanov

V. Pokrovsky, TAMU
Speaker: V. Pokrovsky, TAMU Title: Bose-Einstein condensation and superfluidity of magnons in ferromagnetic films under pumping Abstract: We present a brief review of the theory and experiment of quasi-equilibrium Bose-Einstein condensation and superfluidity of magnons in a film of Yttrium Iron Garnet (YIG). The Bose-Einstein condensation of magnons in YIG film at room temperature under the RFf pumping was discovered in 2006 by the Munster experimental team led by S. Demokritov. We describe their experiments [1,2] and explanation of their results by theory [3,5]. There are two equal minima of energy in the magnon spectrum of YIG film at non-zero momenta and therefore two condensates. The same experimental group has discovered the interference of the two condensates [2] thereby proving their coherence. Theory [3] attracted attention to the strong degeneration of the ground state of non-interacting magnons with respect to distribution of them between two minima. This degeneration is lifted by the magnon interaction. Theory predicts that interaction leads to spontaneous violation of the reflection symmetry and non-equal number of magnons in two condensates for thick films. In thin films the condensate is symmetric at low magnetic field and transits to to non-symmetric state at higher field. Dipolar interaction depends on the phase of the condensate wave function. In quasi-equilibrium it traps the phase. Two obstacles for the magnon superfluidity are the dominance of the normal magnon density over the condensate approximately 100 times and the phase trapping. We show that the velocity of the superfluid part of magnon gas is by 5-7 decimal orders larger than the velocity of normal part at typical values of magnetic field and its gradient. Thus, the spin current is mainly superfluid. The phase trapping is a consequence of non-conservation of the spin angular moment that turns into the orbital moment by dipolar interaction. Therefore, the number of magnons is not conserved locally, stationary spin current becomes inhomogeneous. But it conserves globally due to the remaining discrete symmetry [4]. We propose to observe magnon superfluidity creating a soliton bouncing between two reflecting boundaries. We discuss theory of superfluid solitons and prospects to reproduce all these phenomena in nano-scale samples [5]. 1. S.O. Demokritov et al., Nature 443, 406 (2006) 2. P. Nowik-Boltyk et al., Sci. Rep. 2, 482 (2012) 3. F. Li et al., Sci. Rep. 3, 1372 (2013) 4. C. Sun et al., Phys. Rev. Lett. 116, 257205 (2016) 5. C. Sun et al., J. Phys. D, 2017 Host: A. Akimov

7 Mar 2017, 11:45AM | MPHY 578
Hosted By: A. Akimov

Karen Michaeli, Weizmann
Speaker: Karen Michaeli, Weizmann Title: Electron teleportation in multi-terminal Majorana islands: Kondo effect at high temperature Abstract: Recent advances in growing complex structures of topological superconducting nanowires have paved the way for exploring new physics beyond the immediate application of finding Majorana modes. In our work we studied setups involving a superconducting island with multiple Majorana modes connected to normal leads. I will explain how coherent effects give rise to a family of non-Fermi liquid states similar to the multiple channel Kondo problem. These states are remarkably stable and persist even in the resonant regime, defying conventional wisdom. As a direct consequence we predict a universal, gate-voltage independent conductance in these systems that develops at relatively high temperatures. Our results make the implementation of various devices more feasible, and we expect our observation to shift the paradigm in designing protocols for braiding Majorana modes and quantum computation away from focusing solely on the off-resonant regime. Host: A. Finkelstein

10 Mar 2017, 4:00PM | MIST M102
Hosted By: A. Finkelstein

Speaker: SPRING BREAK, MARCH MEETING, New Orleans, LA Title: Abstract: Host:

17 Mar 2017, 4:00PM | MIST M102

Hugh Churchill, University of Arkansas
Speaker: Hugh Churchill, University of Arkansas Title: 2D Materials: What Now? Abstract: For its first 10 years, the atomically thin 2D materials community was largely unified in its choice of materials, first with graphene and more recently with 2D semiconductors, primarily transition metal dichalcogenides and black phosphorus. An explosion in the availability of diverse 2D materials, combined with a relatively short attention span of the community lead to the question, what now? In this talk, I will describe two ways that my lab is approaching this question: first, to select the most compelling properties of more well-known materials to target particular physical effects or device types; and second, to investigate new materials that may add to the list of things that can be done with 2D materials and their heterostructures. In the first category, we are making gate-defined quantum dots based on transition metal dichalcogenides and exploring excitonic effects in black phosphorus heterostructures. In the second category, we are investigating layered group-IV monochalcogenides, which are potential multiferroic phase-change materials, and layered transition metal thiophosphates, which add the possibility of gate-controlled ferromagnetism to the 2D material toolbox. Host: Ar. Abanov

24 Mar 2017, 4:00PM | MIST M102
Hosted By: Ar. Abanov

A. Akimov, AMO, TAMU
Speaker: A. Akimov, AMO, TAMU Title: Towards Light Matter Interface for NV Center in Diamond Abstract: The nitrogen-vacancy (NV) center in diamond is attracting a lot of attention in quantum information processing community. A spin system in the NV center is positioned in a clean and well-controlled environment. As a result, it shows outstanding performance as quantum memory, even at room temperature, allows for spin control with a single shot optical readout, and offers a possibility of building quantum registers on a single NV center. At low temperature, the NV centers have narrow optical transitions, which enable interfacing between optical photons and the NV center spin states. Recently, the entanglement of two independent NV centers has been demonstrated. This makes the NV center a promising candidate for realization of quantum repeaters. Moreover, the NV centers could be used as sensitive elements of detectors of magnetic or electric fields, temperature etc. For all of these applications collection of the light emitted by the NV center is a crucial point. Recent developments in the field of metamaterials allowed creating hyperbolic metamaterials which may provide an efficient interface for coupling light into and out of the NV centers and, in particular, enable a single photon source based on NV. In this work, I will present our work on using CMOS-compatible hyperbolic metamaterials and optical fibers to construct efficient single photon sources and sensing elements using NV centers in diamond. Host: A. Zheltikov

31 Mar 2017, 4:00PM | MIST 102
Hosted By: A. Zheltikov

Abhay Pasupathy, Columbia University
Speaker: Abhay Pasupathy, Columbia University Title: Materials under Strain: an Atomic-Scale Perspective. Abstract: What is the effect of stretching a crystal along a given direction by a small amount? In general, one might not expect much: a change in lattice constant, accompanied by corresponding changes in the electronic and vibrational properties of a crystal. I will describe a few cases of materials where the effect of stretching (ie, uniaxial strain) lead to large and unexpected effects. These include the iron-based superconductors (where large electronic nematic effects are seen), layered transition-metal dichalcogenides (where we can observe the formation of strain solitons) and two-dimensional semiconductors (where we can cause large changes in band gap and even cause structural phase transitions). I will discuss these from an experimental perspective - in particular, I will describe new experimental techniques where we can apply calibrated uniaxial strain to crystals and measure their response with atomic-resolution scanning tunneling microscopy techniques. Host: Ar. Abanov

7 Apr 2017, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Justin Wilson, Caltech
Speaker: Justin Wilson, Caltech Title: Quantum phases of disordered three-dimensional Majorana-Weyl fermions Abstract: The gapless Bogoliubov-de Gennes (BdG) quasiparticles of a clean three-dimensional spinless p+ip superconductor provide an intriguing example of a thermal Hall semimetal (ThSM) phase of Majorana-Weyl fermions in class D of the Altland-Zirnbauer symmetry classification; such a phase can support a large anomalous thermal Hall conductivity and protected surface Majorana-Fermi arcs at zero energy. We study the effect of quenched disorder on such a topological phase with both numerical and analytical methods. Using the kernel polynomial method, we compute the average and typical density of states for the BdG quasiparticles; based on this, we construct the disordered phase diagram. We show for infinitesimal disorder, the ThSM is converted into a diffusive thermal Hall metal (ThDM) due to rare statistical fluctuations. Consequently, the phase diagram of the disordered model only consists of ThDM and thermal insulating phases. Nonetheless, there is a cross-over at finite energies from a ThSM regime to a ThDM regime, and we establish the scaling properties of the avoided quantum critical point which marks this cross-over. Additionally, we show the existence of two types of thermal insulators: (i) a trivial thermal band insulator (ThBI) [or BEC phase] supporting only exponentially localized Lifshitz states (at low energy), and (ii) a thermal Anderson insulator (AI) at large disorder strengths. We determine the nature of the two distinct localization transitions between these two types of insulators and ThDM. Based on arXiv:1612.05648 Host: Ar. Abanov

14 Apr 2017, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Bill Gannon, Texas A&M University
Speaker: Bill Gannon, Texas A&M University Title: Evolution of Spinons and the Emergence of a Longitudinal Mode in One Dimensional Yb2Pt2Pb Abstract: The Yb3+ ions in Yb2Pt2Pb form large, seemingly classical Ising magnetic moments, with the large spin-orbit coupling of the 4f-electrons and the crystal electric field forming a J = +/-7/2 Yb ground state doublet [1]. However, from this unlikely host, emerges a continuum of quantum excitations — spinons on one dimensional chains — in good agreement with the behavior expected for nearly isotropic, S = +/-1/2, d-electron magnetic moments [2]. These spinons, in a system who\ Host: Alexey Akimov

18 Apr 2017, 12:00PM | MPHY 578
Hosted By: Alexey Akimov

Da-Wei Wang, AMO, TAMU
Speaker: Da-Wei Wang, AMO, TAMU Title: Quantum optical simulation of the Haldane model in superradiance lattices and cavity QED Abstract: The Haldane model played an important role in the quantum Hall effect and the development of topological insulators. It has been recently simulated in cold atoms. By combining single photon superradiance and electromagenetically induced transparency, we can use timed Dicke states to construct momentum-space tight-binding lattices, coined the superradiance lattices. In this talk, I will introduce the simulation of the Haldane model in superradiance lattices. In addition, a variation of the Haldane model can also be simulated in a cavity QED systems. This research offers a new platform for designing novel optical devices that can serve as the elementary components in quantum information. Host: A. Zheltikov

21 Apr 2017, 4:00PM | MIST M102
Hosted By: A. Zheltikov

Ar. Abanov, TAMU, CM/AMO
Speaker: Ar. Abanov, TAMU, CM/AMO Title: Magnetization texture production by current. Abstract: This is the second installment of the talk. In this part I will show how to produce new domain walls in nano-wires and new skyrmions (skyrmion-antiskyrmion pairs) in magnetic films by all-electric means. Host: Ar. Abanov

25 Apr 2017, 12:00PM | MPHY 578
Hosted By: Ar. Abanov

Alexey Belyanin, TAMU
Speaker: Alexey Belyanin, TAMU Title: Optics of Dirac and Weyl fermions Abstract: Relativistic Dirac and Weyl fermions were extensively studied in quantum field theory. Recently they emerged in the nonrelativistic condensed-matter setting as gapless quasiparticle states in some types of crystals. Notable examples of 2D systems include graphene and surface states in topological insulators such as Bi2Se3. Their 3D reincarnation is Dirac and Weyl semimetals that were recently discovered experimentally. Most of the research has been focused on their topological properties and electron transport. However, their optical properties are no less exciting. Moreover, optical phenomena can provide unique insight into fascinating physics of these materials. I will discuss several examples illustrating this point. They include: plasmons and normal modes in Weyl semimetals, nonlocal optical response of graphene and topological insulators, and optical properties of chiral Dirac/Weyl fermions in a quantizing magnetic field. Host: A. Belyanin

28 Apr 2017, 4:00PM | MIST M102
Hosted By: A. Belyanin

Mikhail Raikh, the University of Utah
Speaker: Mikhail Raikh, the University of Utah Title: Spin dynamics of a hopping carrier in a random hyperfine field Abstract: In course of carrier motion between the sites which host random hyperfine fields, its spin experiences random precessions. Normally, this leads to the exponential decay of average spin, , with a rate proportional to the square of the magnitude of the field. We demonstrate that in low dimensions, d = 1, 2, this orthodox scenario is violated: the decay does not follow the simple-exponent behavior at all times. The origin of the effect is that for d = 1, 2 a typical random-walk trajectory exhibits numerous self-intersections. Multiple visits of the carrier to the same site accelerates the relaxation since the corresponding partial rotations of spin during these visits add up. Another consequence of self -intersections of the random-walk trajectories is that, in all dimensions, , becomes sensitive to a weak magnetic field directed along z-axis. Remarkably, for random fields located in the (x, y) - plane, the decay of in d = 1 is accompanied by the reversal. We develop an approximate analytical description of the behavior. This description is in a very good agreement with numerical simulations. * In collaboration with Yue Zhang, Robert Roundy, and Vagharsh Mkhitaryan Host: Ar. Abanov

5 May 2017, 4:00PM | MIST M102
Hosted By: Ar. Abanov

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