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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


Open
Speaker: Open Title: Abstract: Host:

3 Mar 2017, 4:00PM | MIST M102


Olena Gomonay, SPICE, Mainz
Speaker: Olena Gomonay, SPICE, Mainz Title: TBA Abstract: Host: Ar. Abanov

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


SPRING BREAK, MARCH MEETING, New Orleans, LA
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: TBA Abstract: Host: Ar. Abanov

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


Alexey Belyanin, TAMU
Speaker: Alexey Belyanin, TAMU Title: TBA Abstract: Host: A. Belyanin

31 Mar 2017, 4:00PM | MIST M102
Hosted By: A. Belyanin


Abhay Pasupathy, Columbia University
Speaker: Abhay Pasupathy, Columbia University Title: TBA Abstract: Host: Ar. Abanov

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


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

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


Open
Speaker: Open Title: Abstract: Host:

21 Apr 2017, 4:00PM | MIST M102


A. Akimov, AMO, TAMU
Speaker: A. Akimov, AMO, TAMU Title: TBA Abstract: Host: A. Zheltikov

28 Apr 2017, 4:00PM | MIST 102
Hosted By: A. Zheltikov


Mikhail Raikh, the University of Utah
Speaker: Mikhail Raikh, the University of Utah Title: TBA Abstract: Host: Ar. Abanov

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


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