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Condensed Matter Seminar

Joseph Ross, TAMU
Speaker: Joseph Ross, TAMU Title: NMR studies of Dirac Semimetals and Related Materials Abstract: Much of my talk will focus on our results for the topological quantum material ZrTe5. Long studied as a candidate for charge-density wave transformations, layered ZrTe5 has recently been recognized as a Dirac electron system. Its behavior includes a topological phase transition joining two different massive Dirac regimes, although there have been a number of conflicting reports about the nature of this transformation. NMR spectroscopy provides a local probe of the fluctuating Dirac carriers, and we have used this to provide a quantitative measure of the T-induced gap separating the Dirac cones. Combined with DFT-based electronic structure calculations, we also showed that the transition involves a change from weak to strong topological insulator as temperature increases, the reverse of what had been proposed. Given the tunability and 2D nature of this material, these properties may lend themselves to new quantum electronic applications. Finally, I will also discuss our recent results on half-Heusler type semiconductor materials, including signatures of the topological inversion from trivial to inverted-band configuration in Bi-based superconducting materials. Host: Joseph Ross

30 Aug 2019, 4:00PM | MIST M102
Hosted By: Joseph Ross

Koushik Biswas, Arkansas State University.
Speaker: Koushik Biswas, Arkansas State University. Title: Hybrid and All-Inorganic Perovskite Halides: A First-Principles Perspective Abstract: The remarkable success of organic-inorganic Pb-halide perovskites as solar absorbers has drawn attention towards a broader family of ionic semiconductors having fascinating optoelectronic properties. Many of these compounds have favorable characteristics in terms of their band gap, carrier mobility, lifetime, and tolerance towards deleterious effect of deep defects. The crystals, loosely termed as perovskites, encompass a large group of structures built from metal-halogen octahedral framework, [MX6] (M = metal, X = halogen) that may lead to interesting properties and applications. Here, we will discuss the structure-property relation of CH3NH3PbI3 (MAPbI3) as representative of hybrid perovskites and Cs4PbBr6/CsPbBr3 as that of the all-inorganic variety. The hybrid halides feature organic cations whose molecular orientations clearly influence the fundamental electronic band gap and Rashba-type energy band splitting resulting in indirect gap behavior. Among the all-inorganics, highly luminescent CsPbBr3 nanocrystals and Cs4PbBr6/CsPbBr3 composite crystals have been reported as bright green emitters with fast radiative lifetime. CsPbBr3 (band gap ~2.3 eV) is built from corner sharing 3-dimensional (3D) network of [PbBr6], whose bulk crystals are not known as efficient light emitters. Cs4PbBr6 structure is comprised of almost disjointed [PbBr6] octahedra held together by Cs-Br bridges. Using density functional calculations, we will discuss the electronic consequences of the two different crystal structures. The emergence of strongly bound excitonic features in Cs4PbBr6 and Type-I band alignment with CsPbBr3 support the notion of carrier confinement causing fast, green luminescence in samples containing CsPbBr3 nano-islands embedded within Cs4PbBr6. Host: A. Belyanin

6 Sep 2019, 4:00PM | MIST M102
Hosted By: A. Belyanin

Fan Zhang, UT Dallas
Speaker: Fan Zhang, UT Dallas Title: A Tale of Two Majoranas Abstract: We are witnessing and participating the grand discovery of various topological states of matter. In this talk, I will introduce time-reversal-invariant topological superconductivity and its several ongoing experimental realizations. Uniquely, each 0D boundary of such a topological superconductor hosts one Majorana Kramers pair, producing tabletop supersymmetry, quantized tunneling conductance, and fractional Josephson effects beyond Kitaev's classification. I will then highlight the tantalizing roles played by the many-body interactions in such a platform. One prime example is the emergence of quadruple periodicity and fractionalized parafermions in a Josephson effect. Another paradigmatic setup is a weakly probed floating topological superconductor, in which a two-channel Kondo ground state becomes stabilized without fine tuning. Host: Ar. Abanov

13 Sep 2019, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Qi Zhou, Purdue University. CANCELED.
Speaker: Qi Zhou, Purdue University. CANCELED. Title: Synthetic quantum matters in ultracold atoms: from topological defects to curved spaces. Abstract: The well controllable atomic samples have allowed physicists to access a wide range of quantum phenomena unattainable in other systems. I will discuss a few such examples. The first example includes topological defects in high dimensions, ranging from Yang monopoles to continuous topological defects in five dimensions. A Yang monopole as a fundamental ingredient in non-abelian gauge theories was proposed back to seventy years ago. It was recently delivered for the first time in an experiment by tailoring laser-atom couplings in a five-dimensional parameter space. Moreover, highly tunable interactions between atoms could turn a Yang monopole into a much broader range of continuous topological defects. In another example, I will show how to create synthetic curved spaces, including a Hall cylinder and a Hall torus. Coupling internal degrees of freedom and orbital motions of ultracold atoms allows us to bypass constraints from physical laws and thread a finite effective magnetic flux through the surface of a torus or a cylinder. This development opens the door to exploring intriguing quantum phenomena inherent to the topology of spaces. Host: Ar. Abanov

20 Sep 2019, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Jeff Thompson, Princeton
Speaker: Jeff Thompson, Princeton Title: Spin dynamics of individually addressed Er3+ ions in a nanophotonic circuit Abstract: Individually addressed rare earth ions are a promising platform for quantum information processing. Erbium is particularly attractive as a single photon source and quantum memory for quantum networks, owing to its optical transition at 1.5 um, in the lowest-loss telecom band. A central challenge to utilizing individual rare earth ions is their low photon emission rates, which results from the dipole-forbidden nature of the intra-4f optical transitions. We have demonstrated a solution to this problem by coupling single Er3+ ion dopants in a Y2SiO5 crystal to a silicon nanophotonic circuit, where a photonic crystal cavity tuned to the ions’ resonance enhances the emission rate by nearly three orders of magnitude [1]. This has enabled the observation of single-photon emission from single Er3+ ions for the first time. More recently, we have leveraged the strong cavity modification of the spontaneous emission to control the spin selection rules, which enables single-shot quantum nondemolition measurement of the ion’s spin with 95% fidelity [2]. We have measured the spin T1 of single Er3+ ions for the first time and find that it exceeds 40 seconds. The spin coherence is limited by the nuclear spin bath, and I will discuss two strategies to mitigate this: dynamical decoupling and control of the spin bath, as well as the development of a new host material that has lower abundance of nuclear spins, TiO2 [3]. I will conclude with prospects for creating arrays of strongly interacting Er3+ spins using ion implantation, with sub-wavelength optical addressing using frequency multiplexing. [1] A. M. Dibos, M. Raha, C. M. Phenicie, and J. D. Thompson, Phys. Rev. Lett. 120, 243601 (2018). [2] M. Raha, S. Chen, C. M. Phenicie, S. Ourari, A. M. Dibos, and J. D. Thompson, Arxiv 1907.09992 (2019). [3] C. M. Phenicie, P. Stevenson, S. Welinski, B. C. Rose, A. T. Asfaw, R. J. Cava, S. A. Lyon, N. P. De Leon, and J. D. Thompson, ArXiv 1909.06304 (2019). Host: Ar. Abanov

27 Sep 2019, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Katja Nowack, Cornell
Speaker: Katja Nowack, Cornell Title: Spatially modulated superconductivity in microstructures Abstract: Although crystals of strongly correlated metals exhibit a diverse set of electronic ground states, few approaches to spatially modulate their properties exist. In this talk, I will discuss how we achieve spatial modulation of the superconducting state in focus ion beam (FIB) defined microstructures fabricated from single crystals of the heavy-fermion superconductor CeIrIn5. In a nutshell, differential thermal contraction of the substrate and the microstructures induces a non-trivial strain field in the device. This strain field results in a complex pattern of superconductivity due to the dependence of the superconducting transition temperature on the strength and direction of strain. We directly image the spatially modulated superconductivity using scanning superconducting quantum interference device microscopy. Devices with different geometry show that the obtained spatial modulation of superconductivity can be tailored in agreement with predictions based on finite element simulations. These results offer a new approach to manipulate strain-sensitive electronic order on micrometer length scales in strongly correlated matter. Host: Ar. Abanov

4 Oct 2019, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Liang Wu, University of Pennsylvania
Speaker: Liang Wu, University of Pennsylvania Title: Linear and nonlinear optical responses in chiral topological semimetals. Abstract: The fundamental difference between electrons in a solid and those in high-energy physics is the absence of Poincare symmetry in lattice systems. This gives rise to a much larger number of possible low energy excitations (known as multi-fold fermions) in solid-state physics. Recent theory and experiments show that semimetals with three-fold degeneracies can exist in nonmagnetic materials with chiral crystal structures. In my talk, I would like to discuss how do we use light to probe these low-energy excitations near the three-fold degenerate points by linear and nonlinear optical conductivity measurements. Host: Ar. Abanov

11 Oct 2019, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Vedika Khemani, Harvard, CANCELED.
Speaker: Vedika Khemani, Harvard, CANCELED. Title: Abstract: Host: Ar. Abanov

18 Oct 2019, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Francois Amet, Appalachian State University
Speaker: Francois Amet, Appalachian State University Title: Signatures of superconductivity in the quantum Hall regime. Abstract: In the quantum Hall regime, chiral electron and hole edge states can hybridize along a superconducting interface, as a result of the Andreev reflection process. This yields exciting electronic phenomena which are within experimental reach given recent progress in the fabrication of superconducting graphene heterostructures. Indeed, a robust quantum Hall effect can be induced at low field in these devices, well below the upper critical field of the superconductor. The two phases can therefore coexist. In this talk, I will discuss how the control of the electrostatic potential near the edges of the device allows the experimental realization of a superconducting interferometer relying on quantum Hall edge states. I will also discuss experimental signatures of chiral Andreev edge states at the interface of the superconductor. Host: Ar. Abanov

25 Oct 2019, 4:00PM | MPHY 578
Hosted By: Ar. Abanov

Nikolai Sinitsyn, LANL
Speaker: Nikolai Sinitsyn, LANL Title: Integrable Time-Dependent Quantum Hamiltonians Abstract: Time-dependence of parameters provides a new dimension for engineering quantum systems with unusual behavior. The limits of fast and slow (adiabatic) time-dependence are well studied. However, the intermediate regime is very poorly understood today because of the lack of proper theoretical methods and efficient numerical algorithms. This is the place where robust and unusual effects are still waiting to be uncovered. In this talk, I will describe the method to study dynamics of interacting spins with simple, e.g., ~t or ~1/t time-dependence of some of the parameters without any approximation. I define what it means for such a model to be integrable and then show three examples that reveal robust dynamic collective effects. One is the BCS model with decaying superconducting gap, which shows emergence of a fully thermalized state. Another is the model of cavity QED with linear optical frequency chirp, which demonstrates a sharp phase transition between fast and slow phases. The third model shows the effect of “dynamic spin localization” in a hysteresis loop of interacting spins. This effect has no counterpart in any known classical or quantum spin model. References: [1] NA Sinitsyn, EA Yuzbashyan, VY Chernyak, A Patra, C Sun. Integrable time-dependent quantum Hamiltonians, Phys. Rev. Lett. 120 190402 (2018) [2] F Li, VY Chernyak, and NA Sinitsyn. Quantum annealing and thermalization: insights from integrability, Phys. Rev. Lett. 121, 190601 (2018) Host: V. Pokrovsky

1 Nov 2019, 4:00PM | MIST M102
Hosted By: V. Pokrovsky

Nathaniel Gabor, UC Riverside
Speaker: Nathaniel Gabor, UC Riverside Title: Electron-hole liquid in a van der Waals heterostructure photocell at room temperature. Abstract: Condensation - the familiar process underlying the formation of clouds and the distillation of ethyl alcohol into whiskey - is the phase transition of gas into liquid droplets. In semiconductors, at sufficiently high electron-hole (e-h) densities or low temperatures, the gas of non-equilibrium electrons and holes may undergo condensation into one of several potential liquid-like phases. In this talk, I present recent results on the gas-to-liquid phase transition of electrons and holes in ultrathin van der Waals heterostructure photocells revealed through multi-parameter dynamic photoresponse microscopy (MPDPM). By combining rich visualization with comprehensive analysis of very large data sets acquired through MPDPM, we find that ultrafast laser excitation at a graphene-molybdenum ditelluride-graphene interface leads to the abrupt formation of ring-like spatial patterns in the photocurrent response as a function of increasing optical power. These patterns, together with extreme sublinear power dependence and picosecond-scale photocurrent dynamics, provide strong evidence for the formation of a two-dimensional e-h liquid. While our imaging experiments mark the first observation (in over 50 years of study) of an e-h liquid at room temperature, I will discuss our results within the greater context of strongly correlated electronic condensates. Host: Ar. Abanov

8 Nov 2019, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Ming Yi, Rice University
Speaker: Ming Yi, Rice University Title: A walk towards orbital-selectivity in iron-based superconductors Abstract: Electron correlation effects give rise to a variety of emergent phenomena in quantum materials-high temperature superconductivity, electronic nematicity, Mott insulating phase, and magnetism. In the multi-orbital iron-based superconductors, electronic correlations are manifested in an orbital-dependent way, realizing all of the above in different parameter regimes. In this talk, I will present experimental evidence from angle-resolved photoemission spectroscopy on systematic evolution of multi-orbital correlation effects across the iron-based superconductor material basis. In particular, I will present spectroscopic evidence for tendencies towards an orbital-selective Mott phase in the iron-chalcogenides as tuned via both temperature and isovalent substitution, as manifested in the vanishing of coherent quasiparticle spectral weight, divergence of effective mass, and reorganization of low energy electronic states, revealing the strong presence of correlation effects in this class of multi-orbital superconductors. Host: Ar. Abanov

15 Nov 2019, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Peter Armitage, Johns Hopkins University, RESCHEDULED for 2020-04-17
Speaker: Peter Armitage, Johns Hopkins University, RESCHEDULED for 2020-04-17 Title: Abstract: Host: Ar. Abanov

22 Nov 2019, 4:00PM | MIST M102
Hosted By: Ar. Abanov

Speaker: Thanksgiving Title: Abstract: Host:

29 Nov 2019, 4:00PM | MIST M102

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