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

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


Nikolai Sinitsyn, LANL
Speaker: Nikolai Sinitsyn, LANL Title: TBA Abstract: Host: V. Pokrovsky

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


Nathaniel Gabor, UC Riverside
Speaker: Nathaniel Gabor, UC Riverside Title: TBD Abstract: Host: Ar. Abanov

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


Ming Yi, Rice University
Speaker: Ming Yi, Rice University Title: TBA Abstract: Host: Ar. Abanov

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


Peter Armitage, Johns Hopkins University
Speaker: Peter Armitage, Johns Hopkins University Title: TBA Abstract: Host: Ar. Abanov

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


Thanksgiving
Speaker: Thanksgiving Title: Abstract: Host:

29 Nov 2019, 4:00PM | MIST M102


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