Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B04: Driven Topological Quantum MaterialsInvited

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Sponsoring Units: DCMP DMP Chair: JianXin Zhu, Los Alamos Natl Lab Room: LACC 151 
Monday, March 5, 2018 11:15AM  11:51AM 
B04.00001: Using Ultrashort Light Pulses to Probe and Control Quasiparticle Dynamics in Topological Materials Invited Speaker: Rohit Prasankumar Low energy excitations can shed light on the interplay between different degrees of freedom in complex materials. Ultrashort optical and terahertz (THz) pulses can be used to both drive and probe these excitations. This is particularly useful in topological materials, since coupling these excitations to light can potentially enable control of their properties. Here, the use of ultrafast THz spectroscopy to separately probe the dynamics of surface and bulk carriers and drive nonlinear phonon dynamics in the topological insulator Bi_{2}Se_{3} will be discussed. We will also describe recent experiments using nonlinear optical techniques to examine quasiparticle dynamics in the Weyl semimetals WTe_{2} and TaAs. 
Monday, March 5, 2018 11:51AM  12:27PM 
B04.00002: Light controlled topological phase transitions in multiorbital and frustrated magnetic systems Invited Speaker: Thomas Devereaux Spurred by recent progress in melting, enhancement and induction of electronic order out of equilibrium, a tantalizing prospect concerns instead accessing transient Floquet steady states via broad pump pulses, to affect electronic properties. Here, we consider a twopronged approach to manipulate the topology of a band insulator, as well as topological order in a Mott insulator. We first consider monolayer transitionmetal dichalcogenides (TMDCs), and show that their lowenergy description as massive 2D relativistic fermions fails to hold for optical pumping. Instead, the added complexity of a realistic materials description leads to a novel mechanism to optically induce topologicallyprotected chiral edge modes, facilitating opticallyswitchable conduction channels that are insensitive to disorder. We develop a strategy to understand nonequilibrium FloquetBloch bands and topological transitions directly from ab initio calculations, and illustrate for the example of WS2 that control of chiral edge modes can be dictated solely from symmetry principles and is not qualitatively sensitive to microscopic materials details. Second, we extend these ideas to strongly correlated systems and show that pumping frustrated Mott insulators with circularlypolarized light can drive the effective spin system across a phase transition to a chiral spin liquid (CSL). We show that the transient time evolution of a Kagome lattice Hubbard model is well captured by an effective spin description, where circular polarization promotes a staggered scalar spin chirality directly to the Hamiltonian level. We fingerprint the ensuing phase diagram and find a stable photoinduced CSL in proximity to the equilibrium ground state. The results presented suggest new avenues to marry dynamical symmetry breaking, strong interactions, and ab initio materials modelling, to access elusive phase transitions that are not readily accessible in equilibrium. 
Monday, March 5, 2018 12:27PM  1:03PM 
B04.00003: Photoemission Studies of Topological Superconducting Materials Invited Speaker: Madhab Neupane A threedimensional (3D) topological insulator (TI) is a crystalline solid, which is an insulator in the bulk but features spinpolarized Dirac electron states on its surface. In 2007, the first 3D TI was discovered in a bismuthbased compound. The discovery of the first TI tremendously accelerated research into phases of matter characterized by nontrivial topological invariants. Not only did the 3D TI itself attract great research interest, it also inspired the prediction of a range of new topological phases of matter. Primary examples include the topological Kondo insulator, the topological 3D Dirac, Weyl, and nodalline semimetals, the topological crystalline insulator, and the topological superconductor. Specifically, recent predictions have claimed that longsoughtout Majorana fermions can be realized at the interface between a topological insulator and a superconductor and may shift our scientific trajectory from research to applications in topological quantum computing. In this talk, I will discuss the electronic properties of topological superconducting materials obtained by using momentum, spin, and timeresolved photoemission spectroscopy. Developing our understanding of topological superconducting materials will guide us on a path to realize the properties of Majorana fermion quasiparticle states associated with topological superconductivity. 
Monday, March 5, 2018 1:03PM  1:39PM 
B04.00004: Spincharge conversion in topological materials via THz emission spectroscopy Invited Speaker: Ee Min Chia In this talk I will show THz emission data from spintocharge conversion processes occuring in topological material (TM)/ferromagnet (FM) heterostructures, where the TM is the Bi_{2}Se_{3} topological insulator or the MoS_{2} 2D transition metal dichalcogenide, both of which possess large spinorbit coupling. By measuring the emitted THz pulses in the time and frequency domain, as a function of pump polarization angle, pump fluence, magnetic field direction, sample direction, TM thickness, FM thickness and temperature, both in the pure TM film and the TM/FM heterostructure, we demonstrate highlyefficient THz emission from both Bi_{2}Se_{3}/Co and MoS_{2}/Co. In Bi_{2}Se_{3}/Co, we conclude that the Bi_{2}Se_{3} surface states play a dominant role in THz emission via the inverse RashbaEdelstein effect. Compared to the instantaneous shift current THz emission mechanism in pure Bi_{2}Se_{3}, we observe a temporal delay in the spincurrentrelated component ~0.12 ps, which characterizes the spintocharge conversion timescale in topological insulators. In MoS_{2}/Co, we observe and disentangle the contributions of inverse spin Hall effect and inverse RashbaEdelstein effect to the spintocharge conversion. Our analysis gave estimates for the spin Hall angle, which is a measure of the spincharge conversion efficiency, and the spin diffusion length, that enables us to resolve the spin diffusion dynamics from the Co layer into the TM surface. 
Monday, March 5, 2018 1:39PM  2:15PM 
B04.00005: Topological frequency conversion in strongly driven quantum systems Invited Speaker: Gil Refael When a small quantum system is subject to multiple periodic drives, it may realize multidimensional topological phases. In my talk, I will explain how to make such constructions, and show how a spin1/2 particle driven by two ellipticallypolarized light beams could realize the BernevigHughesZhang model of 2 topological insulators. The observable consequence of such a construction is quantized pumping of energy between the two drive sources. I will also show that the topological pumping quantization is not only robust against smooth temporal disorder, but even enforced by it. 
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