Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session X14: Topological Materials  Heterostructures and spectroscopyFocus

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Sponsoring Units: DMP Chair: Guang Bian, Univ of Missouri  Columbia Room: LACC 304B 
Friday, March 9, 2018 8:00AM  8:36AM 
X14.00001: Interplay of orbital effects and nanoscale strain in topological crystalline insulators Invited Speaker: Vidya Madhavan Orbital degrees of freedom can have pronounced effects on the fundamental properties of electrons in solids. In addition to influencing bandwidths, gaps, correlation strength and dispersion, orbital effects have been implicated in generating novel electronic and structural phases, such as JahnTeller effect and colossal magnetoresistance. In this talk I will describe how we use a combination of STM studies and first principles calculations to reveal the mechnaism by which the orbital nature of bands can result in nontrivial effects of strain on band structure. Using quasiparticle interference we study the influence of strain on the electronic structure of a heteroepitaxial thin film of a topological crystalline insulator SnTe. By mapping the effects of uniaxial strain on the band structure we find a surprising effect where strain applied in one direction has the most pronounced influence on the band structure along the perpendicular direction. Our theoretical calculations indicate that this can be attributed to the orbital nature of the conduction and valence bands. Our results imply that a microscopic model capturing strain effects on band structure must include a consideration of the orbital nature of bands. 
Friday, March 9, 2018 8:36AM  8:48AM 
X14.00002: Generating Dirac mass by local symmetry breaking in SnTe topological crystalline insulator Subhasish Mandal, Omur Dagdeviren, Ke Zou, Chao Zhou, Georg H. Simon, Frederick Walker, Charles H. Ahn, Udo Schwarz, Eric I. Altman, Sohrab IsmailBeigi Topological phases of matter provide an exciting field of research and may be a key to future electronic devices. In a topological insulator, metallic electronic surface states (SS) are protected by time reversal symmetry. Topological crystalline insulators (TCIs) are a newer class of materials for which electronic SS are topologically protected by certain crystallographic symmetries. Breaking global crystal symmetry causes the SS to become gapped and nonmetallic. But what happens when the crystal symmetry is broken locally? 
Friday, March 9, 2018 8:48AM  9:00AM 
X14.00003: The Realization of the Axion Insulator State in Quantum Anomalous Hall Sandwich Heterostructures Di Xiao, Jue Jiang, Jaeho Shin, Wenbo Wang, Fei Wang, YiFan Zhao, ChaoXing Liu, Weida Wu, Moses Chan, Nitin Samarth, CuiZu Chang The topological magnetoelectric (TME) effect is a special version of the magnetoelectric effect, showing quantized response functions. The Z_{2} topological insulators (TIs) with gapped surfaces and time reversal invariant bulk state are predicted to show the TME effect, and are called axion insulators. We present the observation of the axion insulator state in a magnetically doped TI ((Bi_{x}, Sb_{1x})_{2}Te_{3}) sandwich heterostructure, where the top and bottom layers are doped with V and Cr, respectively, and are separated by an undoped TI middle layer [arXiv: 1710.00471]. Magnetic force microscopy measurements confirm the existence of parallel and antiparallel magnetization of the two magnetic layers by sweeping the external magnetic field. Transport measurements unambiguously demonstrate the quantum anomalous Hall (QAH) state (parallel configuration), and the axion insulator state (antiparallel), where we observe a large longitudinal resistance and vanishing longitudinal and Hall conductance. Our findings thus show evidence for a phase of matter distinct from the established QAH state and provide a promising platform for the realization of the TME effect. 
Friday, March 9, 2018 9:00AM  9:12AM 
X14.00004: Optoelectronic detection of magnetization in topological insulator/magnetic insulator heterostructure devices Timothy Pillsbury, Yu Pan, Yunqiu (Kelly) Luo, James Kally, Hailong Wang, Anthony Richardella, Tao Liu, Mingzhong Wu, Roland Kawakami, Nitin Samarth Topological Insulators (TIs) are of interest for ‘topological spintronics’ applications due to the inherent spin texture of the surface states, the large spinorbit coupling, and the high spin currenttocharge current conversion efficiency. Heterostructure devices that interface TIs with magnetic insulators (MIs) are of particular interest because the charge current in the TI layer is not shunted by the MI layer. However, the latter characteristic also makes it challenging to electrically detect the magnetization state of hybrid TI/MI devices that use chargespin conversion for manipulating the magnetization of the MI layer. To solve this problem, we demonstrate that the magnetic state of an insulating yttrium iron garnet thin film can be probed by using a spindependent photocurrent generated optically in a proximal TI ((Bi,Sb)_{2}Te_{3}). We study the magnetic fielddependence of the photocurrent as a function of light intensity, light wavelength, and chemical potential over a temperature range from 25 K to room temperature. We account for our observations using the recently discovered photospinvoltaic effect [Nat. Phys. 12, 861 (2016)]. 
Friday, March 9, 2018 9:12AM  9:24AM 
X14.00005: Ultrafast SpintoCharge Conversion in Topologicalinsulator/Ferromagnet Heterostructures Xinbo Wang, Liang Cheng, Dapeng Zhu, Yang Wu, Mengji Chen, Daming Zhao, JianXin Zhu, Justin Song, Marco Battiato, Hyunsoo Yang, Ee Min Chia Strong spinorbit coupling, resulting in the formation of spinmomentumlocked surface states in topological insulators (TIs), is predicted to possess superior spintocharge conversion (SCC) efficiency via the inverse RashbaEdelstein effect and/or inverse spin Hall effect. The knowledge of the timescale of such SCC processes is crucial for the optimization of TIbased spintronic devices. Here, we investigate the ultrafast SCC in topologicalinsulator/ferromagnet heterostructures via terahertz emission spectroscopy. We demonstrate highlyefficient THz emission from Bi_{2}Se_{3}/Co heterostructures, where the surface states play a dominant role. Compared to the instantaneous shift current terahertz emission mechanism in Bi_{2}Se_{3}, we directly observe a temporal delay in the spincurrentrelated component — 0.12 ps, which characterizes the SCC timescale in TIs. In addition, we show that the SCC efficiency is temperature independent in Bi_{2}Se_{3}_{ }as expected from the nature of the surface states. Our study provides fundamental insights into the ultrafast SCC processes, and demonstrate the practical applications of TIs, especially the surface states, in terahertz spintronic devices at room temperature. 
Friday, March 9, 2018 9:24AM  9:36AM 
X14.00006: Linear magnetoresistance in topological crystalline insulator PbTe/SnTe heterostructure: van der Pauw vs. Corbino measurements ChiehWen Liu, Kasun Viraj Madusanka Nilwala Gamaralalage Premasiri, Shuhao Liu, Feng Wei, Song Ma, Zhidong Zhang, Xuan Gao Nonsaturating linear magnetoresistance (MR) effect is a novel magnetotransport phenomenon widely observed in various topological materials with gapless dispersion. Despite its ubiquitous presence in graphene, topological insulators, topological crystalline insulators, Dirac semimetals, and Weyl semimetals, the origin of the large linear MR effect has not been clearly resolved. The two leading theories explaining the linear MR effect are the quantum linear MR model (by Abrikosov) and the mobility fluctuation induced classical linear MR model (by Littlewood and Parish). Here, we report a magnetotransport study on heterostructures of topological crystalline insulator(TCI) SnTe and trivial insulator PbTe and elucidate the linear MR effect in TCI SnTe by comparing the MR measurement in the standard van der Pauw geometry vs. the Corbino geometry where the longitudinal magnetoconductivity σ_{xx}(B) is directly measured. Our results revealed consistency between the two geometries and show that the Drude model clearly fails to account for the magnetoconductivity data. We will then discuss the implications of our data in the two leading models of linear MR. 
Friday, March 9, 2018 9:36AM  9:48AM 
X14.00007: Van Der Waals Heterostructure of Insulator/Topological Insulator/Insulator for DualGated Quantum Capacitance Su Kong Chong, Kyu Bum Han, Akira Nagaoka, Jared Harmer, Ryuichi Tsuchikawa, Taylor D. Sparks, Vikram Deshpande Threedimensional topological insulator (3D TI) with two important signatures, namely Dirac conelike surface states and nondegenerate momentumlocked spin is an ideal system for fundamental physics studies. Quantum transport of 3D TI shows the integer quantum Hall effect arising from the addition of the top and bottom surface states. This yields the interesting ν=±1 (dissipationless) and ν=0 (dissipative) quantum states. As the top and bottom surface states are always coupled to each other, independent gate tuning on both surfaces are highly desirable to fully understand the quantum Hall states in 3D TI. In this work, I will discuss the different approaches in making the dualgated BiSbTeSe_{2} (BSTS) devices by van der Waals stacking in a sequent of insulator/3D TI/insulator. Such a device configuration can also be treated as a series of plate capacitors formed between the top (bottom)gate electrodes and top (bottom) surface states. I will also present the recent progress on quantum capacitance measurement for our dualgated BSTS devices. 
Friday, March 9, 2018 9:48AM  10:00AM 
X14.00008: Epitaxial Growth of Topological Insulator Bi_{2}Se_{3} Thin Films on the Quantum Spin Liquid Candidate αRuCl_{3} by Molecular Beam Epitaxy Joon Young Park, Janghyun Jo, YoungJune Kim, Miyoung Kim, GyuChul Yi Heterostructures with topological insulators (TIs) serve as a breeding ground for novel states of matter; various classes of materials ranging from semiconductors to magnets and superconductors have been combined with TIs. Quantum spin liquids (QSLs) have also been brought into focus as a new topological material. However, TI/QSL heterostructure, a stage where the two different electronic states can compete, coexist, and collaborate, has remained unexplored. Heteroepitaxial growth, one of the most promising methods of preparing uniform materials with welldefined structural properties, is an essential step in investigating emerging physical phenomena in real TI/QSL heterostructures. Here, we present the epitaxial growth of Bi_{2}Se_{3} thin films on αRuCl_{3} layers using MBE and their structural characteristics. The films with an atomically smooth and uniform surface were prepared through the twostep growth process and examined by AFM. We found, using TEM investigation, a welldefined epitaxial relationship between Bi_{2}Se_{3} and αRuCl_{3} and the formation of commensurate supercells despite a lattice misfit of –60%, which is attributed to the weak van der Waals interaction on the interface. We believe that our work opens the door to further exploration of the physics of the heterostructure. 
Friday, March 9, 2018 10:00AM  10:12AM 
X14.00009: Spinmomentum locked interface states in Bi2Se3Ni bilayer from firstprinciples Kyungwha Park, YiTing Hsu, EunAh Kim Topological insulators (TIs) exhibit one of new quantum states of 
Friday, March 9, 2018 10:12AM  10:24AM 
X14.00010: First Principles Calculations of the Topological Insulator Stanene on an Al_{2}O_{3} Substrate Stephen Eltinge, Stephen Albright, Minjung Kim, Rui Peng, Ke Zou, Frederick Walker, Charles Ahn, Sohrab IsmailBeigi Topological insulators are a class of materials under continuing investigation due to their potential for hosting robust currentcarrying surface or edge states. Stanene, the twodimensional monolayer form of tin, has been predicted to be a 2D topological insulator due to its large spinorbit interaction, but a clear experimental demonstration of this behavior has eluded observation. In part, this is because the choice of growth substrate has a major effect on the electronic and topological properties of stanene due to the imposition of both epitaxial strain and bonding interactions between the substrate and the stanene monolayer. We present firstprinciples density functional theory (DFT) calculations of monolayer stanene grown on the (0001) surface of alumina, Al_{2}O_{3}. We perform a detailed analysis of the binding energy and electronic structure of stanene on Al_{2}O_{3}, and compute the system’s Z_{2} topological invariant. In addition, we describe other potential structures of tin on the alumina surface, and analyze their competition with monolayer stanene. 
Friday, March 9, 2018 10:24AM  10:36AM 
X14.00011: Symmetryprotected degeneracies in the electronic band structure in a phosphorene oxide Sungjong Woo, SeoungHun Kang, Jejune Park, YoungKyun Kwon Phosphorene, a single layer of black phosphorus, has attracted a lot of interest since its isolation from black phosphorus by mechanical exfoliation. Since it is extremely reactive with oxygen owing to the lone pairs of electrons in the phosphorous atoms, oxidation is unavoidable during synthesis and fabrication processes. The oxidation removes inversion symmetry in the system while keeping two nonsymmorphic symmetries allowing rich features in the electronic band structure. We have found two separate double degenerate band lines cause by different symmetry origins as well as a circular line node and 4fold degenerate Dirac point without an inversion symmetry. Even though the electronic filling of the system is not 4n+2, such energy levels may be reached with reasonable electronic doping. In the same system, we have also observed a topological phase transition controlled by tensile strain due to level inversion based on firstprinciples calculations and explicit evaluation of Z_{2} topological invariant. 
Friday, March 9, 2018 10:36AM  10:48AM 
X14.00012: Geometric Protection of Helical Edge States in Trivial Cylindrical Quantum Dots Denis Candido, Michael Flatté, Carlos Egues Here we investigate electronic and transport properties of InAs_{1x}Bi_{x} quantum dots (QDs) in both topological and trivial regimes. We show through calculations that Bialloyed InAs quantum wells become 2D topological insulators with large inverted band gaps ~ 30 meV (> k_{B}T) for well widths larger than 7nm and x=0.15. By solving the proper BHZ model we find for cylindrical soft and hard walls confinement analytical expressions for the wave functions and circulating currents with energy levels determined from a transcendental equation. Interestingly, we find that trivial QDs have counterpropagating helical edgelike valence states that are shown to be "geometrically protected" over a wide range of QD radii. We calculate the circulating current densities for both topological and trivial edge states, where we find a higher density peak for trivial QDs while the integrated currents over half of QD cross section show no substantial difference. We have also calculated via Green's function the twoterminal linear conductance and find distinctive features between both regimes due to the energy degeneracy of the bulk and edgelike states in trivial QDs. 
Friday, March 9, 2018 10:48AM  11:00AM 
X14.00013: Electrostatic tuning of strongly coupled superconductorsemiconductor heterostructures Andrey Antipov, Arno Bargerbos, Bela Bauer, Enrico Rossi, Roman Lutchyn We study the effect of gateinduced electric fields on the properties of semiconductorsuperconductor heterostructures. Using a selfconsistent PoissonSchrodinger approach that describes the semiconductor and the superconductor on equal footing, we are able to access the strong coupling regime and identify the impact of the applied gate voltage on the coupling between semiconductor and superconductor. We show how the induced superconducting gap is modified by redistributing the density of states across the interface upon application of gate voltage. We show that these changes of the heterostructure wave functions can significantly modify the effective gfactor. We map out the topological phase diagrams of InAs/Al and InSb/Al wires and show the redistribution of Majorana states across SM/SC interface. 
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