Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F3: Invited Session: Helical Edge States in INAs/GaSb Bilayers |
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Sponsoring Units: DCMP Chair: Kathryn Moler, Stanford University Room: 002AB |
Tuesday, March 3, 2015 8:00AM - 8:36AM |
F3.00001: Quantized Edge Transport in InAs/GaSb quantum Spin Hall Insulator Invited Speaker: Rui-Rui Du |
Tuesday, March 3, 2015 8:36AM - 9:12AM |
F3.00002: Imaging current in quantum spin Hall insulator InAs/GaSb Invited Speaker: Eric Spanton Scanning superconducting quantum interference device (SQUID) microscopy allows us to visualize how currents flow in materials by imaging magnetic fields. I will give an overview of our technique and focus on the quantum spin Hall edge states we observed in Si-doped InAs/GaSb quantum wells. The main feature of 2D topological insulators is the topologically-protected edge modes that are a result of their special band structure. We used a SQUID to image current in the edge modes, which are present when the chemical potential lies in or near the insulating gap. The unique spin-texture of the edge states restricts how electrons can backscatter, leading to ballistic transport in small enough devices. In more resistive, longer devices which we study, the temperature dependence of the resistance of the edges due to backscattering is flat and does not match any of the allowed backscattering mechanisms which have been theoretically investigated. [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:48AM |
F3.00003: Magnetotransport in the topological insulator candidate InAs/GaSb Invited Speaker: Thomas Ihn InAs/GaSb quantum wells have been proposed as an electrically tunable two-dimensional topological insulator system. Transport can be tuned from the electron to the hole regime whereby the Fermi-energy crosses the hybridization gap, where topological edge states are predicted to exist. We have investigated this material system using dc magneto-transport measurements at cryogenic temperatures. In high-mobility large-area samples, a resistivity maximum is observed at the charge-neutrality point. It increases strongly with magnetic field. At the same time, a strong non-local resistance appears which we describe by a model of helical and dissipative quantum Hall edge channels shorted by residual bulk conductivity. In an attempt to reduce the bulk conductivity, we have grown samples with slightly impure Gallium. Large-area devices show a peak resistance at the charge neutrality point enhanced by almost three orders of magnitude compared to the high-mobility samples. A requirement for observing the topological insulator state is the fabrication of devices smaller than the inelastic scattering length. We have developed an optimized fabrication recipe by comparing samples produced with wet and dry etching. The former turns out to be favorable for obtaining smooth edge potentials and a width-independent electron density. With this fabrication technology we have produced small-area Hall bar devices with widths and contact separations in the micrometer range. Surprisingly these devices do not exhibit a resistance maximum at charge neutrality like large-area devices, but rather show a plateau-like local resistivity at negative gate voltages. At the same time the Hall density saturates in these devices at finite electronic densities instead of turning into the hole regime, and a systematic non-local resistance signal appears in all devices. We discuss these findings in view of conflicting interpretations involving either fabrication issues or the proposed helical edge modes. [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:24AM |
F3.00004: One-dimensional topological edge states of bismuth bilayers Invited Speaker: Ilya Drozdov A quantum spin Hall (QSH) state of mater, also known as a two-dimensional (2D) topological insulator, is distinguished by one-dimensional (1D) chiral edge modes propagating along the perimeter of the system without backscattering. Among the first systems predicted to be a 2D topological insulator are bilayers of Bismuth (Bi) [1]. Despite being a well-known QSH candidate system which should in principle be accessible to scanning tunneling microscopy (STM) techniques, the experimental attempts carried out so far have suffered from edge imperfections and coupling of the edge states to the substrate. In this talk I will present recent STM experiments on bulk Bi crystals [2] which show that a subset of the predicted Bi-bilayers' topological edge states are in fact decoupled from the states of the substrate which makes it possible to probe the 1D electronic channels experimentally using spectroscopic STM techniques. Spectroscopic features observed in STM are directly compared to model calculations. Furthermore, unique electronic structure of topological edge modes of Bi allows for quantum interference of edge-mode quasi-particles in confined geometries, which is visualized by STM and allows to reveal the absence of backscattering of electrons in the 1d edge channels - the key property of the QSH systems resulting in their remakable coherent propagation. Additionally, I will present comparison to theoretical models of the edge state along with supporting experimental study of Bi(111) surface state's electronic structure. \\[4pt] [1] Murakami, S.~Quantum spin Hall effect and enhanced magnetic response by spin--orbit coupling.~Phys. Rev. Lett. 97, 236805 (2006).\\[0pt] [2] Ilya K. Drozdov, A. Alexandradinata, Sangjun Jeon, Stevan Nadj-Perge, Huiwen Ji, R. J. Cava, B. Andrei Bernevig, and Ali Yazdani, One-dimensional topological edge states of bismuth bilayers, Nature Physics 10, 664--669 (2014) [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 11:00AM |
F3.00005: Quantum spin Hall effect in InAs/GaSb bilayers subject to exciton condensation and magnetic field Invited Speaker: Dmitry Pikulin Motivated by the recent experiments we study the phase diagram of the bilayer InAs/GaSb quantum wells in the presence of electron-electron interactions. The interactions lead to formation of thermodynamically stable exciton condensate. We show that in the presence of condensate but without external magnetic field the bilayer can be in three distinct insulating phases: trivial, topological, and spontaneously breaking time-reversal symmetry ones. In the applied magnetic field the bilayer remains gapped and undergoes a series of phase transitions changing from quantum spin Hall-like state to trivial insulator. We suggest transport and spectroscopic measurements for future experiments to substantiate our picture. [Preview Abstract] |
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