Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F08: Two-dimensional Topological Insulators: Transport (II) |
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Sponsoring Units: DCMP Room: LACC 153C |
Tuesday, March 6, 2018 11:15AM - 11:27AM |
F08.00001: Zitterbewegung and Bulk-Edge Landau-Zener Tunneling in 2D Topological Insulators Poliana Penteado, Gerson Ferreira, Renan Maciel, Carlos Egues We investigate the ballistic dynamics and the Landau-Zener tunneling between edge and bulk states in 2D topological insulators. In bulk, we use the Ehrenfest theorem to show that an external in-plane electric field not only drifts the packet longitudinally but also induces a trembling motion (Zitterbewegung) and a transverse side-jump, whose direction, although dependent on the gap sign, is finite for both the trivial and nontrivial topological regimes. For finite ribbons of width W, we show that the Landau-Zener tunneling between bulk and edge states vanish for large W as their electric field induced coupling decays with W^{-3/2}. This is demonstrated by expanding the time-dependent Schrödinger equation in terms of Houston states. We conclude that, contrary to what has been proposed, we cannot picture the quantum spin Hall states as arising from Zitterbewegung bulk trajectories ‘leaking’ into the edge states. |
Tuesday, March 6, 2018 11:27AM - 11:39AM |
F08.00002: Temperature dependence of the quantized spin Hall conductance in high mobility HgTe quantum wells Saquib Shamim, Pragya Shekhar, Kalle Bendias, Andreas Budewitz, Raimund Schlereth, Philipp Leubner, Hartmut Buhmann, Laurens Molenkamp The theoretically predicted helical edge states of a quantum spin Hall insulator were observed experimentally in the HgTe quantum wells (QWs), leading to the discovery of a new topological state of matter. Since then, the research has evolved dramatically with several theoretical and experimental investigations on HgTe QWs as well as other topological insulator materials. Here, we present temperature dependent measurements of the quantum spin Hall effect in HgTe QWs. The micro Hall bars of HgTe QWs have been fabricated using a refined wet etching process, which results in high carrier mobility for sample dimensions of the order of few microns. The spin Hall conductance is quantized to 2e^{2}/h in the temperature range 30mK to 15 K. At T > 15 K, the conductance decreases following a power law dependence on temperature. Our results are significant for understanding the nature of backscattering mechanisms relevant in the quantum spin Hall regime in HgTe QWs. |
Tuesday, March 6, 2018 11:39AM - 11:51AM |
F08.00003: Microwave Photocurrent From the Edge States of InAs/ GaSb and InAs/InGaSb Bilayers Jie Zhang, Tingxin Li, Gerard Sullivan, Rui-Rui Du We measure microwave photocurrent in devices made from InAs/GaSb and InAs/InGaSb bilays, where both insulating bulk state and conducting edge state were observed in the inverted-band regime, consistent with the theoretical prediction for a quantum spin Hall (QSH) insulator. It has been theoretically proposed that microwave photocurrent could be a unique probe in studying the properties of QSH edge states. To distinguish possible photoresponse between the bulk state and the edge state, we prepare two types of samples, i.e., Hall bar and Corbino disk, from the same wafer; in the latter the edge states are shunt by metal electrodes. A back gate is used to tune Fermi level into the bulk gap. Results show that the Corbino disk samples have a negligible photocurrent in the bulk gap. On the contrary we observe clear photocurrent signals from the Hall bar samples in the bulk gap. This finding suggests that the photocurrent may carry information concerning the electronic properties of the edge states. We have studed the frequency, temperature, and magnetic field dependence of the photocurrent. A brief discussion along with the data will be presented in the talk. |
Tuesday, March 6, 2018 11:51AM - 12:03PM |
F08.00004: Transport in InAs/GaSb Composite Quantum Wells with an Oxide Top Barrier Xiaoxue Liu, Tingxin Li, Gerard Sullivan, Rui-Rui Du InAs/GaSb composite quantum wells (CQWs) have now attracted much attentions with the exciting prospect that a variety of topological phases could be hosted in this material. Typical CQWs incorporate AlGaSb as top and bottom barriers by molecular epitaxy, providing nearly lattice - matched interfaces. On the other hand it may be desirable to replace AlGaSb barriers by oxide/ high - k* dielectrics for the purpose of enhancing gating functionality and device reliability. Here we report preliminary results of low temperature quantum transport on devices with an oxide top barrier, where the MBE grown AlGaSb barrier was removed by selective etching and an oxide layer was subsequently deposited by ALD. We will show the transport results of devices with ALD dielectrics including HfO_{2}/ZrO_{2}/Al_{2}O_{3}. |
Tuesday, March 6, 2018 12:03PM - 12:15PM |
F08.00005: Gapless insulating edges of dirty interacting topological insulators Yang-Zhi Chou, Rahul Nandkishore, Leo Radzihovsky We demonstrate that a combination of disorder and interactions in a two-dimensional bulk topological insulator can generically drive its helical edge insulating. We establish this within the framework of helical Luttinger liquid theory and exact Emery-Luther mapping. The gapless glassy edge state spontaneously breaks time-reversal symmetry in a `spin glass' fashion, and may be viewed as a localized state of solitons which carry half integer charge. Such a qualitatively distinct edge state provides a simple explanation for heretofore puzzling experimental observations. This phase exhibits a striking non-monotonicity, with the edge growing less localized in both the weak and strong disorder limits. |
Tuesday, March 6, 2018 12:15PM - 12:27PM |
F08.00006: Excitonic Correlation in Coulomb-Coupled Electron-Hole Double Layers Wu Xing-Jun, Rui-Yuan Liu, Wenkai Lou, Kai Chang, Gerard Sullivan, Rui-Rui Du In a spatially separated electron-hole system such as InAs-GaSb-based quantum wells, a number of phases and phase transitions are theoretically predicted to occur, including quantum spin Hall insulator, excitonic insulator, and broken symmetry states, such as nematic phase or Chern insulator. Motivated by this exciting prospect and with the advant of high quality semiconductor material grown by MBE technique we explore low temperature electrical transport properties in InAs/GaInSb bilayers with a AlSb middle barrier. The devices were made with flip-chip technique to facilitate dual-gate control of carrier densities. Preliminary results concerning excitonic correlation in this system will be presented. |
Tuesday, March 6, 2018 12:27PM - 12:39PM |
F08.00007: Conductance Oscillations in the Hybridization Gap of InAs/GaSb Quantum Spin Hall Insulators Zhongdong Han, Tingxin Li, Long Zhang, Gerard Sullivan, Rui-Rui Du Motivated by recent theoretic works [1, 2] we study the electrical transport properties of InAs/GaSb quantum spin Hall (QSH) insulator under perpendicular magnetic fields up to 35 T. In this talk, we report anomalous conductance oscillations in the hybridization gap for both InAs/GaSb and InAs/GaInSb systems. The bulk oscillations, which are approximately periodic in 1/B, originate from the Landau spectrum in inverted band structure, and the observed period is closely related to the degree of band inversion. Simultaneously, we observe a strong conductance peak emerging at field B > 11 T. The strong peak does not follow the above-mentioned 1/B pattern and is accompanied by increasing noise at its onset. We interpret that the strong peak represents a metallic phase arising from gap-closing during the topological transition between the QSH phase and the integer quantum Hall phase. In tilted magnetic fields, the conductance peak shows distinctly different behaviors, depending on shallow inverted or deeply inverted regime. |
Tuesday, March 6, 2018 12:39PM - 12:51PM |
F08.00008: Tuning the charge states in InAs/GaSb and InAs/GaInSb composite quantum wells by persistent photoconductivity Tingxin Li, Bingbing Tong, Zhongdong Han, Chi Zhang, Gerard Sullivan, Rui-Rui Du Inverted InAs/GaSb quantum wells (QWs) and InAs/GaInSb QWs have been proved to support the quantum spin Hall states. In this talk, we will show how to use the persistent photoconductivity (PPC) to tune the bulk band structure and carrier densities in inverted InAs/GaSb QWs and InAs/GaInSb QWs. Experimentally, we used light-emitting diodes (LEDs) with different light-emitting wavelengths to illuminate the samples at low temperature. We found the persistent photoconductivity in these system could be negative or positive, depending on the specific doping structure and the illuminating photon energy. Compared to the widely-used electrostatically gating method, our findings provide a more flexible and non-invasive way to control the band structures and charge states in InAs/GaSb QWs and InAs/GaInSb QWs. |
Tuesday, March 6, 2018 12:51PM - 1:03PM |
F08.00009: Electric-Field Driven Topological Phase Transition in InAs/InGaSb Composite Quantum Wells Hiroshi Irie, Takafumi Akiho, Yukio Takahashi, Francois Couedo, Kyoichi Suzuki, Koji Onomitsu, Koji Muraki Band-inverted InAs/(In,Ga)Sb composite quantum wells (CQWs) have been under intensive study as a 2D topological insulator. Among other 2D topological insulators, the CQW hosts an electrically tunable band alignment thanks to its double layer structure wherein electron and hole gases are spatially separated. This work presents electric-field-driven topological-to-normal insulator transition for recently-developed strained InAs/In_{x}Ga_{1-x}Sb CQWs [1, 2] with a large hybridization gap up to 35 meV. By measuring the energy gap as a function of electric field, we observed gap closing and reopening as expected for a topological phase transition. [1] T. Akiho et al., Appl. Phys. Lett. 109, 192105 (2016). [2] L. Du et al., Phys. Rev. Lett. 119, 056803 (2017). |
Tuesday, March 6, 2018 1:03PM - 1:15PM |
F08.00010: Current Noise in InAs/GaInSb Quantum Well Interfaces Loah Stevens, Tingxin Li, Rui-Rui Du, Douglas Natelson We report preliminary shot noise measurements in InAs/GaInSb quantum well interfaces. The band structure of these composite quantum wells is inverted, and the electron and hole densities are equal at the cross point of the valence and conduction bands. The holes in the strained GaInSb layer couple strongly to the electrons in the InAs layer, opening a gap in the spectrum of the 2d bulk. When gated into the appropriate regime, this allows only topologically-protected 1d helical edge states to contribute to charge transport. Using a back gate to tune the Fermi level, we can sweep into the hybridized bulk gap, recognized by a peak in the longitudinal resistance. If the helical edge length is within the coherence length of the sample, only ballistic transport should occur along the edge, and one would expect the shot noise to be dominated by the resistance of the contacts. We will compare current noise measurements of samples in the nominal Quantum Spin Hall Insulator state and in the lower-resistance, bulk transport state. |
Tuesday, March 6, 2018 1:15PM - 1:27PM |
F08.00011: Hall Conductivity of Chern Insulators in a Static Inhomogeneous Electric Field Shudan Zhong, Joel Moore We study the Hall conductivity of insulators in a static inhomogeneous electric field to the second order of wave number q by using the Kubo formula. For a Chern insulator with two flat bands, we show that the q^{2} term in hall conductivity is determined by the quantum metric tensor and the Berry curvature. Using a wave-packet semiclassical derivation we provide an intuitive picture to show how the quantum metric tensor comes into play. We also do numerical studies for Chern insulators with nearly flat bands and the Hofstadter model in the small flux limit. |
Tuesday, March 6, 2018 1:27PM - 1:39PM |
F08.00012: Noise-induced Backscattering in a Helical Quantum-spin-Hall Edge Jukka Vayrynen, Dmitry Pikulin, Jason Alicea In a 2D topological-insulator edge scalar potential disorder does not induce elastic electron backscattering, implying a quantized zero-temperature conductance. At finite temperature, electron-electron interactions generate inelastic backscattering, though the resulting conductance modification vanishes as a large power of temperature due to small scattering phase-space volume. We study a novel mechanism for conductance suppression: backscattering caused by electromagnetic noise. Noise leads to disorder potentials that fluctuate randomly in time, and can backscatter electrons inelastically without constraints faced by electron-electron interactions. We quantify the noise-induced correction to the time-averaged linear conductance under a variety of possible regimes. |
Tuesday, March 6, 2018 1:39PM - 1:51PM |
F08.00013: Large-Spin Magnetic Impurity near a 2D Topological Edge Igor Burmistrov, Vladislav Kurilovich, Pavel Kurilovich, Moshe Goldstein 2D topological insulators have attracted much attention lately, due to their gapless helical edge modes, which should display maximal charge-spin entanglement and be protected from time-reversal-invariant backscattering at low temperatures. However, significant low-energy backscattering was measured in recent experiments, an observation that has hitherto remained elusive. In this work we study the possible role that may be played by magnetic impurities with spin larger than 1/2, such as the ubiquitous S=5/2 Mn in HgTe. For the first time we treat the case of arbitrary isotropy in the impurity-edge exchange, as well as the self-exchange (local anisotropy) of the impurity. We find the latter may strongly enhance backscattering, not only at low temperatures and voltages (where it can exponentially suppress Kondo screening), but also at relatively high energies. The resulting rich behavior of the current-voltage characteristics may allow experiments to reveal the complex internal structure of the magnetic impurities. |
Tuesday, March 6, 2018 1:51PM - 2:03PM |
F08.00014: Hidden edge Dirac point and robust quantum edge transport in InAs/GaSb quantum wells Changan Li, Song-Bo Zhang, Shun-Qing Shen The robustness of quantum edge transport in InAs/GaSb quantum wells raises an issue on the fate of topological phases of matter under time-reversal symmetry breaking. Here a peculiar band structure evolution in InAs/GaSb quantum wells is revealed: the electron subbands cross the heavy hole subbands but anticross the light hole subbands. The topologically protected band crossing point of the helical edge states, i.e., the Dirac point, is pulled to be close to and even buried in the bulk valence bands when the system is in a deeply inverted regime, which is attributed to the existence of the light hole subbands. A sizable Zeeman energy gap verified by the effective g-factors of edge states opens at the Dirac point by an in-plane or perpendicular magnetic field, however it can also be hidden in the bulk valance bands. This provides a plausible explanation for the recent observation on the robustness of quantum edge transport in InAs/GaSb quantum wells subjected to strong magnetic fields. |
Tuesday, March 6, 2018 2:03PM - 2:15PM |
F08.00015: Tuning thermometric transport in Floquet Topological insulator Muhammad Tahir We study thermoelectric transport in Floquet topological insulators under the application of circularly polarized off-resonant light. We derive analytical expressions for the band structure, orbital magnetization, and the thermal and Nernst conductivities. Reversing the light polarization from right to left leads to an exchange of the conduction and valence bands of the symmetric and antisymmetric surface states and to a sign change in magnetization, Nernst and thermal conductivities. Varying the sample thickness or amplitude of light leads to a strong enhancement of magnetization and Nernst conductivity. These effects are accessible to experiments, open the possibility for selective, state-exchanged excitations under light and the conversion of heat to electric energy. |
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