Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session V3: Quantum Spin Hall Edge States: Transport |
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Sponsoring Units: DCMP Chair: Debaleena Nandi, Harvard University Room: 262 |
Thursday, March 16, 2017 2:30PM - 2:42PM |
V3.00001: Current noise in the edge states of InAs/GaSb quantum well interfaces. Loah Stevens, Pavlo Zolotavin, Tingxin Li, Rui-Rui Du, Douglas Natelson We present an investigation of current noise in the edge states of InAs/GaSb quantum wells using a broadband RF measurement approach. In these systems, a hybridization of the bound quantum well states at the interface of InAs and GaSb layer can open a gap in the spectrum of the bulk 2d electron gas, such that only topologically protected 1d edge states are available to contribute to charge transport. In preliminary measurements of a 2-terminal edge state device, at temperatures comparable or above the hybridization gap (40-60K), shot noise is linear with bias, as expected for a short diffusive conductor. At lower temperatures the noise power displays an anomalous bias dependence. The noise power is observed to decrease with increasing bias until a threshold bias level is reached, after which, noise magnitude begins to grow. The threshold bias and rate of noise decrease appear to be dependent on device length, temperature, magnetic field, and gate voltage. The leading candidate mechanism behind this phenomenon is the contribution of the silicon dopants, used to quell residual bulk conduction, to generation-recombination noise in the quantum wells. Investigations of structures without such dopants are underway. [Preview Abstract] |
Thursday, March 16, 2017 2:42PM - 2:54PM |
V3.00002: Current noise from a magnetic moment in a helical edge Jukka Vayrynen, Leonid Glazman We calculate the two-terminal current noise generated by a magnetic moment coupled to a helical edge of a two-dimensional topological insulator. When the system has in-plane $U(1)$ spin rotation symmetry, the noise $S(\omega)$ is given by the fluctuation-dissipation theorem even in the presence of a voltage bias $V$. The noise is strongly dependent on frequency on a small scale $\tau_{K}^{-1}\ll T$ set by the Korringa relaxation rate of the local moment. Exchange components breaking the symmetry give rise to shot noise in the limit of high bias. The differential noise $dS/dV$, commonly measured in experiments, is dominated by the symmetric component up to potentially large bias and disperses strongly with $\omega$ at low frequencies $\omega\sim\tau_{K}^{-1}$, unlike in the case of conventional elastic scatterer where $dS/dV$ is given by white shot noise. [Preview Abstract] |
Thursday, March 16, 2017 2:54PM - 3:06PM |
V3.00003: Nuclear spin-induced localization of the edge states in two-dimensional topological insulators Peter Stano, Chen-Hsuan Hsu, Jelena Klinovaja, Daniel Loss We investigate the influence of nuclear spins on the transport properties of the helical edge states in two-dimensional topological insulators. The nuclear spins couple to the edge-state electrons through the hyperfine interaction, and cause spin-flip backscattering between the edge-state electrons. In a system with long edges, the edge states are localized due to disordered nuclear spins below the localization temperature. On the other hand, the edge states mediate anisotropic RKKY interaction, which induces a cycloidal nuclear spin order below the transition temperature. At zero temperature, where the nuclear spins are completely ordered, the nuclei-induced backscattering is suppressed in clean systems. However, we demonstrate that other backscattering processes can be caused by magnons at finite temperature, and charge impurities in the presence of the Overhauser field, which is induced by the ordered nuclei. The coexistence of the charge impurities and the Overhauser field leads to Anderson-type localization of the edge states, whereas the magnon-mediated backscattering depends on the magnon energy. We provide the RG analysis for the backscattering mechanisms caused by disordered nuclei, magnons, and the coexistence of charge impurities and an Overhauser field. [Preview Abstract] |
Thursday, March 16, 2017 3:06PM - 3:18PM |
V3.00004: Edge reconstruction in correlated quantum spin Hall insulators Giorgio Sangiovanni, Adriano Amaricci, Jan Budich, Massimo Capone, Bjoern Trauzettel Symmetry-protected helical edge states live at the boundary of a topologically non-trivial quantum spin Hall insulator. We consider a repulsive local interaction on each site of a two-dimensional ribbon and study the edge states upon increasing the Hubbard-$U$ strength.Due to its reduced coordination the surface has the tendency to become Mott insulating before the bulk. This activates a correlation-driven reconstruction mechanism in the intermediate-$U$ regime, before the whole system has become a Mott insulator itself. We analyze this phenomenon within the Bernevig-Hughes-Zhang model, which we solve by means of Dynamical Mean Field Theory at zero temperature [1-3]. Our results hints at viewing the influence of Coulomb interaction on the helical edge states from a combined bulk-edge perspective, challanging the customary approach based on model Hamiltonians with one spatial dimension by construction. \\ {[1]} A. Amaricci, J. C. Budich, M. Capone, B. Trauzettel and G. Sangiovanni. {\it Phys.~Rev.~Lett.} {\bf 114}, 185701 (2015). \\ {[2]}A. Amaricci, J. C. Budich, M. Capone, B. Trauzettel and G. Sangiovanni. {\it Phys.~Rev.~B} {\bf 93}, 235112 (2016). \\ {[3]} A. Amaricci, {\it et al.}, in preparation. [Preview Abstract] |
Thursday, March 16, 2017 3:18PM - 3:30PM |
V3.00005: Edge states of HgTe/CdTe quantum wells beyond critical magnetic field Zewei Chen, Ting Pong Choy, Tai Kai Ng 2D topological insulators have caught considerably attention because of its time reversal symmetry (TRS) projected topology. It was believed that when TRS is broken, the helical edge states will be gapped out. However, this picture is challenged by a transport measurement in HgTe quantum well which seems to indicate the existence of edge states under strong magnetic where band inversion of the topological insulator has been destroyed.~ In this paper, we study the HgTe quantum well under perpendicular magnetic field. In the presence of strong spin-orbit coupling and magnetic field, we find edge-like states at the first electron-like and hole-like Landau levels leading to edge conductance without the Landau level being completely filled. The origin of this edge-like state is explained.~ The experimental results can be explained by our calculation. [Preview Abstract] |
Thursday, March 16, 2017 3:30PM - 3:42PM |
V3.00006: Visualization of chiral edge states in a magnetic topological insulator Monica Allen, Yongtao Cui, Eric Yue Ma, Masataka Mogi, Minoru Kawamura, David Goldhaber-Gordon, Yoshinori Tokura, Zhi-Xun Shen Topological insulators with ferromagnetic ordering exhibit the quantum anomalous Hall effect, in which a chiral one-dimensional edge state encloses an insulating interior. We provide a real-space visualization of the local conductivity profile in Cr modulation doped (Bi,Sb)2Te3 using microwave impedance microscopy (MIM). Well-defined edge states appear in the quantum anomalous Hall regime, which is robust at magnetic fields exceeding the coercive field. Our images reveal a dramatic change in the edge state pattern and microwave response near the topological phase transition between the Chern number N$=$1 and N$=$0 states. By mapping the non-monotonic evolution of the complex microwave response in magnetic field, we construct a phase diagram of competing topological states and unveil the microscopic nature of dissipation and conductivity in each regime. [Preview Abstract] |
Thursday, March 16, 2017 3:42PM - 3:54PM |
V3.00007: Probing helical edge states in InAs/GaSb supperlattices by low-temperature scanning tunneling spectroscopy Xiaohu Zheng, Bingbing Tong, Zhongdong Han, Po Zhang, Xingjun Wu, Xiaoxue Liu, Jianmin Zheng, Chi Zhang, Rui-Rui Du We report work in progress for measurements of edge states in an InAs/GaSb supperlattice using a low temperature (400 mK) scanning tunneling microscopy/spectroscopy equipped with vector magnets. The superlattices samples are consisting of 60 or 100 units of InAs/GaSb bilayer, with wide or narrow barriers separating the units. The sample surface that is being studied is prepared by UHV in situ cleavage. The atomic images were taken at the cross-section; the local density of states where the helical edge states are proposed to exist are analyzed by recording the differential conductance as a function of sample bias, temperature, and vector magnetic fields. The results should help us to understand the topological edge states in InAs/GaSb quantum wells in a clean environment. [Preview Abstract] |
Thursday, March 16, 2017 3:54PM - 4:06PM |
V3.00008: Transport Studies of Strained-layer InAs/GaInSb Quantum Spin Hall Insulators Tingxin Li, Lingjie Du, Wenkai Lou, Xingjun Wu, Xiaoxue Liu, Zhongdong Han, Chi Zhang, Gerard Sullivan, Amal Ikhlassi, Kai Chang, Rui-Rui Du We report on a new class of quantum spin Hall insulators (QSHI) in strained-layer InAs/GaInSb quantum wells, in which the bulk gaps can be achieved to \textasciitilde 20 meV, enhancing by up to five folds as compared to the binary InAs/GaSb QSHI. We will present bulk gap measurement results using Corbino devices, and compare the results with band structure calculations. Remarkably, with consequently increasing edge velocity, the edge conductance at zero and applied magnetic fields clearly manifests time reversal symmetry -protected properties consistent with Z$_{\mathrm{2\thinspace }}$topological insulator. The InAs/GaInSb bilayers offer a much sought-after building block for a single-mode platform of Majorana bound states and parafermions. [Preview Abstract] |
Thursday, March 16, 2017 4:06PM - 4:18PM |
V3.00009: Interplay of helical and chiral states in a 2D-Topological Insulator lateral junction M. Reyes Calvo, Fernando de Juan, Roni Ilan, Jing Wang, Eli Fox, Andrew J. Bestwick, Philipp Leubner, Christopher Ames, Mathias Muehlbauer, Christoph Bruene, Laurens W. Molenkamp, David Goldhaber-Gordon A 2D topological insulator has an inverted band structure. Quantum Spin Hall states cross the gap at the material's edge, giving rise to helical edge modes where backscattering is forbidden by time reversal symmetry. This situation is predicted to persist near zero-density, unprotected from backscattering, up to a critical magnetic field where band inversion is lifted. When 2D carriers are accumulated, magnetic field drives the system to the Quantum Hall regime and chiral states propagate at the edge. Here, we study electrostatically-gated lateral junctions in a HgTe quantum well, the canonical 2D topological insulator, at zero and under applied magnetic field. At finite carrier densities out and inside the junction, we observe fractional plateaus of conductance characteristic of equilibration between different number of chiral Quantum Hall modes. When the junction is tuned near zero density, we observe coherent oscillations that we attribute to Fabry-Perot interference induced by backscattering of the helical edge mode. Zero field oscillations are also observed at zero field and in the bipolar regime, which we attribute to interference from bulk carriers. [Preview Abstract] |
Thursday, March 16, 2017 4:18PM - 4:30PM |
V3.00010: Robustness of a topological insulator state probed by quantum Hall systems Xinqiang Cai, Vinicio Tarquini, Talbot Knighton, Zhe Wu, Jian Huang, Loren Pfeiffer, Ken West This study concerns the origin of the robustness of a topological insulator studied via dilute quantum Hall systems. Low carrier density is chosen to minimize the density gradient at the edge in order to examine how the edge-reconstruction is related to the protection mechanism of the topological order. An anti-Hall bar geometry is adopted to facilitate simultaneous probes to the edge (via the magnetoresistance and the Hall resistance) and bulk (via measurement between independent edges) states. The fully gapped bulk states are found only at exact integer filling factors down to a few percent accuracies. Fixing the magnetic filed at the fully gapped point, the breakdown is studied as a function of the external dc$+$ac voltage bias. A large breakdown threshold is far beyond the Landau level spacing. Moreover, extremely sharp discontinuous steps spaced at exactly the Landau level spacing appears following the threshold voltage, confirming a breakdown via a resonance-like edge-bulk tunneling, in contrast to the well-known QUILL theory. A model based on edge reconstruction will be discussed. [Preview Abstract] |
Thursday, March 16, 2017 4:30PM - 4:42PM |
V3.00011: Semimetal-Metal Transition and Quantum Electronic Transport in Ultrathin WTe$_{\mathrm{2}}$ Valla Fatemi, Sanfeng Wu, Quinn D. Gibson, Kenji Watanabe, Takashi Taniguchi, Robert J. Cava, Pablo Jarillo-Herrero We report low-temperature electronic transport measurements of ultrathin WTe$_{\mathrm{2}}$. By encapsulating samples inside a glove-box, we can approach the intrinsic electronic transport physics of the system. We find that tri-layer samples remain semimetallic, similar to bulk crystals. We tune the system from an intrinsic semimetallic state into a simple metallic state by doping via an electrostatic gate, which results in total suppression of magnetoresistance and disappearance of the hole pocket in Shubnikov-de Haas oscillations. However, the magnetoresistance of semimetallic 2D WTe$_{\mathrm{2}}$ follows a distinctly sub-quadratic power law, in contrast to bulk crystals which show nearly quadratic behavior, suggesting new physics in the 2D limit. Finally, we report on our progress towards electronic transport in the monolayer limit. [Preview Abstract] |
Thursday, March 16, 2017 4:42PM - 4:54PM |
V3.00012: Control of the Edge-State in Quantum Spin Hall System with a Transverse Electric Field and Edge-Functionalization. Hyeonjin Doh, Hyoung Joon Choi The Kane-Mele model is one of the typical model for the quantum spin Hall system. The edge state of the Kane-Mele model has a finite width depending on the energy, the momentum, and the various parameters including spin-orbit coupling and sub-lattice potential. Here, we systematically study the effect of the in-plane transverse electric field to the edge and the edge-functionalization on the edge-state width in the Kane-Mele model. We calculate the edge-state width for the semi-infinite honeycomb lattice with a zigzag- and an armchair-edge. Our results show the edge-state width is increased by the edge-functionalization and by the transverse electric field regardless of its direction. We discuss the control of the edge-state gap with the relation between the sample size and the edge-state width by comparing with the results of the honeycomb nano-ribbon results. [Preview Abstract] |
Thursday, March 16, 2017 4:54PM - 5:06PM |
V3.00013: Plateau-Plateau Transitions in Disordered Topological Chern Insulators Ying Su, Yshai Avishai, Xiangrong Wang Occurrence of the topological Anderson insulator (TAI) in the HgTe quantum well demonstrates that topological phase transition can be driven by disorder, where re-entrant $2e^2/h$ quantized conductance is contributed by helical edge states. Within a certain extension of the disordered Kane-Mele model for magnetic materials that violate time-reversal symmetry and inversion symmetry, it is shown that the physics of TAI becomes even richer due to lifted spin and valley degeneracies. Tuning either disorder or Fermi energy (in both topologically trivial and nontrivial phases) makes it possible to drive plateau-plateau transitions between distinct TAI phases characterized by different Chern numbers, marked by jumps of the quantized conductance from 0 to $e^2/h$ and from $e^2/h$ to $2e^2/h$. An effective medium theory based on the Born approximation yields an accurate description of different TAI phases in parameter space. [Preview Abstract] |
Thursday, March 16, 2017 5:06PM - 5:18PM |
V3.00014: Dyon proliferation in interacting quantum spin Hall edges Shu-Ping Lee, Joseph Maciejko We show that a quantum spin Hall system with intra-edge multiparticle backscattering and inter-edge exchange interactions exhibits a modular invariant zero-temperature phase diagram. We establish this through mapping to a classical 2D Coulomb gas with electrically and magnetically charged particles; strong coupling phases in the quantum edge problem correspond to the proliferation of various dyons in the Coulomb gas. Distinct dyon proliferated phases can be accessed by tuning the edge Luttinger parameters, for example using a split gate geometry. [Preview Abstract] |
Thursday, March 16, 2017 5:18PM - 5:30PM |
V3.00015: The Haldane model under uniaxial strain Yen-Hung Ho, Eduardo Castro, Maria Vozmediano, Miguel Cazalilla The topological features of Haldane model under strain are studied using a tight-binding approach. Strain induces a time-reversal preserving pseudo-magnetic field that quantizes the bulk states inducing pseudo Landau levels. Unlike the time-reversal symmetry breaking induced by a real magnetic field, strain lifts the degeneracy of the zero-energy Landau level in each valley. The degeneracy is also lifted in the presence of a potential that breaks the sublattice inversion symmetry. In addition, strain also modifies locally the Fermi velocity and induces a tilt of the Landau levels. [Preview Abstract] |
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