Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session F18: Two Dimensional Topological Insulators I |
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Sponsoring Units: DCMP Chair: Yong Chen, Purdue University Room: 320 |
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F18.00001: Imaging currents in HgTe quantum wells in the quantum spin Hall regime Katja Nowack, Eric Spanton, Matthias Baenninger, Markus K\"onig, John Kirtley, Beena Kalisky, Christopher Ames, Philipp Leubner, Christoph Br\"une, Hartmut Buhmann, Laurens Molenkamp, David Goldhaber-Gordon, Kathryn Moler Dissipationless edge channels are a key feature of the quantum spin Hall (QSH) state, which was predicted and experimentally demonstrated to exist in HgTe quantum wells. The existence of the edge channels has been inferred from local and non-local transport measurements. Here we image the current in Hall bars made from HgTe quantum wells by probing the magnetic field generated by the current using a scanning superconducting quantum interference device. We observe that the current flows mainly along the edge of the device in the QSH regime, and furthermore that an identifiable edge channel exists even in the presence of disorder and considerable bulk conduction as the device is gated and its temperature is raised. Our results represent a versatile method for the characterization of new quantum spin Hall materials systems. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F18.00002: Electronic properties of HgTe/CdTe heterostructure under perturbations preserving time reversal symmetry Tome Schmidt, Jonas Anversa, Paulo Piquini, Adalberto Fazzio Using first principles calculations, the Dirac cone of HgTe/CdTe heterostructure is identified at the interface, inside the valence band. The spin texture of the 2D Dirac states is totally in-plane for all interface directions, different from the 3D topological insulators, where there is always some out-of-plane spin components. The masless Dirac states are strongly affected by applying positive or negative biaxial pressure. While negative pressure turns the system metallic, suppressing the Dirac states, positive pressure maintains the protected topological states, but dislocates the Dirac cone upward in energy. The protected Dirac states are kept up to a contraction of 3\% in the lattice parameter. Larger compressive pressures leads to suppression of the protected metallic states. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F18.00003: Quantized Conductance in InAs/GaSb Quantum Wells Lingjie Du, Ivan Knez, Rui-Rui Du, Gerald Sullivan We have studied electrical transport in inverted InAs/GaAs quantum wells (QWs) made by molecular beam epitaxy, in which the evidences for helical edge modes were observed in messoscopic samples with either normal or superconductor contacts. Here we report on measurements of QWs that are doped with Si at the InAs/GaSb interface, where Si is a donor in InAs and an acceptor in GaSb. The influences of induced disorder in the quantum Spin Hall effect as well as outside this regime are systematically studied and results will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F18.00004: Scanning SQUID measurements of current flow in InAs/GaSb Quantum Wells Eric Spanton, Lingjie Du, Katja Nowack, Gerry Sullivan, Rui-Rui Du, Kathryn Moler InAs/GaSb quantum wells have been predicted theoretically to exhibit the quantum spin hall phase in the inverted regime. In this phase, spin-polarized helical edge modes are expected to exist. Previous published results on length and width dependence of InAs/GaSb 4-terminal devices suggests these helical edge states coexist with a residual bulk conductivity when the device is tuned into the minigap. We probe the spatial distribution of currents in devices using a scanning SQUID to measure the resulting magnetic fields. Specifically, we find that when the device is tuned into the gap with a front gate, current flows along the edge and coexists with bulk current. We also look at dependence on back gate voltage and temperature dependence. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F18.00005: Quantum Transport near the Charge Neutrality Point in Inverted Type-II InAs/GaSb Field-Effect Transistors W. Pan, J.F. Klem, J.K. Kim, M. Thalakulam, M.J. Cich, S.K. Lyo We present here our recent quantum transport results around the charge neutrality point (CNP) in a type-II InAs/GaSb field-effect transistor. At zero magnetic field, a conductance minimum close to $4e^2/h$ develops at the CNP and it follows semi-logarithmic temperature dependence. In quantized magnetic ($B$) fields and at low temperatures, well developed integer quantum Hall states are observed in the electron as well as hole regimes. Electron transport shows noisy behavior around the CNP at extremely high B fields. When the diagonal conductivity $\sigma_{xx}$ is plotted against the Hall conductivity $\sigma_{xy}$, a conductivity circle law is discovered, suggesting a chaotic quantum transport behavior. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F18.00006: Quantum Anomalous Hall Effect in 2D Organic Topological Insulators Zhengfei Wang, Zheng Liu, Feng Liu Quantum anomalous Hall effect (QAHE) is a fundamental transport phenomenon in the field of condensed-matter physics. Without external magnetic field, spontaneous magnetization combined with spin-orbit coupling give rise to a quantized Hall conductivity. So far, a number of theoretical proposals have been made to realize the QAHE, but all based on inorganic materials. Here, using first-principles calculations, we predict a family of 2D organic topological insulators (OTIs) for realizing the QAHE. Designed by assembling molecular building blocks of triphenyl-transition-metal compounds into a hexagonal lattice, this new classes of organic materials are shown to have a nonzero Chern number and exhibit a gapless chiral edge state within the Dirac gap. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F18.00007: Quantum anomalous Hall effect with in-plane magnetization in HgMnTe Hsiu-Chuan Hsu, Xin Liu, Chao-Xing Liu A quantum anomalous Hall (QAH) insulator carries quantized Hall conductance which is similar to Quantum Hall (QH) effect. However, it originates from the exchange coupling of magnetization instead of Landau levels. It was proposed that QAH effect can be realized in HgTe quantum wells doped with Mn ({\bf Phy. Rev. Lett. 101, 146802 (2008)}) and evidenced by recent experiments. However, Mn is paramagnetic and an external magnetic field, which also leads to Landau levels, is required to obtain Mn polarization. Thus, it is essential to find an experimentally feasible way to distinguish between the two effects. In this study, we propose to distinguish QH effect and QAH effect by inducing the in-plane magnetization of Mn with an in-plane magnetic field. The in-plane magnetic field reduces the QAH effect by tilting the magnetization of Mn into the quantum well plane and reducing the out-of-plane magnetization. In contrast, the in-plane magnetic field has little influence on the conventional QH effect which only depends on the out-of-plane magnetic field. The phase diagram is identified based on the band structure calculation and Landau level calculation with the realistic material parameters of HgMnTe quantum wells, which can serve as the guidance for the future transport experiment. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F18.00008: Engineering quantum anomalous Hall phases with orbital and spin Hongbin Zhang, Frank Freimuth, Gustav Bihlmayer, Stefan Bl\"ugel, Yuriy Mokrousov Combining tight-binding models and first principles calculations, we demonstrate that under external exchange fields, non-zero Chern numbers and nontrivial QAH effect could be induced by on-site spin-orbit coupling (SOC) in buckled honeycomb lattices with $sp$ orbitals. In the Haldane model [1], the occurrence of QAH effect is attributed to complex next nearest neighbor hopping. Detailed analysis of a generic tight-binding models reveals that there exist different mechanisms giving rise to complex hoppings, utilizing both orbital and spin degrees of freedom of electrons on a lattice. Furthermore, it is shown that in Bi/Sb(111) bilayers [2], different topological phases exist as function of the magnitude of SOC and external exchange fields. These phases are characterized using Chern and spin Chern numbers [3] together with transverse charge and spin conductivities. At last, we show that introducing ferromagnetic dopants provides a practical way to induce nontrivial topological phases, whereas the physics is modified due to incompletely filled d states around the Fermi energy.\\[4pt] [1] F.D.M. Haldane, Phys. Rev. Lett. 61, 2015 (1988).\\[0pt] [2] H. Zhang, {\it et al.}, Phys. Rev. B 86, 035104 (2012).\\[0pt] [3] E. Prodan, Phys. Rev. B {\bf 83}, 195119 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F18.00009: Effective field theory of nematic QAH for interacting fermions Yizhi You, Eduardo Fradkin We consider 2D fermionic lattice models with quadratic band touching. By turning on a marginal relevant interaction of this system, the system~condenses into a state that spontaneously breaks time reversal and/or rotation (point-group) symmetry. When both symmetries are broken the state is a nematic quantum anomalous Hall (QAH) phase. We derive an effective field theory which describes the quantum phase transition into this state from a spontaneous QAH state. The effective field theory has the form of Maxwell-Chern-Simons action for the hydrodynamic degrees of freedom of the spontaneous QAH state with a coupling to the nematic order-parameter field that induces a spatial anisotropy. The fluctuations of the nematic field modify the local spatial geometry and couples to the Maxwell term as the spatial components of a metric tensor. We will discuss the behavior at quantum criticality and the relation with recent theories that associate transitions of this type with quantum Lifshitz criticality [1]. We will also discuss extensions of our theory to nematic fractional QAH state. [1] M. Mulligan, C. Nayak, and S. Kachru, Phys. Rev. B 82, 085102 (2010); Phys. Rev. B 84,195124 (2011) This work was supported in part by the NSF grant DMR-1064319 at the University of Illinois. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F18.00010: Symmetry protected Spin Quantum Hall phases Zheng-Xin Liu, Xiao-Gang Wen Symmetry protected topological (SPT) states are short-range entangled states with symmetry. Nontrivial SPT states have symmetry protected gapless edge excitations. Topological insulators are examples of nontrivial SPT phases. We study Bosonic SPT phases protected by $SU(2)$ or $SO(3)$ symmetry in 2D. There are infinite number of such phases, which can be described by $SU(2)/SO(3)$ nonlinear-sigma models with a quantized topological $\theta$-term. At open boundary, the $\theta$-term becomes the Wess-Zumino-Witten term and consequently the boundary excitations are decoupled gapless left movers and right movers. Only the left movers (if $\theta>0$) carry the $SU(2)/SO(3)$ quantum numbers. As a result, the $SU(2)$ SPT phases have a half-integer quantized spin Hall conductance and the $SO(3)$ SPT phases have an even-integer quantized spin Hall conductance. Both the $SU(2)/SO(3)$ SPT phases are symmetric under their $U(1)$ subgroup and can be viewed as $U(1)$ SPT phases with even-integer quantized Hall conductance. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F18.00011: Bloch Model Wavefunctions and Pseudopotentials for All Fractional Chern Insulators Yang-Le Wu, N. Regnault, B. Andrei Bernevig We introduce a Bloch-like basis in a $C$-component lowest Landau level fractional quantum Hall effect (FQH), which entangles the real and internal degrees of freedom and preserves an $N_x\times N_y$ full lattice translational symmetry. We implement the Haldane pseudopotential Hamiltonians in this new basis. Their ground states are the model FQH wavefunctions, and our Bloch basis allows for a mutatis mutandis transcription of these model wave functions to the fractional Chern insulator (FCI) of arbitrary Chern number $C$, obtaining wavefunctions different from all previous proposals. For $C>1$, our wavefunctions are related to color-dependent magnetic-flux inserted versions of Halperin and non-Abelian color-singlet states. We then provide large-size numerical results for both the $C=1$ and $C=3$ cases. This new approach leads to improved overlaps compared to previous proposals. We also discuss the adiabatic continuation from the FCI to the FQH in our Bloch basis, both from the energy and the entanglement spectrum perspectives. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F18.00012: First-principles study on quantum valley Hall effects in silicene Youngkuk Kim, Hosub Jin, Keunsu Choi, Jisoon Ihm Silicene is a two-dimensional honeycomb lattice of silicon atoms, similar to graphene. Based on first-principles calculations, we suggest that silicene is an ideal host for realization of quantum valley Hall effects. We show that the intrinsic buckled structure allows the formation of topological domain walls in silicene under a uniform applied electric field and valley-polarized kink states emerge on the domain walls. Peculiar behaviors of the kink states under various applied electric fields are demonstrated, and simulated scanning tunneling microscopy images are presented to show that they can be used to identify the topological domain walls as well as valley-polarized kink states. Our findings suggest that the one-dimensional domain wall may be used as an electrical wire through which valley-polarized current can flow, and silicene can be used as a valley polarizer. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F18.00013: Quantum Topological Hall Effect in Kagome Ice Yukitoshi Motome, Hiroaki Ishizuka The quantum Hall state was originally discovered in two-dimensional electron systems associated with the formation of quantized Landau levels in external magnetic field. Later, a quantum anomalous Hall effect without Landau levels was proposed, and the idea has been generalized to topological insulators in the presence of the spin-orbit coupling. Besides, a noncoplanar magnetic order was shown to give rise to the quantum anomalous Hall effect through the Berry phase mechanism. Here, we present yet another example of the quantum anomalous Hall state which emerges in the absence of Landau levels, spin-orbit coupling, and magnetic ordering. The new state is realized in itinerant electrons coupled with local spin textures subject to geometrical frustration of lattice structure. Considering the double-exchange model with spin-ice type Ising spins on a kagome lattice, we numerically show that a local spin correlation called kagome-ice opens a charge gap, resulting in quantization of the Hall conductivity in the absence of magnetic ordering. By Monte Carlo simulation, we discuss the stability of the anomalous Hall insulating region in the magnetic phase diagram. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F18.00014: Critical behavior of the transport coefficients at the Chern-to-normal insulator transition Yu Xue, Emil Prodan Using the non-commutative Kubo formula for disordered lattice systems, we mapped the conductivity tensor $\sigma_{xx}$($E_F$,$T$) and $\sigma_{xy}$($E_F$,$T$) as function of Fermi level $E_F$ and temperature $T$, for a model of a Chern insulator in the presence of strong disorder. In line with previous studies, $\sigma_{xy}$ displays a quantized non-trivial value near the half-filling, value that changes rapidly to a trivial value as $E_F$ crosses a critical value $E^c_F$. As expected, the $T$-dependence of $\sigma_{xx}$ display the typical signature of the insulating behavior, except at $E^c_F$. Examining the resistivity tensor $\hat\rho=\hat\sigma^{-1}$, we found that the data looks extremely similar to the experimental data for the plateau-insulator transition in the Integer Quantum Hall Effect: 1)$\rho_{xx}$($E_F$,$T$) vs $E_F$ plots for various temperatures intersect each other at precisely one point; 2) At this $E^c_F$, $\rho_{xx} \approx 1$ and $\sigma_{xy} \approx 0.5$; 3) The plots near $E^c_F$ for different temperatures collapse into one curve when rescaled with an exponent that is consistent with the universally accepted value. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F18.00015: Topological protection of localization against the hybridization Bohm-Jung Yang, Mohammad Saeed Bahramy, Naoto Nagaosa Localization of electronic wave functions is governed by their topological nature as well as the symmetry and dimensionality of the system. Two prominent examples are the presence of the extended states in two-dimensional quantum Hall systems and the absence of localization for the surface states of three dimensional topological insulators. In these cases, the extended states are protected by topological invariants. Here, we show that the two-dimensional quantum Hall system put on the three dimensional trivial insulator manifests a new class of localization phenomena of topological origin, where the two dimensional states remain localized along the surface normal direction in spite of the hybridization with the continuum extended states. The one-dimensional edge channel is also localized along the same direction as long as its energy is within the band gap. This finding demonstrates that the localized states are protected by the topological invariants against the hybridization with the continuum. [Preview Abstract] |
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