Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H29: Two-dimensional Topological Insulators: InAs/GaSb Quantum Wells and BeyondFocus
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Sponsoring Units: DCMP DMP Chair: Xiao Li, University of Maryland Room: 328 |
Tuesday, March 15, 2016 2:30PM - 2:42PM |
H29.00001: Topologically nontrivial Fermi regions and their novel electromagnetic response properties Ching Hua Lee, Xiao Zhang In the last decade, there has been a surge of interest in the application of topology to condensed matter physics. So far, most studies have been concerned with the novel properties that arise due to nontrivial band topology, i.e Quantum Anomalous Hall and Z2 topological insulators (TIs). In this talk, I shall describe another context where nontrivial topology also leads to interesting, measurable effects. Within the semi-classical Boltzmann approach, it can be shown that a topologically nontrivial Fermi sea region generically exhibits a non-monotonic nonlinear electromagnetic response in the limit of low chemical potential. Such topologically nontrivial regions of filled states can arise in experimentally realized TI heterostructures or materials with large Rashba splitting, i.e. BiTeI, where the Fermi sea is not simply connected. A non-monotonic electromagnetic response implies regimes of negative differential resistance, which have important applications in technologies involving microwave generation, like motion sensing and radio astronomy. We hope that nontrivial Fermi sea topology will hence provide another route for the realization of such technologies. [Preview Abstract] |
Tuesday, March 15, 2016 2:42PM - 2:54PM |
H29.00002: Response of Helical Luttinger Liquid in InAs/GaSb Edges to a Magnetic Field Tingxin Li, Bingbing Tong, Xiaoxue Liu, Zhongdong Han, Chi Zhang, Gerard Sullivan, Rui-Rui Du Electron-electron interactions have been shown to play an important role in InAs/GaSb quantum spin Hall (QSH) edge states, leading to power-law behaviors of the helical edge conductance as a function of temperature and bias voltage (Li \textit{et al}, Phys. Rev. Lett.\textbf{ 115} 136804). A variety of inelastic and/or multiparticle backscattering processes could occur in helical edges when taking electron-electron interactions into account. On the other hand, in the presence of an external magnetic field, single-particle elastic backscattering is also allowed in QSH edge due to the breaking of time-reversal symmetry (TRS). It would be interesting to pursue experimental investigations for the combined effect of electron-electron interactions and TRS breaking on QSH edge transport. We report work in progress for low temperature conductance measurements of the helical edge in InAs/GaSb under perpendicular or in-plane magnetic fields. We found that the magnetic field responses are generally correlated with the interaction strength in the edge states. [Preview Abstract] |
Tuesday, March 15, 2016 2:54PM - 3:06PM |
H29.00003: Mapping the topological-to-normal insulator phase transition in InAs/GaSb bilayers by heterostructure variation Borzoyeh Shojaei, Anthony P. McFadden, Joon Sue Lee, Mihir Pendharkar, Chris J. Palmstrøm When 2D electron and hole subbands in InAs/GaSb bilayers are tuned to the inverted regime the system is predicted to exhibit an insulating bulk and counter propagating helical 1D edge states. This work presents a dual-gate mapping of the topological-to-normal insulator phase transition for several InAs/GaSb bilayers wherein the InAs and GaSb layer thicknesses are varied. In-plane and out-of-plane magnetotransport experiments reveal the effect of heterostructure geometry on the magnitudes of the longitudinal and Hall magnetoresistances and on the shape and temperature dependence of the gate-tuned resistance map in the vicinity of the phase transition. [Preview Abstract] |
Tuesday, March 15, 2016 3:06PM - 3:18PM |
H29.00004: Gate-Tuned Mott Transition in Dilute InAs/GaSb Quantum Wells Lingjie Du, Wenkai Lou, Kai Chang, Gerard Sullivan, Rui-Rui Du We investigate the origin of the bulk gap in inverted InAs/GaSb quantum wells (QWs) that host spatially-separated electrons and holes using charge-neutral point (CNP) density (n\textunderscore o\textasciitilde p\textunderscore o) in gated devices as a tuning parameter. We find two distinct gap regimes: for I), n\textunderscore o \textgreater \textgreater 5×1010/cm2, a soft gap opens predominately by hybridization, which closes under B// \textgreater \textasciitilde 10T; for II), approaching the dilute limit n\textunderscore o\textasciitilde 5×1010/cm2, a hard gap opens leading to a true bulk insulator with quantized helical edges, continuously for B// up to 35T. Our results confirm that hard gap is associated with the Quantum Spin Hall (QSH) effect but cannot be explained by single-particle band theory. Instead it originates from many-body correlations. The data are remarkably consistent with a Mott insulator bulk state in the dilute InAs/GaSb bilayers. Specifically, spontaneous exciton binding is a viable mechanism for driving the Mott transition. Our results point to the importance of charge interactions in properties of QSHE in InAs/GaSb, in addition to single-particle band theories. The work in Rice was supported by DOE (measurements) and NSF (materials). [Preview Abstract] |
Tuesday, March 15, 2016 3:18PM - 3:30PM |
H29.00005: Spin-orbit interaction in InAs/GaSb heterostructures Fanming Qu, Arjan J. A. Beukman, Fokko K. de Vries, Jasper van Veen, Stevan Nadj-Perge, Michael Wimmer, Rafal J. Skolasinski, David de Vries, Binh-Minh Nguyen, Wei Yi, Jacob Thorp, Marko Sokolich, Michael J. Manfra, Charles M. Marcus, Leo P. Kouwenhoven We investigated spin-orbit interaction (SOI) in InAs/GaSb double quantum wells. A combination of dual-gating and spatially separated electron and hole gases allows for in situ engineering of the band structure. In both the trivial and inverted band alignment regimes, zero-field spin splitting due to SOI was extracted from the beating of the Shubnikov-de Haas oscillations. Deep in the electron regime, we observed anomalous magnetoresistance that points to a highly anisotropic Fermi surface as a result of the intermixing of Dresselhaus and Rashba SOI. In the inverted regime close to the hybridization gap, we obtained an oscillating spin-splitting as a function of electron density, as expected from the band structure calculation. [Preview Abstract] |
Tuesday, March 15, 2016 3:30PM - 3:42PM |
H29.00006: Electric control of inverted gap and hybridization gap in type II InAs/GaSb quantum wells Lun-Hui Hu, Chao-Xing Liu, Dong-Hui Xu, Fu-Chun Zhang, Yi Zhou The quantum spin Hall effect has been predicted theoretically and observed experimentally in InAs/GaSb quantum wells as a result of inverted band structures, for which electron bands in InAs layers are below heavy hole bands in GaSb layers in energy. The hybridization between electron bands and heavy hole bands leads to a hybridization gap away from \(\mathbf{k}=0\). A recent puzzling observation in experiments is that when the system is tuned to more inverted regime by a gate voltage (a larger inverted gap at \(\mathbf{k}=0\)), the hybridization gap decreases. Motivated by this experiment [ref. 1], we explore the dependence of hybridization gap as a function of external electric fields based on eight-band Kane model. We identify two regimes when varying electric fields: (1) both inverted and hybridization gaps increase and (2) inverted gap increases while hybridization gap decreases. We analyze the effective model and find that light-hole bands in GaSb layers play an important role in determining hybridization gap. In addition, large exernal electric field can induce strong Rashba splitting and also influence hybridization gap. Our results are consistent with experimental observations. Reference: \(\lbrack 1\rbrack\) Lingjie Du, et.al., arXiv:1508.04509 (2015). [Preview Abstract] |
Tuesday, March 15, 2016 3:42PM - 3:54PM |
H29.00007: Image potential states in the topological semimetal Sb Yau Chuen Yam, Yang He, Pengcheng Chen, Zhihuai Zhu, Mohammad Hamidian, Marcel Franz, Jennifer Hoffman Topological materials host protected surface states with locked spin and momentum degrees of freedom that have been predicted to give rise to several exotic excitations such as Majorana fermions and magnetic monopoles. The topological semimetal antimony (Sb) offers a pristine platform in which to search for these excitations. Here we present scanning tunneling microscopy and spectroscopy studies of Sb at high energy where quantized image potential states form due to the binding between the tunneling electron and the polarized surface. These states allow exploration of the image charge geometry necessary to realize a magnetic monopole. [Preview Abstract] |
Tuesday, March 15, 2016 3:54PM - 4:06PM |
H29.00008: Two-Dimensional Oxide Topological Insulator With LiFeAs Structure Qiunan Xu, Zhida Song, Simin Nie, Hongming Weng, Zhong Fang, Xi Dai Using first-principles calculations, we propose that ZrSiO can be viewed as a three-dimensional (3D) oxide weak topological insulator (TI) with Z$_{2}$ idiocies (0; 001). Further calculations show that the single layer of such material is a long-sought-after 2D oxide TI with a band gap around 10 meV. Furthermore, we also find that there are many other isostructural compounds, which can host similar electronic structure and form a `WHM' material family with `W' being Zr, Hf or La, `H' being group IV or group V element, and `M' being group VI one. [Preview Abstract] |
Tuesday, March 15, 2016 4:06PM - 4:18PM |
H29.00009: ab initio based tight-binding investigation of quantum spin Hall effect in InAs/GaSb quantum wells QuanSheng Wu, Alexey Soluyanov, Matthias Troyer Quantum spin Hall state is a topologically non-trivial quantum state, which can be used for designing various quantum devices including those potentially useful for quantum computing. Type-II InAs/GaSb semiconductor quantum well was predicted to realize this state of matter. In this work, we systematically investigate topological properties of this system using symmetrized Wannier-based tight-binding models. The model parameters are derived from first-principles hybrid functional calculations, which capture the right band gap and effective masses of both InAs and GaSb. By varying the thickness of InAs and GaSb layers, three possible phases are obtained: normal insulator, quantum spin Hall insulator, and semimetal, allowing us to optimize the growth conditions for the quantum spin Hall phase realization. Most importantly, we identify optimal growth directions, showing that a significant increase of the topological gap can be obtained by growing the quantum well in the [111]-direction. Phase diagrams are obtained for different layer thicknesses and growth directions. Effects of strain and applied electric fields are also discussed. [Preview Abstract] |
Tuesday, March 15, 2016 4:18PM - 4:30PM |
H29.00010: Zero bias conductance peak on the surface of topological semimetal Sb. Pengcheng Chen, Yang He, Yau Chuen Yam, Shiang Fang, Marcel Franz, Mohammad Hamidian, Jennifer Hoffman Topological materials host protected surface states with locked spin and momentum degrees of freedom that have been predicted to give rise to several exotic excitations such as Majorana fermions and magnetic monopoles. The topological semimetal antimony (Sb) offers a pristine platform in which to search for these excitations. Sb has a bilayer crystal structure; here we obtained both inter-bilayer cleaved and intra-bilayer cleaved terminations. Using scanning tunneling spectroscopy techniques, we observed a robust zero bias conductance peak on the rarer intra-bilayer cleaved surface. [Preview Abstract] |
Tuesday, March 15, 2016 4:30PM - 4:42PM |
H29.00011: Understanding Magnetic Proximity in Topological Insulators with Raman Gavin Osterhoudt, Kenneth Burch The magnetic proximity effect in Topological Insulators has been extensively studied due to predictions of quantum anomalous Hall effect and numerous applications in spintronics. Nonetheless, the origin of the proximity effect remains unclear. To uncover the role of the lattice we have used Raman scattering to investigate the magnetic proximity effect of thin film ferromagnetic insulator EuS grown by MBE on the topological insulator Bi$_{2}$Se$_{3}$. Through these measurements we are able to probe the magnetic fluctuations in the EuS. We will discuss the results of our measurements and their implications for the role of strain in ferromagnetic/topological insulator heterostructures. [Preview Abstract] |
Tuesday, March 15, 2016 4:42PM - 4:54PM |
H29.00012: Visualization of superparamagnetic dynamics in magnetic topological insulators E. Lachman, A. F. Young, A. Richardella, J. Cuppens, N. HR, Y. Anahory, A. Y Meltzer, A. Kandala, S. Kempinger, Y. Myasoedov, M. E. Huber, N. Samarth, E. Zeldov Magnetically doped topological insulators have recently been shown to host a quantum anomalous Hall (QAH) state at low temperatures. Using scanning nanoSQUID magnetic imaging on a Cr-doped $(Bi,Sb)_2Te_3$ thin film$^{[1]}$, we reveal that the magnetic structure of magnetically doped topological insulators is not ferromagnetic as assumed so far. In fact it is superparamagnetic, formed by weakly interacting magnetic domains. These domains have a characteristic size of a few tens of nanometers, and undergo random reversals which drive the electronic state from one Hall plateau to the other. The superparamagnetic state is metastable, with small energy barriers to relaxation. We observe magnetic relaxation even at 300 mK, evident also in transport measurements. Unexpectedly, magnetic relaxation can also be induced by varying the gate voltage, and we propose a mechanism for the influence of the electronic phase on the magnetic relaxation. We speculate that the dynamic nature of magnetic disorder in QAH systems may contribute to the observed fragility of the QAH state at elevated temperatures. [1] Lachman \it{et al}, arXiv:1506.05114 [Preview Abstract] |
Tuesday, March 15, 2016 4:54PM - 5:06PM |
H29.00013: 1-D Modes on Step-edges of the Putative Weak Topological Insulator BI2TeI Nurit Avraham, Andrew Norris, Lin Pan, Shu-Chun Wu, Claudia Felser, Binghai Yan, Haim Beidenkopf Weak topological insulators are layered materials that possess surfaces with an even number of Dirac cones and surfaces that are fully gapped. This inherent anisotropy provides them with unique properties such as sensitivity to the parity of the number of layers and absence of localization of their surface states. We use scanning tunneling microscopy to study the topological properties of stacked Bi$_{\mathrm{2}}$TeI, a promising candidate for weak topological insulator. We report the observation of the bulk energy gap on terraces perpendicular to the stacking direction and signatures of 1D intra-gap topological edge states along step-edges. The rich structure of quasi 2D terraces and Islands obtained on such cleaved Bi$_{\mathrm{2}}$TeI surfaces provides an excellent playground to explore some of the most fundamental concepts of TIs such as their Z2 classification, ``partner switching'' of Kramer's degenerate pairs, and helical modes along dislocation lines. [Preview Abstract] |
Tuesday, March 15, 2016 5:06PM - 5:18PM |
H29.00014: Optical conductivity of topological surface states with emergent supersymmetry Joseph Maciejko, William Witczak-Krempa Topological states of electrons present new avenues to explore the rich phenomenology of corre- lated quantum matter. Topological insulators (TIs) in particular offer an experimental setting to study novel quantum critical points (QCPs) of massless Dirac fermions, which exist on the sample’s surface. Here, we obtain exact results for the zero- and finite-temperature optical conductivity at the semimetal-superconductor QCP for these topological surface states. This strongly interacting QCP is described by a scale invariant theory with emergent supersymmetry, which is a unique symmetry mixing bosons and fermions. We show that supersymmetry implies exact relations between the op- tical conductivity and two otherwise unrelated properties: the shear viscosity and the entanglement entropy. We discuss experimental considerations for the observation of these signatures in TIs. [Preview Abstract] |
Tuesday, March 15, 2016 5:18PM - 5:30PM |
H29.00015: Proximity-induced global time-reversal symmetry (TRS) breaking and enhanced surface ferromagnetism mediated by Dirac fermions in bilayers of magnetic topological insulators (TIs) C.-C. Chen, M. L. Teague, W. Fan, N.-C. Yeh, L. He, X. Kou, M. Lang, K.-L. Wang Proximity-induced magnetic effects on the surface Dirac spectra of TIs are
investigated by scanning tunneling spectroscopic (STS) studies of bilayer
structures consisting of an undoped TI layer
Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ and
(Bi$_{\mathrm{1-x}}$Sb$_{\mathrm{x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ on
top of a Cr-doped, magnetic TI of 6 quintuple-layer (QL)
thickness.$^{\mathrm{1}}$ For all samples with the top layer thinner than
4-QL, a surface gap $\Delta $ opens up below $T_{c}^{2D} $, much higher than
the bulk Curie temperature $T_{c}^{3D} $ derived from the anomalous Hall
resistance. The temperature ($T)$ evolution of $\Delta $ shows an initial
increase below $T_{c}^{2D} $, followed by a `dip' near $T_{X}$, and then
rises again, reaching maximum at $T\ll T_{c}^{3D} $. The gap is spatially
inhomogeneous, and its average value and spatial homogeneity at low $T$
increases with applied magnetic field $H$ and Cr-doping level $x$. The appearance
of massive Dirac spectra below $T_{c}^{2D} $is the result of global TRS
breaking in the surface state of TIs. The non-monotonic $T$-dependence of
$\Delta $ and the finding of $T_{c}^{2D} \gg T_{c}^{3D}
_{\mathrm{\thinspace }}$may be attributed to proximity magnetism induced
by a 3D contribution from the bulk magnetism that dominates at low $T$, and a
2D contribution from the RKKY interaction mediated by surface Dirac
fermions, which dominates at $T_{c}^{3D} \ll T_{X} |
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