Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session M10: Anomalous Quantum Hall Effect |
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Sponsoring Units: DCMP Chair: James Williams, University of Maryland Room: 007A |
Wednesday, March 4, 2015 11:15AM - 11:27AM |
M10.00001: Precise quantization of anomalous Hall effect near zero magnetic field Andrew Bestwick, Eli Fox, Xufeng Kou, Lei Pan, Kang Wang, David Goldhaber-Gordon The quantum anomalous Hall effect (QAHE) has recently been of great interest due to its recent experimental realization in thin films of Cr-doped (Bi, Sb)$_2$Te$_3$, a ferromagnetic 3D topological insulator. The presence of ferromagnetic exchange breaks time-reversal symmetry, opening a gap in the surface states, but gives rise to dissipationless chiral conduction at the edge of a magnetized film. Ideally, this leads to vanishing longitudinal resistance and Hall resistance quantized to $h/e^2$, where $h$ is Planck's constant and $e$ is the electron charge, but perfect quantization has so far proved elusive. Here, we study the QAHE in the limit of zero applied magnetic field, and measure Hall resistance quantized to within one part per 10,000. Deviation from quantization is due primarily to thermally activated carriers, which can be nearly eliminated through adiabatic demagnetization cooling. This result demonstrates an important step toward dissipationless electron transport in technologically relevant conditions. [Preview Abstract] |
Wednesday, March 4, 2015 11:27AM - 11:39AM |
M10.00002: Universal scaling of the quantum anomalous Hall plateau transition Jing Wang, Biao Lian, Shou-Cheng Zhang We study the critical properties of the quantum anomalous Hall (QAH) plateau transition in magnetic topological insulators. We introduce a microscopic model for the plateau transition in the QAH effect at the coercive field and then map it to the network model of quantum percolation in the integer quantum Hall effect plateau transition. Generally, an intermediate plateau with zero Hall conductance could occur at the coercive field. $\sigma_{xx}$ would have double peaks at the coercivity while $\rho_{xx}$ only has single peak. Remarkably, this theoretical prediction is already borne out in experiment. Universal scaling of the transport coefficients $\rho_{xy}$ and $\rho_{xx}$ are predicted. [Preview Abstract] |
Wednesday, March 4, 2015 11:39AM - 11:51AM |
M10.00003: Tunable Anderson metal-insulator transition in quantum spin Hall insulators Chui-Zhen Chen, Haiwen Liu, Hua Jiang, Qing-feng Sun, Ziqiang Wang, X.C. Xie We study disorder effects in Bernevig-Hughes-Zhang (BHZ) model (unitary system), and find that Anderson transition of quantum spin Hall insulator (QSHI) is determined by model parameters. In contrast to the common belief that 2D unitary system scales to insulator except at certain critical points, we find that an exotic metallic phase emerges between QSHI and normal insulator phases in InAs/GaSb-type BHZ model. On the other hand, direct transition from QSHI to normal insulator is found in HgTe/CdTe-type BHZ model. Furthermore, we show that the metallic phase originates from the Berry phase and can survive both inside and outside the gap. [Preview Abstract] |
Wednesday, March 4, 2015 11:51AM - 12:03PM |
M10.00004: Spin Texture and Mirror Chern number in Hg-Based Chalcogenides Qing-Ze Wang, Shu-Chun Wu, Claudia Felser, Binghai Yan, Chao-Xing Liu One special feature of surface states in topological insulators is the so-called spin-momentum locking, which means that electron spin is oriented along a fixed direction for a given momentum and forms a texture in the momentum space. In this work, we study spin textures of two typical topological insulators in Hg-Based Chalcogenides, namely HgTe and HgS, based on both the first principle calculation and the eight band Kane model. We find opposite helicities of spin textures between these two materials, originating from the opposite signs of spin-orbit couplings. Furthermore, we reveal that different mirror Chern numbers between HgTe and HgS characterize different topological natures of the systems with opposite spin textures and guarantee the existence of gapless interface states. [Preview Abstract] |
Wednesday, March 4, 2015 12:03PM - 12:15PM |
M10.00005: Edge states in twisted bilayer graphene: quantum spin Hall and electron-hole bilayers Javier D. Sanchez-Yamagishi, Jason Luo, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero Twisted bilayer graphene offers a unique platform for studying 1d edge states in a bilayer 2-dimensional electron gas. Despite being spaced by only 0.34 nm, a large interlayer twist decouples the layers in the bulk, while opening the door for interesting interactions at the edges. To probe this physics, we study the electronic transport through quantum Hall edge modes in twisted bilayer graphene devices. Using dual electrostatic gates, we independently control the filling factor of each layer to form different combinations of bilayer edge states while measuring their conductance. The most dramatic transport effects are observed when the layers are doped to have edge states of opposite chiralities, resulting in coexisting electron- and hole-like states. We will present evidence that, in this regime, the twisted bilayer graphene can form a quantum spin Hall state where edge states in each layer counter-propagate in opposite directions with opposite spin polarizations. This bilayer realization offers a flexible system to study quantum spin Hall edge transport as well as to build more complex 1d circuits. We will also discuss the possibility for fractional generalizations of this edge physics and our measurements of the fractional QHE in twisted bilayer graphene. [Preview Abstract] |
Wednesday, March 4, 2015 12:15PM - 12:27PM |
M10.00006: ABSTRACT WITHDRAWN |
Wednesday, March 4, 2015 12:27PM - 12:39PM |
M10.00007: Interaction induced topological phases in partially flat band graphene-like systems J\"orn W.F. Venderbos, Liang Fu In this work we show how periodically modulated strain fields lead to partially flat bands resembling Landau level quantization due to pseudo-magnetic fields. These partially flat pseudo-Landau levels occur in systems such as graphene or at the interface between a trivial and topological crystalline insulator, where in both cases strain fields couple as pseudo-gauge fields. With these examples in mind, we discuss several different physical implementations. Periodically modulated strain fields hold the promise of realizing Quantum Hall physics on a large scale, whereas constant pseudo-magnetic fields are generally limited to the nanoscale. The electronic properties of these systems being fundamentally altered by the formation of these flat bands, we investigate the effect of electron-electron interaction and find that they induce topological phases, notably the Quantum Anomalous Hall state. [Preview Abstract] |
Wednesday, March 4, 2015 12:39PM - 12:51PM |
M10.00008: Optimizing proximity induced anomalous Hall effect in (Bi$_{x}$Sb$_{1-x})_{2}$Te$_{3}$/YIG heterostructures Zilong Jiang, Chi Tang, Jing Shi, Cuizu Chang, Peng Wei, Jagadeesh S. Moodera The spontaneously broken time reversal symmetry leads to an energy gap in the Dirac spectrum of the surface states of a topological insulator (TI) which gives rise to the quantized anomalous Hall effect (QAHE). QAHE has been observed in TI doped with Cr. Here we explore an alternative route by coupling the surface states of TI with yttrium iron garnet (YIG) ferrimagnetic insulator (FI). Just as in Cr-doped TI, a major challenge is to reduce the bulk conduction which overwhelms the surface state contribution. We have successfully grown 5 quintuple layer thick ternary TI (Bi$_{x}$Sb$_{1-x})_{2}$Te$_{3}$ on atomically flat YIG films, in which the Fermi level of TI can be controlled by the Bi:Sb ratio. We have observed the anomalous Hall effect (AHE) in TI/YIG heterostructure over a wide range of carrier density and in both electron and hole types induced by varying the Bi:Sb ratio from 0:1 to 1:0. Both R$_{xx}$ and R$_{AH}$ undergo systematic and dramatic changes as the Bi:Sb ratio is varied. The maximum R$_{AH}$ occurs near the p-n cross-over region at Bi:Sb ratio \textasciitilde 0.2:0.8, which is nearly two orders of magnitude greater than the minimum value at Bi:Sb ratio \textasciitilde 1:0. As the Bi:Sb ratio is varied, we find that R$_{AH}$ scales quadratically with R$_{xx}$, indicating the scattering rate independent AHE. The electric field effect study further demonstrates the existence of robust AHE while the Fermi level of TI is tuned. This research was supported by UC Lab fees program and a DOE/BES award at UCR, and by NSF/DMR at MIT. [Preview Abstract] |
Wednesday, March 4, 2015 12:51PM - 1:03PM |
M10.00009: Quantized Hall effect from the surface states of topological insulator thin films in a strong magnetic field Anna Pertsova, Carlo M. Canali, Allan H. MacDonald Topological insulators (TIs) is a new class of quantum matter characterized by insulating bulk and topologically protected edge or surface states with peculiar properties. Although simple low-energy models based on the Dirac Hamiltonian are widely used to describe topological surface states, more accurate microscopic models are needed to capture Landau level spectra [1,2] and gate-voltage responses [3] in 3D TIs. In particular, inter-surface hybridization [1] and the inevitability of different electrostatic environments [2] on the opposite surfaces lead to a complex Landau level structure in 3D TIs. In addition, the presence of metallic states on the side surfaces of finite-thickness 3D TI thin films has a significant effect on quantum Hall effect measurements [2]. In this work, we use a microscopic approach to study the properties of topological surface and edge states under a strong quantizing magnetic field in thin films of Bi2Se3 3D TI arranged in a Hall bar geometry, and to predict transport properties in the quantum Hall regime. Our approach is based on magnetic bandstructure calculations for a multi-band tight-binding model of Bi2Se3.\\[4pt] [1] Yang and Han PRB 83,045415(2011); Pertsova et al. arXiv:1411.0831;\\[0pt] [2] Br\"{u}ne et al.PRL 106,126803(2011);\\[0pt] [3] Baum et al.PRB 89,245136(2014) [Preview Abstract] |
Wednesday, March 4, 2015 1:03PM - 1:15PM |
M10.00010: Honeycomb lattice with multiorbital structure: Topological and quantum anomalous Hall insulators with large gaps Gu-Feng Zhang, Yi Li, Congjun Wu We construct a minimal four-band model for the two-dimensional topological insulators and quantum anomalous Hall insulators based on the $p_x$- and $p_y$-orbital bands in the honeycomb lattice. The multiorbital structure allows the atomic spin-orbit coupling which lifts the degeneracy between two sets of on-site Kramers doublets $j_z=\pm \frac{3}{2}$ and $j_z=\pm\frac{1}{2}$. Because of the orbital angular momentum structure of Bloch-wave states at $\Gamma$ and $K(K')$ points, topological gaps are equal to the atomic spin-orbit coupling strengths, which are much larger than those based on the mechanism of the $s-p$ band inversion.The energy spectra and eigen wave functions are solved analytically based on Clifford algebra. The competition among spin-orbit coupling $\lambda$, sublattice asymmetry $m$, and the N\'eel exchange field $n$ results in band crossings at $\Gamma$ and $K(K')$ points, which leads to various topological band structure transitions. The quantum anomalous Hall state is reached under the condition that three gap parameters $\lambda$, $m$, and $n$ satisfy the triangle inequality. Flat bands also naturally arise which allow a local construction of eigenstates. The above mechanism is related to several classes of solid state semiconductor. [Preview Abstract] |
Wednesday, March 4, 2015 1:15PM - 1:27PM |
M10.00011: Observation of topological surface state quantum Hall effect in an intrinsic three-dimensional topological insulator Yang Xu, Ireneusz Miotkowski, Yong Chen A three-dimensional (3D) topological insulator (TI) is a novel quantum matter with a gapped insulating bulk yet a conducting surface hosting topologically-protected gapless surface states of Dirac fermions. One of the most distinct electronic transport signatures predicted for such topological surface states (TSS) is a half-integer quantum Hall effect (QHE) in a magnetic field. We have observed well-developed QHE arising from TSS in an intrinsic TI of $BiSbTeSe_2$[1]. Our samples can exhibit surface dominated conduction even close to room temperature, while the bulk conduction is negligible. At low temperatures and high perpendicular magnetic fields, the Hall conductance shows well quantized integer plateaux in exfoliated flake devices on $SiO_2/Si$ substrates, where the top and bottom surface each contributing a half integer $e^{2}/h$ Hall conductance, accompanied by vanishing longitudinal resistance. We have also studied dual-gated devices where both the top and bottom surfaces can be independently gated. Such intrinsic 3D TI materials exhibiting no measurable bulk conduction and well-developed surface state QHE pave the way for further applications of topological quantum electronics.\\[4pt][1]Yang Xu et al, Nature Physics, doi: 10.1038/nphys3140 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 1:27PM - 1:39PM |
M10.00012: Giant Band Gap Quantum Spin Hall Phase with Weak Spin-Orbit Coupling Zhigang Wu, Marc Dvorak The typical quantum spin hall (QSH) insulator relies on spin-orbit (SO) coupling to open a band gap in the bulk material. However, the intrinsic SO coupling is often rather weak, especially in graphene, and most researchers have focused on enhancing the SO interaction, e.g., by adsorbing heavy adatoms, to increase the bulk band gap. We have demonstrated that if patterned properly, periodic defects on graphene are able to induce intervalley scattering between Dirac points and then open a large ($\sim 1$ eV) bulk band gap. Using both tight-binding method and density functional theory, we explore the possibility of creating a QSH insulator in a graphene nanomesh. We find that an arbitrarily weak SO coupling is able to induce the spin-filtered edge states to traverse the bulk band gap. Here the SO coupling is not responsible for band gap opening, but only serves to connect the K and K' points, leading to a QSH phase with a giant band gap without the existence of strong SO coupling. [Preview Abstract] |
Wednesday, March 4, 2015 1:39PM - 1:51PM |
M10.00013: Prediction of Near-Room-Temperature Quantum Anomalous Hall Effect on Honeycomb Materials Binghai Yan, Shu-Chun Wu, Guangcun Shan Recently, this long-sought quantum anomalous Hall effect was realized in the magnetic topological insulator. However, the requirement of an extremely low temperature ($\sim$ 30 mK) hinders realistic applications. Based on honeycomb lattices comprised of Sn and Ge, which are found to be 2D topological insulators, we propose a quantum anomalous Hall platform with large energy gap of 0.34 and 0.06 eV, respectively. The ferromagnetic order forms in one sublattice of the honeycomb structure by controlling the surface functionalization rather than dilute magnetic doping, which is expected to be visualized by spin polarized STM in experiment. Strong coupling between the inherent quantum spin Hall state and ferromagnetism results in considerable exchange splitting and consequently an ferromagnetic insulator with large energy gap. The estimated mean-field Curie temperature is 243 and 509 K for Sn and Ge lattices, respectively. The large energy gap and high Curie temperature indicate the feasibility of the quantum anomalous Hall effect in the near-room-temperature and even room-temperature regions. Ref: S.-C.Wu, G. Shan, B. Yan, arXiv:1405.4731 (2014). [Preview Abstract] |
Wednesday, March 4, 2015 1:51PM - 2:03PM |
M10.00014: Electronic Structure of Quantum Spin Hall Parent Compound CdTe Guang Bian Cadmium telluride, a compound widely used in devices, is a key base material for the experimental realization of the quantum spin Hall phase. The electronic structure of CdTe has been studied by various theoretical and experimental methods. However, high-resolution band mapping has been lacking to this date. The detailed low-energy electronic structure of CdTe is thus unavailable, but it is of fundamental importance for understanding the topological properties and trends of this type of materials. We report herein, for the first time, a systematic study of the electronic structure of CdTe by angle-resolved photoemission spectroscopy from well-ordered (110) surfaces. The results are compared with first-principles calculations to illustrate the topological distinction between CdTe and a closely related compound HgTe. In addition, topological phase transition from CdTe to HgTe upon alloying and the massless Dirac-Kane semimetal phase at the critical composition are illustrated by computations based on a mixed-pseudopotential simulation. [Preview Abstract] |
Wednesday, March 4, 2015 2:03PM - 2:15PM |
M10.00015: Unexpected edge conduction in HgTe quantum wells under broken time reversal symmetry Eric Yue Ma, M. Reyes Calvo, Jing Wang, Biao Lian, Matthias Muehlbauer, Christoph Br\"une, Yongtao Cui, Keji Lai, Worasom Kundhikanjana, Yongliang Yang, Matthias Baenninger, Markus K\"onig, Christopher Ames, Hartmut Buhmann, Philipp Leubner, Laurens Molenkamp, Shou-Cheng Zhang, David Goldhaber-Gordon, Michael Kelly, Zhi-Xun Shen A key prediction of quantum spin Hall (QSH) theory that remains to be experimentally verified is the breakdown of the edge conduction under broken TRS by a magnetic field. Here we use a unique cryogenic microwave impedance microscopy (MIM) on two HgTe QW devices, corresponding to a trivial (5.5 nm) and an inverted (7.5 nm) band structure, to find unexpectedly robust edge conduction under broken TRS. At zero field and low carrier densities, clear edge conduction is observed only in the local conductivity profile of the 7.5 nm device, consistent with QSH theory. Surprisingly, the edge conduction persists up to 9 T with little effect from the magnetic field, as confirmed by both transport and real space MIM images. This indicates physics beyond current simple QSH models, possibly associated with material-specific properties, other symmetry protection and/or electron-electron interactions. [Preview Abstract] |
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