Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session W9: QHE and topological insulators |
Hide Abstracts |
Sponsoring Units: DCMP Chair: Marcel Franz, University of British Columbia Room: A105 |
Thursday, March 18, 2010 11:15AM - 11:27AM |
W9.00001: Fractional topological insulators Adiel (Ady) Stern, Michael Levin We analyze generalizations of two dimensional topological insulators which can be realized in interacting, time reversal invariant, electron systems. These states, which we call fractional topological insulators, contain excitations with fractional charge and statistics in addition to protected edge modes. In the case of $s^z$ conserving toy models, we show that a system is a fractional topological insulator if and only if $\sigma_{sH}/e^*$ is odd, where $\sigma_{sH}$ is the spin-Hall conductance in units of $e/2\pi$, and $e^*$ is the elementary charge in units of $e$. [Preview Abstract] |
Thursday, March 18, 2010 11:27AM - 11:39AM |
W9.00002: Robustness of topological invariants: Implications for localization Emil Prodan A class of bulk and edge topological invariants that includes the spin-Chern number has been recently shown to be robust against smooth deformations of the models and disorder [Phys. Rev. B \textbf{80}, 125327 (2009), J. Math. Phys. \textbf{50}, 083517 (2009), Journal of Phys. A: Math. Theor. \textbf{42} 082001 (2009), Journal of Phys. A: Math. Theor. \textbf{42}, 065207 (2009)]. Such topological invariants are useful for classifying the topological phases, but in some instances their robustness have non-trivial consequences for the localization of the bulk and edge states. For Integer Quantum Hall Effect, for example, the robustness of the Chern number implies existence of bulk states that resist localization. In this talk, I will discuss what topological invariants can tell us about the localization of the bulk and edge states in Quantum spin-Hall Insulators. [Preview Abstract] |
Thursday, March 18, 2010 11:39AM - 11:51AM |
W9.00003: Discovery of spin-textured single-Dirac-cone pi-Berry's phase Topological Insulator states in Bi2Se3, Bi2Te3 and Sb2Te3 and related new materials M. Zahid Hasan, David Hsieh, Yuqi Xia, L. Andrew Wray, Dong Qian, J. Hugo Dil, Fabian Meier, Luc Patthey, Jurg Osterwalder, Alexei Fedorov, Hsin Lin, Arun Bansil, David Grauer, Yewsan Hor, Robert Cava The topological insulator is a fundamentally new state of quantum matter that exhibits exotic quantum-Hall-like behavior even in the absence of an applied magnetic field. In this talk, I will present new results on the topological insulator in Bi1-xSbx beyond Ref-[1,2], and then report our discovery and findings of a new generation of topological insulators with a single spin-helical surface Dirac cone that can be gated by chemical tuning of the surface [3,4,5]. A method would be discussed how superconducting and magnetic interactions [6] can be realized within the topological matrix. [1] ``A topological Dirac insulator in a quantum spin Hall phase'', D. Hsieh et al., Nature 452, 970 (2008). [2] ``Observation of unconventional quantum spin textures in topological insulators'', D. Hsieh et al., Science 323, 919 (2009). [3] ``Observation of a large-gap topologicalinsulator class with a single Dirac cone on the surface'', Y. Xia et al., Nature Phys. 5, 398 (2009). [4] D. Hsieh et al., Phys. Rev. Lett., 103, 146401 (2009). [5] ``A tunable topological insulator in the spin helical Dirac transport regime'', D. Hsieh et al., Nature 460, 1101 (2009). [6] Preprint (2009). [Preview Abstract] |
Thursday, March 18, 2010 11:51AM - 12:03PM |
W9.00004: Wormhole effect in a strong topological insulator M. Franz, G. Rosenberg, H.-M. Guo When the surface of a strong topological insulator (STI) is coated by a ferromagnetic film the surface state acquires a gap and becomes a quantum Hall liquid with the Hall conductance $(n+1/2)e^2/h$, $n$ integer. Applying the Laughlin flux-insertion argument to such a surface implies the existence of quasiparticles with fractional charge $\pm e/2$. This result however contradicts the microscopic theory of the surface state, given by a simple massive Dirac Hamiltonian with odd number of fermion species, which only has excitations with integral charge. The paradox is resolved by noticing that an infinitesimally thin flux tube employed in the Laughlin argument is not innocuous inside a STI; we show that when the flux is equal to $hc/2e$ the flux tube carries topologically protected one-dimensional gapless modes that form a conducting `wormhole' through the STI bulk. The wormhole allows the excess charge to escape to the other side of the STI and, in the end, no fractional charge is accumulated at the surface. [Preview Abstract] |
Thursday, March 18, 2010 12:03PM - 12:15PM |
W9.00005: Topological Insulators on kagome and pyrochlore lattices H.-M. Guo, M. Franz Electrons hopping on the sites of a two-dimensional kagome and a three-dimensional pyrochlore lattice are shown to form topologically non-trivial insulating phases when the spin-orbit (SO) coupling and lattice distortions are present. On the kagome lattice SO coupling produces a quantum spin Hall state at both 1/3 and 2/3 fillings. On the pyrochlore lattice 9 topological classes representing both strong and weak topological insulators are realized for various ranges of model parameters and fillings. Other interesting phases, such as the dimerized, trimerized and Kekule insulators are also obtained. Possible realizations of this physics in crystalline solids are discussed. [Preview Abstract] |
Thursday, March 18, 2010 12:15PM - 12:27PM |
W9.00006: Witten effect in a topological insulator Gilad Rosenberg, Marcel Franz It has been established recently that topological insulators exhibit non-trivial electromagnetic response described by the `axion' term ${\Delta{\cal L}_{\rm axion}=\theta (e2/2\pi h){\bf E} \cdot {\bf B}}$ with $\theta=\pi$. The parameter $\theta$ is known to particle physicists as the axion field and whether or not it has a nonzero vacuum expectation value remains a fundamental open question of the Standard Model. A key manifestation of axion electrodynamics is the Witten effect, according to which a unit magnetic monopole placed inside an axion medium is predicted to bind fractional charge $-e(\theta/2\pi)$. I will present a proposal for a first test of the Witten effect. Using a simple model for a topological insulator we demonstrate the existence of a fractional charge bound to a monopole by an explicit numerical calculation. We also propose a scheme for generating an `artificial' magnetic monopole in a topological insulator film that may be used to facilitate an experimental test of Witten's prediction in a solid state system. [Preview Abstract] |
Thursday, March 18, 2010 12:27PM - 12:39PM |
W9.00007: Topological insulators on the Lieb and Perovskite lattices Conan Weeks, Marcel Franz We study a system of tight-binding electrons on both the 2D Lieb and 3D edge-centered cubic (Perovskite) lattices. With spin-orbit coupling between next-nearest neighbor sites these systems become band insulators with non-trivial Z2 topological invariants. We calculate the Z2 invariants for various fillings and study the emergence of topologically protected gapless edge and surface modes. Aside from establishing simple models for topological insulators with cubic symmetry, we also discuss some possible candidates among the naturally occurring Perovskites. [Preview Abstract] |
Thursday, March 18, 2010 12:39PM - 12:51PM |
W9.00008: Inverse spin-galvanic effect in the interface between a topological insulator and a ferromagnet Ion Garate, Marcel Franz When a ferromagnet is deposited on the surface of a topological insulator, the topologically protected surface state develops a gap and becomes a 2-dimensional quantum Hall liquid. We demonstrate that the Hall current in such a liquid, induced by an external electric field, can have a large effect on the magnetization dynamics of the ferromagnet by changing the effective anisotropy field. This change may be substantial even in weakly spin-orbit coupled ferromagnets. We study the possibility of dissipationless current-induced magnetization reversal in monolayer-thin, insulating ferromagnets with a soft perpendicular anisotropy. [Preview Abstract] |
Thursday, March 18, 2010 12:51PM - 1:03PM |
W9.00009: Orbital magnetoelectric coupling in band insulators Andrew Essin, Ari Turner, Joel Moore, David Vanderbilt Magnetoelectric responses are a fundamental characteristic of materials that break time-reversal and inversion symmetries, notably multiferroics, and, remarkably, of topologically nontrivial materials in which those symmetries are unbroken. Previous work has shown how to compute almost all contributions to the magnetoelectric tensor. Here we complete the linear response problem for band insulators by computing the electronic polarization resulting from an applied magnetic field. One part of this response can appear even in time-reversal-symmetric materials and was previously shown to be quantized in ``topological insulators.'' In so doing we present both a useful formalism for dealing with uniform magnetic fields and a treatment based on a long wave approximation of the field. Finally, we discuss properties of band structures that can cause the response to vanish. [Preview Abstract] |
Thursday, March 18, 2010 1:03PM - 1:15PM |
W9.00010: Fractional Quantum Hall Effect and Featureless Mott Insulators Anton Burkov We point out and explicitly demonstrate a close connection that exists between featureless Mott insulators and fractional quantum Hall liquids. Using magnetic Wannier states as the single-particle basis in the lowest Landau level (LLL), we demonstrate that the Hamiltonian of interacting particles (either bosons or fermions) in the LLL maps onto the Hamiltonian of a featureless Mott insulator on triangular lattice, formed by the magnetic Wannier states. The Hamiltonian is remarkably simple and consists only of short-range repulsion and ring-exchange terms. [Preview Abstract] |
Thursday, March 18, 2010 1:15PM - 1:27PM |
W9.00011: Quantum Hall plateau transition as a transition between Anderson and band insulators Dmitri Feldman We consider a system of parallel quantum wires in a magnetic field. Noninteracting electrons can tunnel between neighboring wires and are backscattered by impurities in the wires. In an appropriate range of tunneling constants, an infinite array of such wires forms a band insulator at weak disorder and an Anderson insulator at strong disorder. In a finite 2D array, the band insulator state possesses two delocalized chiral edge modes and thus exhibits the quantum Hall effect. An exact solution for the second order phase transition between Anderson and band insulating states can be obtained for a system of wires arranged in a Bethe lattice with an infinite coordination number $z$. The quantum Hall transition in 2D can be accessed through a $1/z$-expansion with $z=2$. [Preview Abstract] |
Thursday, March 18, 2010 1:27PM - 1:39PM |
W9.00012: Transport Between Supercondutors Coupled to Quantum Hall Edge Modes Stephanie Law, Michael Vissers, Allison Dove, Serena Eley, Nadya Mason, James Eckstein We report IV characteristics and differential resistance measurements on two dimensional electron gases coupled to superconducting electrodes in high magnetic fields. The semiconductor layer is grown by MBE, followed by in-situ deposition of a superconducting NbTi film. For measurement, the samples are fabricated into Hall bars with the superconducting contacts connected to points along the edge. Differential resistance and IV characteristics are then measured in two and four terminal setups at low temperatures in magnetic fields, on and off quantum Hall plateaus. We are thus able to investigate the ability of chiral edge modes to couple to a superconductor and mediate a supercurrent. Samples are made both with high critical field superconducting contacts and with normal metal contacts, allowing us to isolate the effect of injecting Cooper pairs. [Preview Abstract] |
Thursday, March 18, 2010 1:39PM - 1:51PM |
W9.00013: Quantum Transport in Oxide Nanostructures Jeremy Levy, Cheng Cen, Daniela F. Bogorin We describe low-temperature magnetotransport experiments in nanostructures formed at the LaAlO$_3$/SrTiO$_3$ interface using a conducting AFM writing technique.\footnote{C. Cen, S. Thiel, K. E. Andersen, C. S. Hellberg, J. Mannhart, and J. Levy, Nature Materials \textbf{7}, 2136 (2008).}$^,$\footnote{C. Cen, S. Thiel, J. Mannhart, and J. Levy, Science \textbf{323}, 1026 (2009).} Measurements on a 6-nm wide Hall cross containing $N\approx 250$ electrons (density $n=1.6x10^{12}cm^{-2}$) show evidence for quantized conductance, with notable departures from traditional quantized Hall behavior. A pronounced weak antilocalization feature near $B=0$ is exhibited in both the Hall and magnetoresistance channels, indicating the presence of significant spin-orbit (Rashba) coupling. A 14-nm wide nanowire with lower carrier density (density $n=8.5x10^{11}cm^{-2}$) exhibits magnetoresistance plateaus associated with integer Landau level filling factors $\nu$=2,3,...,9, and the fractional filling factors $\nu$=7/3 and 11/5. The ability to fashion conducting structures with extreme nanoscale dimensions and distinct signatures of quantum transport opens new opportunities for the development of novel quantum devices. [Preview Abstract] |
Thursday, March 18, 2010 1:51PM - 2:03PM |
W9.00014: Signatures of surface states in bismuth at high magnetic field Babak Seradjeh, Jiansheng Wu, philip phillips Electrons in a metal subject to magnetic field commonly exhibit oscillatory behavior as the field strength varies, with a period set by the area of quantized electronic orbits. Recent experiments on elemental bismuth have revealed oscillations for fields above 9 tesla that do not follow this simple dependence and have been interpreted as a signature of electron fractionalization in the bulk. We argue instead that a simple explanation in terms of the surface states of bismuth exists when additional features of the experiment are included. These surface electrons are known to have significant spin-orbit interaction. We show the observed oscillations are in quantitative agreement with the surface theory, which we propose to test by studying the effect of the Zeeman coupling in higher fields, dependence on the field orientation, and the thickness of the samples. [Preview Abstract] |
Thursday, March 18, 2010 2:03PM - 2:15PM |
W9.00015: Z$_{2}$ topological classification in BEC-BCS crossover phenomena Mitsuhiro Arikawa, Isao Maruyama, Yasuhiro Hatsugai Recently, the BEC-BCS crossover is experimentally realized in ultracold atomic Fermi gases--- the Bose-Einstein condensation of real space molecules in a strongly attractive system and the BCS superfluidity in a weakly coupled case. They are not clearly distinguished by the standard order parameters. They are just separated as a crossover, that is, the two ground states are adiabatically connected even in the thermodynamic limit. Introducing a topological order parameter as the Z$_{2}$ Berry phase with a local U(1) twist[1], we have discussed the BCS hamiltonian in a real space for the chiral symmetric case. This local Z$_{2}$ Berry phase distinguishes the BEC-BCS crossover as a local quantum phase transition, that is the phases are separated by closing of the energy gap under the local twist, ~although the gap of the translational invariant system is always open. Physically it characterizes the paired electron is itinerant or localized. This comes from the bulk-edge correspondence in the BEC-BCS crossover as is well established in the Quantum Hall effects. [1] ~Y. Hatsugai, J. Phys. Soc. Jpn. 75, 123601 (2006) . [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700