Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z22: Focus Session: Quantum Spin Hall Effect |
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Sponsoring Units: GMAG DMP FIAP Chair: Ron Jansen, University of Twente Room: 324 |
Friday, March 20, 2009 11:15AM - 11:51AM |
Z22.00001: Optical Control of Topological Quantum Transport in Semiconductors Invited Speaker: Spin-orbit coupling enables electrical manipulations of spins, e.g. through the spin Hall effect, but it also causes spin relaxation and thus a rapid loss of information stored in spins. We propose a solution to this dilemma by exploiting light-matter interactions in the reactive regime: light is used as a control knob to switch on/off spin-orbit coupling readily without exciting real carriers. In electron-doped semiconductors, when an off-resonant optical field virtually excites interband transitions, the large spin-orbit coupling in the valence bands can be partially transferred to the photon-dressed conduction band. The adiabatic electronic ground state can thus be reactively controlled by optical pulses, exhibiting anomalous Hall conductivity. By the control of linearly polarized light, a pure spin Hall current of electrons can be driven by an in-plane DC electric field, which results in net spin accumulations at the edges of the optical excitation area. Effectively, one has created a spin battery powered by optical pulses together with DC electric field, which allows the spatial and temporal control of spin generations. The resultant electron spin accumulations can have long lifetime when spin-orbit coupling vanishes with the adiabatic switch off of the control light. Circularly polarized light breaks the time reversal symmetry and can result in spin polarized charge Hall conductance. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z22.00002: Spin resonance and spin-orbit coupling effects in quantum Hall edge channels A.V. Stier, C.J. Meining, V.R. Whiteside, B.D. McCombe, E.I. Rashba, P. Grabs, L.W. Molenkamp We report studies of far-infrared (FIR) photo-response (E$_{FIR}$=3.15meV) of a 2D electron gas in an asymmetric 15nm InAs quantum well in a field/frequency regime where electron spin resonance is expected. Photo-induced changes in the longitudinal resistance were measured in a Hall-bar geometry in a tilted magnetic field (B) whose angle $\theta $ was varied. For $\theta \approx $40\r{ } and Landau Level (LL) filling factor $\nu \approx $7, we observe \underline {several} sharp minima with a dominant central feature. This feature vanishes for $\theta <$38.4\r{ } and splits into two sharper lines at larger angles. The center of gravity of this pair tracks approximately the center of the $\nu $ =7 Quantum Hall (QH) plateau. The appearance of the central feature coincides with the condition of complete filling of the $\nu $=7 LL at an applied B where the Zeeman spin splitting equals the energy of the FIR laser line. We attribute the sharp multiple line structure to EDSR transitions in pairs of QH edge channels whose resonance conditions are modified by Rashba effective fields. A detailed model that describes qualitatively the experimental findings will be discussed. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z22.00003: Tunneling between edge states in a quantum spin Hall system Anders Str\"om, Henrik Johannesson We analyze a quantum spin Hall (QSH) device with a point contact connecting two of its edges. The contact supports a net spin tunneling current that can be probed experimentally via a two-terminal resistance measurement. We find that the low-bias tunneling current and the differential conductance exhibit scaling with voltage and temperature that depend nonlinearly on the strength of the electron-electron interaction. [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z22.00004: Kondo effect in the helical edge liquid of the quantum spin Hall state Joseph Maciejko, Chaoxing Liu, Yuval Oreg, Xiao-Liang Qi, Congjun Wu, Shou-Cheng Zhang Following the recent observation of the quantum spin Hall (QSH) effect in HgTe/CdTe quantum wells, an important question is to understand the effect of impurities on transport in the QSH regime. Using linear response and renormalization group methods, we calculate the edge conductance of a QSH insulator in the presence of a single magnetic impurity. At high temperatures, due to Kondo scattering we find a logarithmic temperature dependence consistent with current experiments. At low temperatures, for weak Coulomb interactions in the edge liquid the conductance is restored to unitarity with unusual power-laws due to the formation of the Kondo singlet, while for strong interactions transport proceeds by weak tunneling through the impurity where only half an electron charge is transferred in each tunneling event. We propose scanning gate and shot noise experiments to search for these effects. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z22.00005: Corner Junction as a Probe of Helical Edge States Chang-Yu Hou, Eun-Ah Kim, Claudio Chamon We propose and analyze inter-edge tunneling in a quantum spin Hall corner junction as a mean to probe the helical nature of the edge states. We show that electron-electron interactions in the one-dimensional helical edge states result in Luttinger parameters for spin and charge that are intertwined, and thus rather different from those for a quantum wire with spin rotation invariance. Consequently, we find that the four-terminal conductance in a corner junction has a distinctive form that could be used as evidence for the helical nature of the edge states. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z22.00006: A topological Dirac insulator in a quantum spin Hall phase David Hsieh, Dong Qian, Lewis Wray, YuQi Xia, Yew San Hor, Robert Cava, Zahid Hasan When electrons are subject to a large external magnetic field, the conventional charge quantum Hall effect dictates that an electronic excitation gap is generated in the sample bulk, but metallic conduction is permitted at the boundary. Recent theoretical models suggest that certain bulk insulators with large spin orbit interactions may also naturally support conducting topological boundary states in the quantum limit, which opens up the possibility for studying unusual quantum Hall-like phenomena in zero external magnetic fields. Bulk Bi$_ {1-x}$Sb$_x$ single crystals are predicted to be prime candidates for one such unusual Hall phase of matter known as the topological insulator. The hallmark of a topological insulator is the existence of metallic surface states that are higher-dimensional analogues of the edge states that characterize a quantum spin Hall insulator. Here, using incident-photon-energy-modulated angle-resolved photoemission spectroscopy, we report the direct observation of massive Dirac particles in the bulk of Bi$_{0.9}$Sb$_{0.1}$ and provide a comprehensive mapping of the Dirac insulators gapless surface electron bands. These findings taken together suggest that the observed surface state on the boundary of the bulk insulator is a realization of the topological metal. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z22.00007: Non-magnetic disorder effects on 3-dimensional $Z_2$ quantum spin Hall systems Ryuichi Shindou, Shuichi Murakami Motivated by the recent discovery of the $Z_2$ quantum spin Hall insulator (QSHI) in the antimony doped bismuth, we have studied the non-magnetic disorder effects onto the quantum critical point (QCP) which always exists between an ordinary insulator and the $Z_2$ QSHI. Namely, intervening the topologically distinct states of matter, such QCP should be generally stable against any perturbations (i.e. disorders), as far as the time-reversal symmetry is preserved. In this talk, I will present a possible microscopic mechanism of this stability, based on simple weak-localization calculations. Specifically, at the QCP between the topological insulator and an ordinary insulator, so-called the {\it parity} degree of freedom also becomes the conserved quantity, in addition to the usual charge. As a result of this, the diffuson near the QCP consists of the {\it two} quasi-degenerate dominant contributions having the diffusion poles; one contributes to the usual charge diffusion, while the other is ascribed to the {\it parity diffusion}. In terms of these two quasi-degenerate low-energy modes, I will construct a possible microscopic picture for the stability of the QCP against no-magnetic disorders. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z22.00008: Magnetic impurities on the surface of a topological insulator Qin Liu, Chao-Xing Liu, Cenke Xu, Xiao-Liang Qi, Shou-Cheng Zhang The surface states of a topological insulator are described by an emergent relativistic massless Dirac equation in 2+1 dimensions. In contrast to graphene, there is an odd number of Dirac points, and the electron spin is directly coupled to the momentum. We show that a magnetic impurity opens up a local gap and suppresses the local density of states. Furthermore, the Dirac electronic states mediate an RKKY interaction among the magnetic impurities which is always ferromagnetic, whenever the chemical potential lies near the Dirac point. These effects can be directly measured in STM experiments. We also study the case of quenched disorder through a renormalization group analysis. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z22.00009: Seeing the magnetic monopole through the mirror of topological surface states Xiao-Liang Qi, Run-Dong Li, Jiadong Zang, Shou-Cheng Zhang Existence of the magnetic monopole is compatible with the fundamental laws of nature, however, this illusive particle has yet to be detected experimentally. In this work, we show that an electric charge near the topological surface state induces an image magnetic monopole charge due to the topological magneto-electric effect. The magnetic field generated by the image magnetic monopole can be experimentally measured, and the inverse square law of the field dependence can be determined quantitatively. We propose that this effect can be used to experimentally realize a gas of quantum particles carrying fractional statistics, consisting of the bound states of the electric charge and the image magnetic monopole charge. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z22.00010: Theoretical study on quantum spin Hall phases in bismuth ultrathin films Shuichi Murakami, Masaki Wada, Frank Freimuth, Gustav Bihlmayer It has been proposed that the (111) 1-bilayer bismuth is in the quantum spin Hall phase [1]. This argument is based on a simple tight-binding model for the 3D bismuth, and it is desirable to examine this result by more realistic methods. In this presentation, we investigate possibilities of the quantum spin Hall phases in two of the bismuth ultrathin films by first- principle calculations and confirmed the result in Ref.[1]. Bulk bismuth is a semimetal, while some of the bismuth ultrathin films have a gap. As proposed by first-principle calculations, among various phases seen in experiments, only two cases are gapped: (i) (111) 1-bilayer film and (ii) \{012\} 2-monolayer film. These two structures are almost degenerate in energy. We calculate the $Z_2$ topological numbers for the two structures. both from the bulk Bloch wavefunctions and from band structure calculations in the geometry with edges. In the calculations we use the maximally localized Wannier orbitals constructed from first-principle calculations. We found that (i) is the quantum spin Hall phase while (ii) is an ordinary insulator. Their difference can be observed in STM/STS and other possible experiments to verify our results are discussed. [1] S. Murakami, Phys. Rev. Lett. 97, 236805 (2006). [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z22.00011: Probing Surface States of the Topological Insulator Bi$_{1-x}$Sb$_{x}$ with Scanning Tunneling Microscopy and Spectroscopy Pedram Roushan, Anthony Richardella, Colin Parker, Kenjiro Gomes, Abhay Pasupathy, Aakash Pushp, Yew San Hor, Robert Cava, Ali Yazdani There is a considerable interest in surface properties of \textit{Bi}$_{1-x}$\textit{Sb}$_{x}$ alloys, for which there is growing evidence that they are topological bulk insulators with novel surface states [1]. We have used a cryogenic scanning tunneling microscope (STM) to probe the surface of \textit{Bi}$_{1-x}$\textit{Sb}$_{x}$ directly, and confirming the presence of surface states within the bulk band gap. Energy resolved conductance mapping of these surface states reveal strong spatial modulations, similar to those observed with the STM for noble metal surface states [2]. Fourier analysis of these maps shows that the spatial modulation of the surface states can be understood within a model for scattering between various k-states of the band structure of the surface. We will present these results in connection with the angle-resolved photoemission measurements of the contours of constant energy. [1] D. Hsieh \textit{et al.}, Nature \textbf{452}, 970 (2008) [2] M. F. Crommie \textit{et al.}, Nature \textbf{363}, 524 (1993) [Preview Abstract] |
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