Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session D35: Focus Session: Spins in Semiconductors -- Topological Insulators |
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Sponsoring Units: GMAG DMP FIAP Chair: Tomasz Dietl, Polish Academy of Sciences Room: E145 |
Monday, March 15, 2010 2:30PM - 2:42PM |
D35.00001: Giant Kerr Effect and Universal Faraday Effect in Thin-film Topological Insulators Wang-Kong Tse, A. H. MacDonald Topological insulators display a novel magneto-electric coupling due to the presence of gapless surface states. In this talk, we present our study on the Kerr and Faraday effects of thin-film topological insulators in the presence of time-reversal symmetry breaking. We find that the low-frequency Faraday rotation is universal completely determined by the fine structure constant $\alpha = e^2/\hbar c = 1/137$. Moreover, we predict a giant infrared Kerr rotation due to the interplay of thin-film cavity confinement and topological surface Hall conductivity of helical quasiparticles. [Preview Abstract] |
Monday, March 15, 2010 2:42PM - 2:54PM |
D35.00002: Spin Aharonov-Bohm effect and topological spin transistor Joseph Maciejko, Eun-Ah Kim, Xiao-Liang Qi Ever since its discovery, the electron spin has only been measured or manipulated through the application of an electromagnetic force acting on the associated magnetic moment. In this work, we propose a spin Aharonov-Bohm effect in which the electron spin is controlled by a magnetic flux while no electromagnetic field is acting on the electron. Such a nonlocal spin manipulation is realized in an Aharonov-Bohm ring made from the recently discovered quantum spin Hall insulator, by taking advantage of the defining property of the quantum spin Hall edge states: the one-to-one correspondence between spin polarization and direction of propagation. The proposed setup can be used to realize a new spintronics device, the topological spin transistor, in which the spin rotation is completely controlled by a magnetic flux of $hc/2e$, independently of the details of the sample. [Preview Abstract] |
Monday, March 15, 2010 2:54PM - 3:06PM |
D35.00003: Well-localized edge states in two-dimensional topological insulator: bismuth film Masaki Wada, Shuichi Murakami, Frank Freimuth, Gustav Bihlmayer We calculate the $Z_2$ topological numbers of bismuth (111) and \{012\} ultrathin films from 2D tight-binding Hamiltonians obtained by first-principle calculation. We find that Bi(111) 1-bilayer is the quantum spin Hall (QSH) phase, while Bi\{012\} 2-monolayer is not. We calculate the QSH edge states of the (111) 1-bilayer film with zigzag edges, and there are three Kramers pairs of edge states at the Fermi energy, resulting in the two terminal conductance $G=6e^2/h$. It will be reduced to $G=2e^2/h$ by increasing nonmagnetic disorder, but will not vanish because of the topological protection. Compared with the known two-dimensional quantum spin Hall systems such as HgTe quantum well, the decay length $\ell$ of edge states of bismuth (111) 1-bilayer system is much shorter and is of the order of a few lattice constant. This short $\ell$ is attributed to the edge-state dispersion traversing over the Brillouin zone. It is in strong contrast with HgTe quantum well, where $\ell$ might be as long as 50nm. [Preview Abstract] |
Monday, March 15, 2010 3:06PM - 3:18PM |
D35.00004: Optical characterization of Bi$_2$Se$_3$ in magnetic field: evidence for magneto-electric coupling Andrew LaForge, Alex Frenzel, Brennan Pursley, Tao Lin, Xinfei Liu, Jing Shi, Dimitri Basov We present an infrared magneto-optical study of Bi$_2$Se$_3$, one of a class of materials recently shown to exhibit topological surface states due to spin-orbit coupling. Far- and mid-infrared (IR) reflectance and transmission measurements have been performed in magnetic fields oriented both parallel and perpendicular to the trigonal $c$ axis of this layered material, and supplemented with UV-visible ellipsometry to obtain the optical conductivity $\sigma_1(\omega)$. With lowering of temperature we observe narrowing of the Drude conductivity due to reduced quasiparticle scattering, as well as the increase of the band gap absorption edge. Magnetic fields $H \parallel c$ produce dramatic effects in the far-IR, revealing significant magneto-electric coupling. For the perpendicular field orientation, a new feature is observed in the far-IR, and the plasma edge is slightly shifted to higher energies. In both cases the band gap edge is softened in magnetic field. [Preview Abstract] |
Monday, March 15, 2010 3:18PM - 3:30PM |
D35.00005: Scanning Tunneling Microscopy of the Topological Insulator Bi$_2$Se$_3$As$_x$ Tess Williams, Anjan Soumyanarayanan, Dillon Gardner, Shaoyan Chu, Young Lee, Jennifer E. Hoffman We use a low temperature scanning tunneling microscope to study the topological insulator Bi$_2$Se$_3$As$_x$. We present atomic resolution topographies displaying three-fold symmetric impurities. We identify features in the density of states. [Preview Abstract] |
Monday, March 15, 2010 3:30PM - 3:42PM |
D35.00006: The $\nu=0$ Quantum Hall state on the surface of a topological insulator Ying Ran, Hong Yao, Ashvin Vishwanath The surface states of a topological insulator such as Bi$_2$Se$_3$ have a single Dirac node, which cannot be realized in any two dimensional system. We study the situation when the chemical potential is at the Dirac point. In this case, it is known that a metal to insulator transition occurs on the surface as interactions are increased. We study the effect of an applied magnetic field over this range of interaction strengths. A phase diagram that is rather different from that of graphene is obtained, by controlled calculations in different limits. Experimental predictions are discussed. [Preview Abstract] |
Monday, March 15, 2010 3:42PM - 3:54PM |
D35.00007: Hexagonal Warping Effects on the Surface of Topological Insulators Liang Fu A single two-dimensinoal Dirac fermion state has been recently observed on the surface of topological insulator Bi$2$Te$3$ by angle-resolved photoemission spectroscopy(ARPES). We study the surface band structure using $k \cdot p$ theory and find an unconventional hexagonal warping term, which is a counterpart of cubic Dresselhaus spin-orbit coupling in rhombohedral latices. We find hexagonal warping naturally explains the observed snowflake shape of the Fermi surface, and extract its magnitude. We predict a number of testable signatures of hexagonal warping in ARPES and STM experiments on Bi$_2$Te$_3$. We also explore the possibility of a spin-density wave due to strong nesting of the Fermi surface. Ref: L. Fu, arXiv:0908.1418, PRL in press [Preview Abstract] |
Monday, March 15, 2010 3:54PM - 4:06PM |
D35.00008: Bulk optical properties of topological insulators Na2IrO3 and Bi2Se3 Don Schmadel, A.B. Sushkov, H.D. Drew, N.P. Butch, J. Paglione, K.R. Choi, Y.J. Choi, S.-W. Cheong Topological insulators have recently attracted much attention due to their unusual electronic properties associated with a topologically protected spin singlet surface state with a Dirac dispersion relation. A novel quantized Kerr/Faraday rotation has been predicted for this system. In preparation for studying this effect, we have measured the bulk optical properties of two topological insulators. We will discuss reflectivity and transmission spectra of single crystals of both compounds in the infrared to UV frequency range and compare with the band structure calculations. [Preview Abstract] |
Monday, March 15, 2010 4:06PM - 4:18PM |
D35.00009: Bulk properties and quantum oscillations in Bi$_2$Se$_3$ topological insulator crystals Nicholas Butch, Kevin Kirshenbaum, Johnpierre Paglione The compound Bi$_2$Se$_3$, a member of the recently heralded class of topological insulators, possesses a spin polarized metallic surface state and is theoretically expected to be a bulk insulator. In practice, stoichiometric Bi$_2$Se$_3$ is metallic, with carriers believed to arise from intrinsic doping by defects. The synthesis of undoped Bi$_2$Se$_3$ will be presented, along with measurements of electrical resistivity, Hall effect, and angle-dependent quantum oscillations. [Preview Abstract] |
Monday, March 15, 2010 4:18PM - 4:30PM |
D35.00010: Electrical transport study of Bi$_{2}$Se$_{3}$ surface states Hadar Steinberg, Dillon Gardner, Young S. Lee, Pablo Jarillo-Herrero We report an electrical transport study of the topological insulator Bi$_{2}$Se$_{3}$ Topological insulators are materials characterized by a gap in the bulk and gapless surface states. The surface states have Dirac dispersions and are protected from back-scattering. The latter protection arises from the fundamental symmetries of the material, and has far reaching consequences for the generation of novel quantum states. We fabricate Bi$_{2}$Se$_{3}$ devices by mechanical exfoliation of 30-80nm thick flakes and standard electron-beam lithography. The devices are investigated using temperature dependence and magnetoresistance. [Preview Abstract] |
Monday, March 15, 2010 4:30PM - 5:06PM |
D35.00011: Dissipationless transport in the quantum spin Hall insulator Invited Speaker: Hartmut Buhmann The increasing understanding of topological phases in condensed matter physics, which was initiated by the quantum Hall effect, has inspired the search for other topological states, especially, in the absence of magnetic fields. As one example the quantum spin Hall (QSH) effect has been proposed for systems with time reversal symmetry and spin-orbit interactions.$^{1,2)}$ In a two-dimensional system this new state is characterized by an insulating bulk and two counter-propagating helical edge states. These Kramers pairs fulfill time reversal symmetry and account for a quantized conductance and spin currents propagate without dissipation. It turned out that HgTe-based quantum well (QW) structures are most suitable candidates for a successful experimental realization.$^{3)}$ In this presentation, the HgTe QW band structure properties and experimental requirements are discussed which lead to the observation of quantized spin polarized edge channel transport, one of the main signatures of the QSH effect.$^{4,5)}$ Experiments will be presented that demonstrate the stability of the quantized conductance and its non-local character.$^{6)}$ Furthermore, it is possible to show evidence for the spin polarization of the QSH edge channels in an all-electrical measurement$^{7)}$ which demonstrates the potential of the QSH effect for possible spin injection and detection application in spintronics devices. 1) C.L. Kane and E.J. Mele, Phys. Rev. Lett \textbf{95}, 226801 (2005). 2) B.A. Bernevig and S.C. Zhang, Phys. Rev. Lett. \textbf{96}, 106802 (2006). 3) B.A. Bernevig, T.L. Hughes, and S.C. Zhang, Science 314, 1757 (2006). 4) M. K\"{o}nig \textit{et al.}, Science \textbf{318}, 766 (2007). 5) M. K\"{o}nig \textit{et al.} Journ. Phys. Soc. Japn. \textbf{77}, 031007 (2008). 6) A. Roth \textit{et al}., Science \textbf{325}, 294 (2009). 7) C. Br\"{u}ne \textit{et al}., in preparation. [Preview Abstract] |
Monday, March 15, 2010 5:06PM - 5:18PM |
D35.00012: Local probing of Quantum Spin Hall edge states Markus K\"onig, Matthias Baenninger, Andrei Garcia, Christoph Br\"une, Hartmut Buhmann, Laurens Molenkamp, David Goldhaber-Gordon Since their recent experimental discovery, topological insulators have attracted a lot of interest. The two- dimensional manifestation of a topological insulator, the Quantum Spin Hall (QSH) state, is characterized by counter- propagating edge states with opposite spin-polarization, while the bulk is insulating. Previous experiments on HgTe quantum well devices have proven the existence of the QSH state in this material and also demonstrated that the transport is indeed due to edge states. We use Scanning Gate Microscopy to study the edge channels of the QSH state. Utilizing the high spatial resolution of this technique, we can get insight into the spatial properties of the edge states. Furthermore, the experiments can yield information regarding the sensitivity of the QSH edge states to local perturbations, which can be useful for future applications. [Preview Abstract] |
Monday, March 15, 2010 5:18PM - 5:30PM |
D35.00013: Decay length of edge/surface states in topological insulators Shuichi Murakami Edge states of 2D quantum spin Hall (QSH) systems or surface states of 3D QSH systems are localized near the boundary of the systems, whereas their decay length $\ell$ may vary. We study their generic behaviors in the presentation. We first note that $\ell^{-1}$ plays the role of the imaginary part of the wavenumber perpendicular to the edge/surface, and $\ell=\infty$ when the edge/surface states are absorbed into the bulk bands. We calculate $\ell$ and discuss their behaviors by using effective models. In accordance to our expectations, we can show that the $\ell$ is shorter when the edge/surface states are far away from the points where the edge/surface states are absorbed into the bulk bands. The edge states in HgTe quantum well or the surface states in Bi$_2$Se$_3 $ have Dirac-like dispersion and longer $\ell$, while in Bi ultrathin films or Bi$_{1-x}$Sb$_x$, $\ell$ is as short as the lattice constant because the edge/surface states spread almost over the Brilloin zone. In particular, in the Dirac-like bands, the minimum $\ell$ corresponds to the inverse of the $k$-space size of the Dirac cone of the edge/surface states. The QSH systems with shorter $\ell$ will be more favorable for real-space observation of edge/surface states. [Preview Abstract] |
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