Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session M13: Focus Session: Topological Materials - Surface Effects |
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Sponsoring Units: DMP Chair: Arun Bansil, Northeastern University Room: 315 |
Wednesday, March 20, 2013 8:00AM - 8:12AM |
M13.00001: Photoelectron spin-flipping and texture manipulation in a topological insulator Chris Jozwiak, Cheol-Hwan Park, Kenneth Gotlieb, Choongyu Hwang, Dung-Hai Lee, Steven G. Louie, Jonathan D. Denlinger, Costel R. Rotundu, Robert J. Birgeneau, Zahid Hussain, Alessandra Lanzara A hallmark characteristic of the recently discovered topological insulators is their protected metallic surface states. Electrons in these surface states are spin polarized with their spins governed by their momentum, resulting in a helical spin texture in momentum space. Spin- and angle-resolved photoemission spectroscopy has been the only tool capable of directly observing this central feature with simultaneous energy, momentum, and spin sensitivity. By using an innovative photoelectron spectrometer with a high-flux laser, we found that the spin polarization of the resulting photoelectrons exhibits rich phenomena previously unobserved. These surprising results provide insight into the physics of these fascinating materials and the use of spin-resolved photoemission in general. [Preview Abstract] |
Wednesday, March 20, 2013 8:12AM - 8:24AM |
M13.00002: Interaction between Dirac fermions and phonons on the (001) surface of the strong 3D topological insulator Bi$_2$Te$_3$ Colin Howard, Michael El-Batanouny, Fang-Cheng Chou, R. Shankar We report on studies of the interaction of Dirac fermion quasiparticles with phonons on the (001) surface of the strong 3D topological insulator Bi$_2$Te$_3$. Studying this coupling is essential for determining the technological viability of this new class of materials. We employed inelastic helium atom scattering to determine surface phonon dispersions along the $\Gamma$M and $\Gamma$KM directions. In contrast to our previous studies on Bi$_2$Se$_3$,\footnote{Zhu, et al. Phys. Rev. Lett. 107, 186102, 2011.} which exhibited a strong Kohn anomaly at $2k_F \approx 0.2$\AA$^{-1}$ in a low-lying optical phonon branch, the current results show a weaker Kohn anomaly at $2k_F \approx 0.1$\AA$^{-1}$ in a similarly low-lying branch. The lower value of $k_F$ is consistent with the smaller carrier concentration in Bi$_2$Te$_3$ as evidenced by Hall conductivity measurements. Our results are further substantiated by lattice dynamical calculations performed within the pseudo-charge model. We also report on a detailed analysis of the electron-phonon coupling as a function of phonon branch index and wave vector utilizing the methods we recently developed.\footnote{Zhu, et al. Phys. Rev. Lett. 108, 185501, 2012. } [Preview Abstract] |
Wednesday, March 20, 2013 8:24AM - 8:36AM |
M13.00003: Polarization-Dependent Scanning Photocurrent Microscopy of Bi2Se3 Behnood Ghamsari, Dohun Kim, Michael Fuhrer, Steven Anlage We measured the spatially-resolved response of Bi2Se3 topological insulator to polarized light by means of scanning photocurrent microscopy. A polarized laser spot of 1 um diameter is raster scanned over a gate-controlled Bi2Se3 two-contact device oriented at 45 degrees to the plane of incidence, and the photo-generated current is measured at each point for varying light polarization states from linearly polarized to right-handed circularly polarized to left-handed circularly polarized. The data is, in turn, used to differentiate the contributions from helicity-dependent spin-orbit coupling effects and helicity-independent photovoltaic and photothermoelectric effects, as well as map their spatial distributions over the device. The experiment is repeated for different carrier densities, through varying the voltage of the back gate, to investigate the dependence of the photoresponse on the carrier density. [Preview Abstract] |
Wednesday, March 20, 2013 8:36AM - 8:48AM |
M13.00004: Investigation of Positron Sticking to the Surfaces of Topological Insulators K. Shastry, P.V. Joglekar, A.Y. Olenga, N.G. Fazleev, A.H. Weiss, B. Barniellini We describe experiments aimed at probing the sticking of positrons to the surfaces of topological insulators. In these experiments, a magnetically beam will be used to deposit positrons at the surface of Bi$_{2}$Te$_{2}$Se. The energy spectra and intensities of electrons emitted as a result of Positron Annihilation induced Auger electron Spectroscopy (PAES) provides a distinct element specific signal which can be used to determine if positrons can be trapped efficiently into a surface localized bound state. The experiments are aimed at determining the practicality of using positron annihilation to selectively probe the critically important top most layer of topological insulator system. [Preview Abstract] |
Wednesday, March 20, 2013 8:48AM - 9:00AM |
M13.00005: Emergent quantum size effects at topological insulator surfaces Phil D.C. King, M.S. Bahramy, G. Balakrishnan, R. Arita, N. Nagaosa, F. Baumberger Bismuth-chalchogenides are model examples of three-dimensional topological insulators. Their ideal bulk-truncated surface hosts a single spin-helical surface state, which is the simplest possible surface electronic structure allowed by their non-trivial Z$_2$ topology. However, real surfaces of such compounds, even if kept in ultra-high vacuum, rapidly develop a much more complex electronic structure\footnote{P.D.C. King et al., Phys. Rev. Lett., 107 (2011) 096802 } whose origin and properties have proved controversial. Here we demonstrate that a conceptually simple model, implementing a semiconductor-like band bending in a parameter-free tight-binding supercell calculation, can quantitatively explain the entire measured hierarchy of electronic states.\footnote{M.S. Bahramy, P.D.C. King et al., Nature Commun. 3 (2012) 1159} In combination with circular dichroism in angle-resolved photoemission experiments, we further uncover a rich three-dimensional spin texture of this surface electronic system, resulting from the non-trivial topology of the bulk band structure. Moreover, our study sheds new light on the surface-bulk connectivity in topological insulators, and reveals how this is modified by quantum confinement. [Preview Abstract] |
Wednesday, March 20, 2013 9:00AM - 9:12AM |
M13.00006: Surface metal doping of topological insulator Bi$_{2}$Se$_{3}$ thin films Y.Y. Li, Y. Liu, M. Weinert, L. Li Three-dimensional topological insulators have attracted much attention due to their spin-momentum locked surface Dirac states, which have been proposed as the basis for spintronics and quantum computing. In the case of Bi$_{2}$Se$_{3}$, thin films grown by molecular beam epitaxy are typically heavily doped n-type, which places the Fermi level outside its band gap, making it challenging to develop devices that rely on the behavior of surface Dirac fermions. In this work, we grow high quality Bi$_{2}$Se$_{3}$ films and tune the topological surface state by metal doping on the surface. The atomic structure and morphology of the metal/Bi$_{2}$Se$_{3}$ are investigated by \textit{in situ} scanning tunneling microcopy. Furthermore, scanning tunneling spectroscopy reveals that the position of Dirac energy can be shifted by as much as 150 meV. These results and comparison with first-principles calculations will be discussed at the meeting. [Preview Abstract] |
Wednesday, March 20, 2013 9:12AM - 9:24AM |
M13.00007: Carrier control via charge transfer at the topological-insulator/organic-molecule interface Katsumi Tanigaki, Yoichi Tanabe, Khuong Kuynh, Takahiro Urata, Ryo Nouchi, Satoshi Heguri, Hidekazu Shimotani A topological insulator is a material that behaves as an insulator as a bulk state, while permitting metallicity on its Dirac cone surface state. One of the most serious issues of recent researches in this field, however, has been the fact that the Fermi levels in many TIs actually fall in either the conduction or valence band due to the naturally occurring defects and must be controlled by further doping. We report here that the major electron carriers on the SS of a Bi$_{2-x}$Sb$_x$Te$_{3-y}$Se$_y$(BSTS) single crystal can be converted to the hole carriers via interface control using 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane(F4-TCNQ), with strong electron affinity. The evidence can be elucidated using a detailed three-carrier model. The results apparently demonstrate that the charge transfer at the TI/organic-molecule interface is very efficient in order to control the carrier density of TIs, particularly on the SS. Our present results will be very important for studying the fundamental aspects of TIs as well as their future device applications. [Preview Abstract] |
Wednesday, March 20, 2013 9:24AM - 9:36AM |
M13.00008: The Effects of Different Ambient Environments on the Electrical Properties of Bi$_{2}$Se$_{3}$ Thin Films over Time Joseph Brom, Malia Kawamura, Joan Redwing It has been recognized recently that the Bi$_{2}$Se$_{3}$ surface is highly susceptible to environmental doping at room temperature when exposed to ambient air. The change in conductivity is correlated to oxidation of the surface; however, the roles of O$_{2}$ and residual H$_{2}$O in the process are not fully understood. In this study, we investigated the effects of different ambient environments (air, O$_{2}$, N$_{2}$, H$_{2}$O) on the electrical properties of Bi$_{2}$Se$_{3}$ thin films grown by hybrid physical-chemical vapor deposition. Hall measurements were performed on samples exposed to each of the gases over a period of several hours to days. The electron concentration of the Bi$_{2}$Se$_{3}$ films initially decreased upon exposure to air but began to rapidly increase and continued to do so over the next several hours. The use of an O$_{2}$ purge resulted in a large initial decrease in electron concentration suggesting that O$_{2}$ rapidly diffuses into Bi$_{2}$Se$_{3}$ and partially compensates the native donors. Over time, however, the electron concentration began to rise rapidly in a similar manner to that observed in air. Exposure of the surface to water vapor resulted in nearly identical behavior to that obtained in air. In contrast, measurements carried out under a N$_{2}$ purge demonstrate a small initial decrease in electron concentration but do not exhibit an appreciable increase in electron concentration even after 24 hours. The mechanism of surface oxidation and conductivity change will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 9:36AM - 9:48AM |
M13.00009: Modeling electron dynamics at the topological insulator-metal interface Susmita Basak, Hsin Lin, Su-Yang Xu, M. Z. Hasan, Arun Bansil The surface environment of the topological insulators possesses ideal properties such as spin-polarized conductivity and suppressed scattering for advanced electronics applications. A major key missing ingredient in this connection is lack of understanding of how topologically ordered electrons respond to the presence of interfaces and various surface terminations that constitute device components at the nanometer scale. To explore these issues we have developed a Green's function implementation of the $k \cdot p$ model to numerically simulate junctions and surfaces of topological insulator $\rm Bi_2Se_3$ based on experimentally measured bulk electron kinetics. Our model explains a number of interesting features observed in ARPES experiments for surface deposition in $\rm Bi_2Se_3$. [Preview Abstract] |
Wednesday, March 20, 2013 9:48AM - 10:24AM |
M13.00010: Interaction of Dirac fermions with surface lattice excitations and electron-phonon coupling on topological insulator surfaces Invited Speaker: Michael El-Batanouny Surface Dirac fermions are robust against backscattering, but other scattering events can affect their anticipated ballistic behavior. Technical improvements may minimize or eventually eliminate surface defects, but phonons are always present. Consequently, coupling to phonons should be the dominant scattering mechanism for Dirac fermions on these surfaces at finite temperatures. Recent measurements of phonon dispersion curves on the (001) surfaces of several binary and ternary topological insulators were carried out using coherent inelastic helium beam surface scattering techniques. The dispersion curves reveal similar features among these materials: first, the absence of long-wavelength Rayleigh waves. Second, the appearance of a low-lying optical phonon branch with isotropic convex dispersive character in the vicinity of the $\Gamma$-point. Lattice dynamics calculations based on the pseudo-charge model show that the optical phonon branch appears with a concave shape when Dirac fermions are absent, but its dispersion changes to a convex shape when Dirac fermions are present. Theoretical analysis attributes this dispersive profile to the renormalization of the surface phonon excitations by the surface Dirac fermions. The contribution of the Dirac fermions to this renormalization is derived in terms of a Coulomb-type perturbation model. Moreover, this optical branch displays a V-shaped minimum at approximately $2\mathbf{k}_F$ that defines a Kohn anomaly. Using a Hilbert transform, we are able to obtain the imaginary part of the phonon self-energy from the real part fitted to the dispersion curve of the surface optical phonon branch. From this imaginary part of the self-energy we obtain a branch-specific electron-phonon coupling constant as a function of wave-vector. The average electron phonon coupling associated with this branch is found to be strong, especially for Bi$_2$Se$_3$, reflecting the pronounced renormalization described above. [Preview Abstract] |
Wednesday, March 20, 2013 10:24AM - 10:36AM |
M13.00011: Direct Real Space Imaging of Quantum Spin Hall Edge States in HgTe Quantum Well Yue Ma, Worasom Kundhikanjana, Jing Wang, Reyes Calvo, Yongliang Yang, Keji Lai, Matthias Baenninger, Markus K\"onig, Christopher Ames, Christoph Br\"une, Hartmut Buhmann, Philip Leubner, Qiaochu Tang, Kun Zhang, Xinxin Li, Laurens Molenkamp, Shou-Cheng Zhang, David Goldhaber-Gordon, Michael Kelly, Zhi-Xun Shen Microscopic real space imaging of the helical edge states is an important milestone to fully elucidate quantum spin Hall effect as a new state of quantum matter. By employing a unique cryogenic microwave impedance microscope, we directly imaged quantum spin Hall edges in a gapped HgTe quantum well. The edge state size increases monotonically as the Fermi level is tuned from p-type across the Dirac point into n-type. Whereas this result is counter-intuitive within any particle-hole symmetric model, it actually agrees well with the 8-band model of real material. Real space evolution of the edge states shows surprising dependence on the magnetic field which could not be explained by Landau level physics assuming a clean system. Alternative scenarios will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 10:36AM - 10:48AM |
M13.00012: Layer-by-layer entangled spin-orbital texture of the topological surface state in Bi$_2$Se$_3$ Zhihuai Zhu, C.N. Veenstra, G. Levy, I.S. Elfimov, A. Damascelli, M.W. Haverkort, A. Ubaldini, P. Syers, N.P. Butch, J. Paglione With their spin-helical metallic surface state, topological insulators (TI) define a new class of materials with a strong application potential in quantum electronic devices. Technological exploitation depends on the degree of spin polarization of the topological surface state (TSS) - assumed to be 100$\%$ in phenomenological models. Yet in real materials, spin- and angle-resolved photoemission spectroscopy (ARPES) showed that the TSS spin polarization varies over a wide range: 20-85$\%$. This striking variation in TSS spin polarization has remained unexplored, leaving an undefined application prospect of TIs. Here we present a light-polarization study of ARPES momentum maps to unveil the entangled spin-orbital texture of the TSS in Bi$_2$Se$_3$. By determining the layer-by-layer evolution of this spin-orbital entanglement, we solve the puzzle of the observed TSS spin polarization and also provide means to manipulate the spin polarization of photoelectrons and photocurrents in TI devices. [Preview Abstract] |
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