Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session V35: Topological Insulators: ARPES & STM |
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Sponsoring Units: DCMP Chair: Shoucheng Zhang, Stanford University Room: C140 |
Thursday, March 24, 2011 8:00AM - 8:12AM |
V35.00001: Local probing of Quantum Spin Hall edge states Markus K\"{o}nig, Andrei Garcia, Matthias Baenninger, 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. We use Scanning Gate Microscopy to demonstrate the edge state nature of transport in the QSH state. Utilizing the high spatial resolution of this technique, we gain 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] |
Thursday, March 24, 2011 8:12AM - 8:24AM |
V35.00002: Visualizing Strong Scattering of Topological Surface States from Magnetic Impurities in Bi$_2$Te$_3$ Haim Beidenkopf, Pedram Roushan, Jungpil Seo, Lindsay Gorman, Y.S. Hor, R.J. Cava, Ali Yazdani Bi$_2$Te$_3$ is a topological insulator with a single Dirac cone in the band structure of its helical surface states. The associated spin texture protected by time reversal symmetry (TRS) is thought to suppress scattering off non-magnetic defects. We tested this using scanning tunneling microscopy and spectroscopy. At high energies, far above the Dirac point, backscattering off non-magnetic defects, such as step-edges, is facilitated by quasi-nesting conditions brought about by the hexagonal warped surface band. At lower energies at which the surface dispersion is linear backscattering is highly suppressed by the helical spin texture protected by TRS. In contrast, in Mn-doped Bi$_2$Te$_3$ the measured quasi- particle interference pattern shows the onset of strong scattering both in the warped region as well as in the conic one. The scattering processes involved are affected both by the spin texture as well as by the geometry of the scattering potential. Furthermore, close to the Dirac point the increased scattering in Mn-doped Bi$_2$Te$_3$ seems to promote localization of the surface states. [Preview Abstract] |
Thursday, March 24, 2011 8:24AM - 8:36AM |
V35.00003: Magnetic Versus Non-magnetic Scattering of Topological Surface States in Bi$_{2}$Te$_{3}$ Lindsay Gorman, Haim Beidenkopf, Pedram Roushan, Jungpil Seo, Yew San Hor, Robert Cava, Ali Yazdani Due to their novel spin texture, the surface states of topological insulators are predicted to be impervious to backscattering from non-magnetic disorder. For impurities which break time-reversal symmetry, however, such backscattering is not forbidden by the topological character of the states. Here we use scanning tunneling microscopy to study scattering from impurities in doped Bi$_{2}$Te$_{3}$. In Mn-doped Bi$_{2}$Te$_{3}$, we have observed an interference pattern from the surface states throughout a broad range of energies, even in the region of linear dispersion near the Dirac point. We contrast these findings of the scattering of topological surface states from magnetic defects with similar measurements on Ca-doped Bi2Te3 using spectroscopic mapping. We will use the results of these experiments to probe whether the presence of magnetic impurities gives rise to backscattering in topological surface states. [Preview Abstract] |
Thursday, March 24, 2011 8:36AM - 8:48AM |
V35.00004: Scanning tunneling spectroscopic (STS) studies of MBE-grown topological insulators of Bi$_{2}$Se$_{3}$ epitaxial films on Si(111) N.-C. Yeh, M.L. Teague, W.-H. Lin, H. Chu, F.X. Xiu, L. He, K.L. Wang We report STS studies of MBE-grown Bi$_{2}$Se$_{3}$ epitaxial films on Si(111) with varying thicknesses. The films were atomically flat on the scale of hundreds of nanometers, with occasional atomic steps of one c-axis lattice constant. In the case of thick Bi$_{2}$Se$_{3}$ films, the tunneling spectra were consistent with those found in single crystalline Bi$_{2}$Se$_{3}$, except that the Dirac point ($E_{Dirac}=-$50 $\sim $ -100 meV) of the MBE-film is generally much closer to the Fermi level ($E$ = 0), in contrast to the large downshift of $E_{Dirac}$ (= -400 $\sim $ -200 meV) commonly found in single crystalline bulk grown Bi$_{2}$Se$_{3}$. The STS spectra of the thinner films deviate from those of the thicker samples, probably the result of strain. Fourier transformed (FT) STS data as a function of energy reveals several quasiparticle scattering interference wave-vectors that are consistent with the topologically protected surface states with chiral spin texture, although the overall FT-STS maps are simpler than those reported on the Bi$_{0.92}$Sb$_{0.08}$ (111) surface due to simpler electronic band-structures of Bi$_{2}$Se$_{3}$. The effect of time reversal symmetry breaking on the FT-STS will be investigated by either magnetic doping or application of magnetic fields. This work was supported by a grant from FENA of FCRP and DARPA. [Preview Abstract] |
Thursday, March 24, 2011 8:48AM - 9:00AM |
V35.00005: Electron interference in the 3D topological insulator Bi$_{2}$Se$_{3}$ probed by scanning tunneling microscope Mao Ye, A. Kimura, S. Kim, K. Kuroda, E.E. Krasovskii, E.V. Chulkov, K. Miyamoto, M. Nakatake, T. Okuda, Y. Ueda, H. Namatame, M. Taniguchi Three-dimensional topological insulators (TIs) have aroused great attention to the new state of quantum matter originating from the surface state that forms a massless Dirac cone. Among the recently discovered TIs, Bi$_{2}$Se$_{3}$ is regarded as the most promising candidate [1]. However, recent magnetotransport measurements showed that the bulk conductance dominates even in low carrier samples [2], which raises the question of possible scattering channels responsible for the reduced surface mobility. Band structure calculations predict the Dirac point of the surface state to be located close to the bulk valence band maximum [1]. In order to clarify the surface state scattering feature, we have performed differential tunneling conductance mapping for the surface of Bi$_{2}$Se$_{3}$. The fast Fourier transformation image shows an electron interference pattern near the Dirac node, which provides the evidence of near-surface scattering of the spin polarized surface electrons at the Dirac point in Bi$_{2}$Se$_{3}$ into the spin-degenerate bulk continuum states. \\[4pt] [1] Y. Xia et al., Nat. Phys. \textbf{5}, 398 (2009). \\[0pt] [2] N. P. Butch, Phys. Rev. B \textbf{81}, 241301(R) (2010). [Preview Abstract] |
Thursday, March 24, 2011 9:00AM - 9:12AM |
V35.00006: Power laws and STM image of standing wave of the topological surface states Bang-Fen Zhu, Jing Wang, Wei Li, Peng Cheng, Canli Song, Tong Zhang, Xi Chen, Xucun Ma, Ke He, Jin-Feng Jia, Qi-Kun Xue We have theoretically and experimentally studied the quasiparticle interference pattern caused by scattering off the step edges of topological surface states in Bi$_{2}$Te$_{3}$ and Bi$_{2}$Se$_{3}$. We propose a general formalism to identify the power law that governs the decaying spatial oscillations of standing wave of the quasiparticle. With strong hexagonal warping of the surface states in Bi$_{2}$Te$_{3}$, the standing wave will have different decay index as the Fermi energy varies; while in Bi$_{2}$Se$_{3}$, the standing wave has only a single decay index due to weak warping effect. Using a scanning tunneling microscope, we directly observe the standing waves in the local density of states on both surfaces, which together with the analysis of such oscillations at different voltage confirms our theoretical predictions. We further show that, the characteristic scattering wavevectors of the standing wave of surface states caused by scattering off the nonmagnetic impurity in both Bi$_{2}$Te$_{3}$ and Bi$_{2}$Se$_{3 }$can also be well explained by this general formalism. [Preview Abstract] |
Thursday, March 24, 2011 9:12AM - 9:24AM |
V35.00007: Multifunctional electronic structure in a topological insulator class Suyang Xu, Zahid Hasan The discovery of topological properties in three dimensional bulk solids have opened up many new research avenues in condensed matter physics. Only a very few compounds have been identified to be topological insulators to this date. However, none of them is proven to be suitable for the majority of experimental configurations including giant magnetoelectric and anomalous optical rotation, unusual exciton condensation, or the neutral half-fermions and interface superconductivity. In fact the realization of even any one of these proposals requires a number of multiply-connected topological compounds with modulated surface band dispersions and naturally tuned in-gap Fermi level, as well as spin variations in the presence of long life-time of the surface states. Here, using conventional and spin-sensitive probes, we report the discovery of several classes of positive band-gap high figure of merit topological insulators with critically important functional properties such as high degree of bulk resistivity and insulation, electronic structure with both in-gap Dirac point and Fermi level crossing, long surface state life-times, as well as chirality inversion through the Dirac node. The unprecedented combinations of electronic, spin, life-time and resistive bulk transport featured by the topological insulators uncovered here not only provide a new platform for research on topological quantum phenomena but also pave the way for functional devices. [Preview Abstract] |
Thursday, March 24, 2011 9:24AM - 9:36AM |
V35.00008: Observation of novel interference patterns in BixFe1-xTe3 by Fourier transform scanning tunneling spectroscopy (FT-STS) Yoshinori Okada, Chetan Dhital, Wen-Wen Zhou, Hsin Lin, Susmita Basak, Arun Bansil, Yaobo Huang, Hong Ding, Ziqiang Wang, Stephen Wilson, Vidya Madhavan We utilize Fourier transform scanning tunneling spectroscopy (FT-STS) to probe the surface of the magnetically doped TI, Bi2-xFexTe3. Our measurements show the appearance of a hitherto unobserved channel of electronic backscattering along the surface q-vector. By referencing the FT-STS with angle-resolved photoemission spectroscopy (ARPES) data, we formulate a simple model showing that these new vectors are fully consistent with spin-flip scattering. Our combined data therefore present compelling evidence for the first momentum resolved measurement of enhanced backscattering due to magnetic impurities in a prototypical TI. [Preview Abstract] |
Thursday, March 24, 2011 9:36AM - 9:48AM |
V35.00009: Local interaction of magnetic impurities and topological surface states Warren Mar, Kenjiro K. Gomes, Wonhee Ko, Hari C. Manoharan Topological insulators have garnered much attention as a vehicle to explore exotic Dirac physics through the projection of unpaired Dirac cones into conducting surface states wrapping a spin-orbit twisted bulk band structure. We use an ultrahigh-vacuum low-temperature scanning tunneling microscope (STM) to gain access to and manipulate the chiral Dirac particles present on the Sb(111) surface. Understanding the interplay between local spins and Dirac fermions represents a key foundation to the development of new spintronic applications. Magnetic moments break time-reversal symmetry and provide an additional local quantum degree of freedom to engineer topological states. By dosing magnetic impurities of varying concentration and species, we show how STM can atomically manipulate individual magnetic adatoms on topological surfaces, and in the process gain insight into the physical bonding arrangement of magnetic impurities on top of and embedded inside the host crystal lattice. Using scanning tunneling spectroscopy, we map in real and momentum space how local spins interact with the chiral surface Dirac carriers. [Preview Abstract] |
Thursday, March 24, 2011 9:48AM - 10:00AM |
V35.00010: Visualizing spin-vortex evolution of a topological insulator Yihua Wang, David Hsieh, Dan Pilon, Liang Fu, Dillon Gardner, Young Lee, Nuh Gedik Charge carriers on the surface of a topological insulator are predicted to form a spin-vortex in momentum space with the direction of spin rotation determined by whether the carriers are electron-like or hole-like. We show that the angular momentum of photon is extremely sensitive to the spin of carriers by performing time-of-flight based angle-resolved-photoelectron spectroscopy (TOF-ARPES) with photons of different helicity. We demonstrate the first reciprocal space volumetric mapping of the vectorial spin-texture of the surface states of $Bi_2 Se_3$ and directly observe spin-vortex evolution from electron-like to hole-like states and the departure from perpendicular momentum-spin locking. [Preview Abstract] |
Thursday, March 24, 2011 10:00AM - 10:12AM |
V35.00011: Spin and angular reoslved photoemission studies of Bi2Se3 Zhihui Pan, E. Vescovo, G.D. Gu, D. Gardner, S. Chu, Y. Lee, A.V. Fedorov, T. Valla Topological insulators (TL) have attracted much attention because of their exotic properties. Bi$_2$Se$_3$ is a model TL with a relative large bulk gap and a simple surface state structure. By depositing various non-magnetic and magnetic impurities on the surface, we were able to fill the topological surface state and higher lying Rashba splitting surface states. The spin texture of the surface electronic structure was determined in spin resolved photoemission measurement. [Preview Abstract] |
Thursday, March 24, 2011 10:12AM - 10:24AM |
V35.00012: Surface states never die by surface impurities in topological insulators Han-Jin Noh, Jinwon Jeong, En-Jin Cho, Han-Koo Lee, Hyeong-Do Kim The metallic surface states in topological insulators are one of the most distinguished features among the characters of this newly discovered quantum state of matter. These states, if properly exploited, may open a new era in spintronics and quantum computing. However, full characterization and understanding of the surface states toward these goals are still far from satisfactory. Here, we focus on the robustness of the metallic surface states in a topological insulator Bi$_{0.9}$Sb$_{0.1}$, and demonstrate their durability over magnetic/non-magnetic surface impurities by measuring the scattering rates of the quasiparticles via angle-resolved photoemission spectroscopy. [Preview Abstract] |
Thursday, March 24, 2011 10:24AM - 10:36AM |
V35.00013: Experimental Realization of Three-dimensional Topological Insulator in Ternary Chalcogenides K. Kuroda, M. Ye, A. Kimura, Y. Ueda, S.V. Eremeev, E.E. Krasovskii, E.V. Chulkov, K. Miyamoto, T. Okuda, K. Shimada, H. Namatame, M. Taniguchi Three-dimensional topological insulators (TIs) featured with spin-helical massless surface state have attracted a great attention. Up to now, the experimentally confirmed topological insulators are limited to some binary compounds, such as Bi$_{2}$Te$_{3}$, Bi$_{2}$Se$_{3}$ and so on. Recently, several ternary chalcogenides have been proposed as a new family of TI. In contrast to the layered binary chalcogenides, in ternary chalcogenides with a more substantial three dimensional character, the surface state depends on the topmost layer because the broken bonds at the surface may give rise also to trivial surface state. Therefore, the experimental realization of non-trivial surface state in TI has been strongly required. In this work, we have performed an angle resolved photoemission spectroscopy by using synchrotron radiation to prove the surface state in the ternary compounds. Especially, for one of the candidate materials, TlBiSe$_{2}$, two important aspects have been revealed: (i) The Dirac cone is more ideal than that of Bi$_{2}$Se$_{3}$. (ii) There are no bulk continuum states that energetically overlap with the Dirac point. This means that the scattering channel from the topological surface state to the bulk continuum is strongly suppressed in TlBiSe$_{2}$. [Preview Abstract] |
Thursday, March 24, 2011 10:36AM - 10:48AM |
V35.00014: STM/STS studies of the surface of Bi2Se3 Megan Romanowich, Stuart Tessmer, Sergei Urazhdin, Duck-Young Chung, Jung-Hwan Song, Mercouri Kanatzidis Building upon previous work,\footnote{Urazhdin S. et al. Physical Review B 69, 085313 (2004); Physical Review B 66, 161306(R) (2002).} we apply scanning tunneling microscopy/spectroscopy to characterize the surface of the topological insulator Bi2Se3. We see clover-like defect states in the topographic scans and a residual image that appears in conductance scans, which we attribute to Bi substitutions in Se lattice sites. Spectroscopy reveals features in the density of states consistent with the topological surface state, with the defect states appearing as an additional enhancement. We will discuss the interaction of the topological surface state with the defect states. [Preview Abstract] |
Thursday, March 24, 2011 10:48AM - 11:00AM |
V35.00015: Scattering on Magnetic and Non-Magnetic Impurities on a Surface of a Topological Insulator T. Valla, Z.-H. Pan, D.R. Gardner, S. Chu, Y.S. Lee Dirac-like surface states on surfaces of topological insulators have a chiral spin structure that supresses back-scattering and protects the coherence of these states in the presence of potential scatterers. In contrast, magnetic scatterers are expected to open the back- scattering channel via the spin-flip processes and to degrade the state's coherence. We present angle-resolved photoemission spectroscopy studies of the electronic structure and the scattering rates upon adsorption of various magnetic and non-magnetic impurities on the surface of Bi$_2$Se$_3$, a model topological insulator. We uncovered an unusual insensitivity of the topological surface state to both non-magnetic and magnetic impurities. The electrons donated by the impurities fill the topological surface state and pairs of higher lying spin-orbit split surface bands, preserving the non-trivial spin texture of the surface. [Preview Abstract] |
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