Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L42: Focus Session: Topological Insulators: Spin Textures |
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Chair: Zahid Hasan, Princeton University Room: Mile High Ballroom 4A |
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L42.00001: Mapping the Spin texture of topological insulators Invited Speaker: Alessandra Lanzara The helical spin texture of surface electrons in topological insulators has attracted a great deal of interest in the past few years. In this talk I will present new results obtained by using an innovative ultra-high efficiency spin-resolved photoemission spectrometer to map the spin texture of Bi$_2$Se$_3$ topological insulator throughout the entire momentum space. We discover a surprising property of these surface electrons, e.g. that the spin polarization of the resulting photoelectrons can be fully manipulated by light in three dimensions. The evolution of spin texture as a function of the light polarization is also studied. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L42.00002: Hedgehog spin texture and competing orders on the surface of strained topological crystalline insulators Cheng-Yi Huang, Wei-Feng Tsai, Yung Jui Wang, Hsin Lin, Arun Bansil We discuss spin reorientation phenomena, which may or may not yield gap formation, on the surface of topological crystalline insulators Pb$_{\mathrm{1-x}}$Sn$_{\mathrm{x}}$(Te, Se) under various applied strains. The low-energy surface electrons on the (001) surface behave like massless Dirac particles with four Dirac points centered along the intersection of the mirror (\textit{xz} or \textit{yz}) plane and the surface plane. We use a four-band k.p model, which captures the spin and orbital texture of the surface states around surface X (or Y) point up to the energy around the Lifshitz transition, and systematically study effects of the applied strain. In contrast to the case without any strain, where the absence of the out-of-the-plane spin component is guaranteed by both the mirror and the time-reversal symmetries, we find that without time-reversal symmetry breaking, the hedgehog-like spin textures associated with a gap formation can be induced by the strain only, breaking the \textit{xz} mirror symmetry. The other cases cannot induce a gap at Dirac points. We also investigate interaction-driven competing orders under the strain and obtain a phase diagram at the mean-field level to reveal the possible novel surface states in the system. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L42.00003: Laser-based spin- and angle-resolved photoemission spectroscopy for rapid, high-resolution measurements Kenneth Gotlieb, Aaron Bostwick, Zahid Hussain, Alessandra Lanzara, Christopher Jozwiak A unique spin-and angle-resolved photoemission spectrometer (spin-ARPES) is coupled with a 6 eV laser to achieve unprecedented measurements of near-EF physics in topological insulators and Rashba systems. The pairing of the spin-ARPES system with the laser allows for energy and angular resolutions never before seen in a spin-ARPES experiment. ~Most importantly, the high efficiency of the system and high photon flux of the laser make measurements very rapid, permitting exploration of a large experimental phase space. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L42.00004: Tunable spin helical Dirac quasiparticles in HgTe surfaces Chang Liu, Guang Bian, Su-Yang Xu, Ilya Belopolski, Hsin Lin, Christian Matt, Irek Miotkowski, Nasser Alidoust, Madhab Neupane, Robert S. Markiewicz, Arun Bansil, Vladimir N. Strocov, Mark Bissen, Alexei V. Fedorov, Taichi Okuda, Yong P. Chen, M. Zahid Hasan We show with photoemission spectroscopy that bulk HgTe is a topologically nontrivial material, possessing a Dirac-cone surface state with clear, unmodulated, left-right imbalanced spin polarization and circular dichroism. This topological surface state maintains its surface character even within the bulk continuum due to topological protection, in drastic contrast with ordinary solid where a surface band usually extends into the bulk and loses its surface character when it degenerates in energy with a bulk state. The Dirac transport regime of HgTe, where the topological surface state is fully exposed and free from influences of the bulk bands, can be easily reached by alkali metal deposition onto the surface. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L42.00005: Photoemission of Bi$_2$Se$_3$ with Circularly Polarized Light: Probe of Spin Polarization or Means for Spin Manipulation? O. Rader, J. S\'anchez-Barriga, A. Varykhalov, J. Braun, S.-Y. Xu, N. Alidoust, O. Kornilov, J. Min\'ar, K. Hummer, G. Springholz, G. Bauer, R. Schumann, L.V. Yashina, H. Ebert, M.Z. Hasan, G. Bauer Topological insulators are characterized by Dirac cone surface states with spins aligned in the surface plane and perpendicular to their momenta. Recent theoretical and experimental work implied that this specific spin texture should enable control of photoelectron spins by circularly polarized light. However, these reports questioned the so far accepted interpretation of spin-resolved photoelectron spectroscopy. We solve this puzzle and show that vacuum ultraviolet photons (50-70 eV) with linear or circular polarization probe indeed the initial state spin texture of Bi$_2$Se$_3$ while circularly polarized 6 eV low energy photons flip the electron spins out of plane and reverse their spin polarization. Our photoemission calculations, considering the interplay between the varying probing depth, dipole selection rules and spin-dependent scattering effects involving initial and final states explain these findings, and reveal proper conditions for light-induced spin manipulation for future applications in opto-spintronic devices. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L42.00006: Photoelectron spin-polarization control in the topological insulator Bi$_2$Se$_3$ Zhihuai Zhu, C.N. Veenstra, S. Zhdanovich, M.P. Schneider, G. Levy, T. Okuda, K. Miyamoto, S.-Y. Zhu, P. Syers, N.P. Butch, J. Paglione, M.W. Haverkort, I.S. Elfimov, A. Damascelli We study Bi$_2$Se$_3$ by angle-resolved photoemission spectroscopy (ARPES) and density functional theory. We find that the topological surface state (TSS) is characterized by a layer-dependent entangled spin-orbital texture, which becomes apparent through photoelectron interference effects in ARPES. This explains the discrepancy between the spin polarization obtained in spin-ARPES--ranging from 20\% to 85\%--and the 100\% value assumed in phenomenological models [1]. We demonstrate how to probe the intrinsic spin texture of TSS by spin-ARPES, and continuously manipulate the spin polarization of photoelectrons and photocurrents all the way from 0 to +/-100\% by an appropriate choice of photon energy, polarization, and angle of incidence [2]. As illustrated by a minimal two-atomic-layer model, photoelectron spin-polarization control is generically achievable in systems with a layer-dependent entangled spin-orbital texture as a direct manifestation of dipole selection rules, photoelectron interference, and TSS complex structure [2].\\[4pt] [1] Z.-H. Zhu et al., Phys. Rev. Lett. 110, 216401 (2013)\\[0pt] [2] Z.-H. Zhu et al., submitted to Phys. Rev. Lett. (2013) [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L42.00007: Observation of a collective mode of the helical liquid on the surface of Bi$_{2}$Se$_{3}$ Anshul Kogar, Sean D. Vig, Alexander Thaler, Man-Hong Wong, Tai-Chang Chiang, Gregory J. MacDougall, Luc Venema, Peter Abbamonte The helical Dirac band structure at the surface of three-dimensional topological insulators has been theoretically predicted to give rise to unusual surface collective modes. Of particular interest is the ``spin-plasmon,'' a coupled collective excitation involving both the spin and charge degrees of freedom. In this talk, I will present data suggesting that we have observed this excitation on the surface of Bi2Se3 using angle-resolved inelastic electron scattering from the surface. In our study, we have grown samples of different dopings and shown, using angle-resolved photoemission, that we can suppress the Fermi Level into the bulk band gap. The evolution from a bulk-band free-carrier surface plasmon into a Dirac band surface spin-plasmon has been observed as a function of doping. The dependence of the spin-plasmon on momentum transfer as well as time will also be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L42.00008: Orbit- and Atom-Resolved Spin Textures of Intrinsic, Extrinsic and Hybridized Dirac Cone States Lin Miao, Zhengfei Wang, Mengyu Yao, Fengfeng Zhu, J. Hugo Dil, Chunlei Gao, Canhua Liu, Feng Liu, Dong Qian, Jinfeng Jia Combining first-principles calculations and spin- and angle-resolved photoemission spectroscopy measurements, we identify the helical spin textures for three different Dirac cone states in the interfaced systems of a 2D topological insulator (TI) of Bi(111) bilayer and a 3D TI Bi$_{2}$Se$_{3}$ or Bi$_{2}$Te$_{3}$. The spin texture is found to be the same for the intrinsic Dirac cone of Bi$_{2}$Te$_{3}$ or Bi$_{2}$Se$_{3}$ surface state, the extrinsic Dirac cone of Bi bilayer state induced by Rashba effect, and the hybridized Dirac cone between the former two states. Further orbit- and atom-resolved analysis shows that $S$ and $P_{z}$ orbits have the conventional helical spin texture; $P_{\mathrm{x}}$ and$ P_{\mathrm{y}}$ orbits show individually radial spin component, while the sum of the two shows a total in-plane helical spins. The orbit-dependent spin structure is a signature property of spin-orbit coupling, independent of topology. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L42.00009: Spin-resolved Andreev levels and parity crossings in hybrid superconductor-semiconductor nanowires Ramon Aguado, Eduardo Lee, Xiaocheng Jiang, Manuel Houzet, Charles Lieber, Silvano De Franceschi I will present measurements and theory of the Zeeman effect on the Andreev levels of a semiconductor quantum dot, based on an InAs nanowire, with large electron g-factors strongly coupled to a conventional superconductor with large critical field. This material combination allows spin degeneracy to be lifted without destroying superconductivity. When the system is in a spin singlet state, a spin-split Andreev level crossing the Fermi energy results in a quantum phase transition to a spin-polarized state, implying a change in the fermionic parity of the system. This crossing manifests itself as a finite-field, zero-bias conductance anomaly [1] whose properties resemble those expected for Majorana modes in a topological superconductor [2-3]. While this resemblance is understood without evoking topological superconductivity, the observed parity transitions could be regarded as precursors of Majorana modes in the long-wire limit [4]. \\[4pt] [1] E. J. H. Lee, X. Jiang, M. Houzet, R.Aguado, C. M. Lieber, and S. De Franceschi, Nature Nanotechnology, in press (2013).\\[0pt] [2] V. Mourik, et al, Science 336, 1003-1007 (2012).\\[0pt] [3] A. Das, et al. Nature Phys. 8, 887-895 (2012).\\[0pt] [4] T. D. Stanescu, et al, Phys. Rev. B 87, 094518 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L42.00010: Orbital-Selective Spin Texture and its Manipulation in a Topological Insulator Zhuojin Xie, Shaolong He, Chaoyu Chen, Ya Feng, Hemian Yi, Aiji Liang, Lin Zhao, Daixiang Mou, Junfeng He, Yingying Peng, Xu Liu, Yan Liu, Guodong Liu, Xiaoli Dong, Li Yu, Jun Zhang, Shenjin Zhang, Zhimin Wang, Fengfeng Zhang, Feng Yang, Qinjun Peng, Ziaoyang Wang, Chuangtian Chen, Zuyan Xu, Xingjiang Zhou Topological insulators represent a new quantum state of matter, possessing a unique electronic structure and spin texture. In the Dirac surface state, the spin is locked with the crystal momentum. Here we report a new phenomenon of the spin texture locking with the orbital texture in a TI Bi2Se3. The laser-based SARPES can directly measure the spin texture of both the upper and lower Dirac cones in Bi2Se3 surface state under different light polarizations. An unexpected spin texture is revealed for the first time in the s-polarization geometry that the upper Dirac cone exhibits the same spin chirality with the lower one, while the opposite spin chirality is observed for the upper and lower Dirac cones in the p-polarization geometry. Because different orbitals and their coupled spin texture are selectively probed by using variable light polarizations, these results constitute strong evidence of the orbital-dependent spin texture in Bi2Se3. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L42.00011: The aging effect in topological insulator Bi2Se3 Kyungwha Park, Christophe De Beule, Bart Partoens Topological insulators (TIs) have attracted a lot of interest due to their topologically protected surface states, as well as exotic proximity-induced phenomena. Since the first experimental data of TIs, angle-resolved photoemission spectra (ARPES) showed that the electronic structure of the topological surface states significantly changes with time after cleavage. The origin and underlying mechanism of this aging effect are still under debate, despite its importance. Here we present our study of the evolution of the surface Dirac cone for Bi2Se3 films upon asymmetric potassium (K) adsorption, using density-functional theory and a tight-binding model. We find that the K adatoms induce short-ranged downward band bending within 2-3 nm from the surface, due to charge transfer from the K to the TI. Our findings are in contrast to earlier proposals in the literature. As the charge transfer increases, we also find that a new Dirac cone, localized slightly deeper into the TI than the original one, appears at the K-adsorbed surface, arising from strong Rashba-split conduction-band states. Our results suggest possible reinterpretations of experiments because the new Dirac cone might have been observed in ARPES measurements instead of the original one that appears just after cleavage. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L42.00012: Evidence for coexistence of Rashba and Dresselhaus effect on semiconductor Wonsig Jung, S.H. Jo, B.Y. Kim, M. Leandersson, B. Thiagarajan, J.S. Hong, J.H. Shim, Changyoung Kim We have performed preliminary circular dichroism angle-resolved photoemission spectroscopy (ARPES) experiments on InSb. Our results show very strong circular dichroism (CD) signal, indicating probable existence of orbital angular momentum (OAM). Non-zero OAM in zincblend semiconductor can appear when there is an inversion symmetry breaking (IBS) in the bulk and on the surface. We find that the dichroism has momentum and band dependence. The CD modulations can be the evidence for coexistence of Rashba and Dresselhaus effect on semiconductor. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L42.00013: Unusual Surface Termination of the Topological Insulator Bi2Se3 Andrew Hewitt, Jingying Wang, Jonathon Boltersdorf, Tianshuia Guan, Paul Maggard, Daniel Dougherty The strong three-dimensional topological insulator has been heralded as a new state of matter with metallic topological surface states (TSS's) of interest for spintronic applications. Recent experiments of the strong 3-D TI Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ have shown surprising discrepancies about whether the atomic termination of Bi2Se3 is either Se or Bi with evidence for both types. We have observed both metallic Bi and Se terminated surfaces on single crystals of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ synthesized by established growth techniques and purchased from commercial suppliers. X-ray Photoelectron Spectroscopy reveals little oxidation for air-cleaves in either the Bi4f or the Se3d core levels. However, a low binding energy component for the Bi4fs is often observed which is indicative of Bi-Bi bonds near the surface. This termination is observed in approximately 50{\%} of air-cleaved samples that have been stored in air, whereas samples stored in rough vacuum have a lower tendency ($\sim$ 10{\%}) to have a Bi-terminated surface. Angle-Resolved Photoelectron Spectroscopy shows unusual valence band structure for Bi-terminated samples which upon annealing revert to a similar band structure for Se-terminated surfaces. Understanding the surface of these materials is essential for interpreting transport measurements on cleaved single crystals. [Preview Abstract] |
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