Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session G42: Focus Session: Novel Effects in Topological Insulator Films |
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Sponsoring Units: DMP Chair: Hsin Lin, National University of Singapore Room: Mile High Ballroom 4A |
Tuesday, March 4, 2014 11:15AM - 11:51AM |
G42.00001: Surfaces, interfaces, and ultrathin films of topological insulators Invited Speaker: Tai Chiang Three-dimensional topological insulators are characterized by an inverted bulk band gap caused by a strong spin-orbit coupling. This gap must close at the surface and reopen outside in vacuum where the gap is noninverted (and infinite). The resulting metallic surface states, or topological states, are spin-polarized and span the bulk gap. They carry a spin current, independent of the details of the surface, which is a feature of strong interest for spintronic applications. This talk will focus on thin films of topological materials (Sb, Bi$_{2}$Se$_{3}$, and Bi$_{2}$Te$_{3})$. Thin films are basic building blocks of devices, which typically involve multilayers of various materials. As the thickness of a topological insulator film is reduced to the nanoscale, the bulk bands are reduced to discrete quantum well states, and the surface/interface states associated with the two faces of the film can interact, resulting in spin mixture and formation of a tunneling gap. The detailed atomic bonding at each face of the film can also affect the overall electronic structure of the system. The interplay of quantum confinement, topological order, spin polarization, and surface/interface bonding and chemistry will be discussed. This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Grant No. DE-FG02-07ER46383. [Preview Abstract] |
Tuesday, March 4, 2014 11:51AM - 12:03PM |
G42.00002: Bulk-Insulating Bi$_{2}$Se$_{3}$ Thin Films and Decoupled Topological Surface States Matthew Brahlek, Nikesh Koirala, Maryam Salehi, Namrata Bansal, Seongshik Oh By applying the simple criteria given by Mott and Ioffe-Regel it is easily seen that even the best TIs are not true insulators in the Mott sense, but at best are weakly-insulating bad metals. However, band-bending effects contribute significantly to the TI transport properties, and we show that utilization of this band-bending effect can lead to a Mott insulating bulk state in the thin regime. This is realized in transport experiments on compensation doped Bi$_{2}$Se$_{3}$ thin-films, where the bulk-insulating picture is supported by enhanced surface mobilities, Hall effect, Shubnikov de-Haas oscillations as well a clear signature of a thickness dependant decoupling of surface states by analyzing the weak anti-localization effect. [Preview Abstract] |
Tuesday, March 4, 2014 12:03PM - 12:15PM |
G42.00003: Electrodynamics of the topological insulator (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}}$)$_{2}$Se$_{3}$ tuned to the brink of quantum criticality Liang Wu, Rolando Valdes Aguilar, Andreas Stier, Christopher Morris, Yuval Lubashevsky, Peter Armitage, Matthew Brahlek, Nikesh Koirala, Namrata Bansal, Seongshik Oh We have utilized time-domain terahertz (THz) spectroscopy to investigate the low frequency optical conductivity in (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}}$)$_{2}$Se$_{3}$ through its topological phase transition from the pure compound (x$=$0) to the topologically trivial strongly insulating material (x$=$0.27). Above a thickness dependent doping threshold we observe a sudden collapse in the transport lifetime that indicates the destruction of the topological phase. We associate this with the doping where the states from opposite surfaces hybridize. As a function of thickness this threshold asymptotically approaches the doping x $\sim$ 0.06 of a maximum in the mid-infrared absorption, which can be identified with the bulk band gap closing and change in topological class. The realization of a topological quantum critical point allows the possible realization of other novel phenomena including the Weyl semi-metal. I will discuss our results on the THz response of these systems in a new generation of materials with greatly suppressed bulk carrier density levels. Reference: Wu, \textit{et al}, \textbf{Nature Physics} 9, 410-414 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 12:15PM - 12:27PM |
G42.00004: Probing Bi2Te3 thin nanoplates by Raman Spectroscopy Rui He, Conor Delaney, Ben Beck, Tim Kidd, Cliff Chancey, ZhenHua Wang, Richard Qiu, Xuan Gao Two infrared (IR)-active vibrational modes centered at 93 and 113 cm$^{-1}$ are observed in Raman spectra from as-grown thin nanoplates (NPs) of topological insulator Bi$_2$Te$_3$. The presence of IR modes in Raman scattering reveals a breakdown of inversion symmetry in thin NPs grown on SiO$_2$. Both Raman and IR modes are preserved after typical device fabrication processes, suggesting the robustness of surface properties. In NPs transferred to another SiO$_2$ substrate, the IR modes are absent, and the Raman spectra are similar to those from bulk samples. These differences could be attributed to interactions between the SiO$_2$ substrate and the as-grown NPs. [Preview Abstract] |
Tuesday, March 4, 2014 12:27PM - 12:39PM |
G42.00005: Effect of film thickness on ultrafast carrier relaxation rates in thin-films of the topological insulator Bi$_{2}$Se$_{3}$ Yuri D. Glinka, Sercan Babakiray, Trent A. Johnson, Alan D. Bristow, Mikel B. Holcomb, David Lederman Transient reflectivity measurements of thin films, ranging from 6 to 40 nm in thickness, of the topological insulator Bi$_{2}$Se$_{3}$ reveal a strong dependence of the ultrafast carrier relaxation rate on the film thickness. We exploit this behavior to distinguish between the contributions from the bulk 3D states and the 2D gapless surface states. Based on experimental observations we conclude that there is a crossover between two carrier relaxation mechanisms associated with the polar phonon (Frohlich) interaction in the bulk insulating phase and the electron-lattice interaction in the surface metallic phase. It is suggested that this crossover could be a result of hybridization of Dirac cone states at the opposite surfaces of the thinnest films. [Preview Abstract] |
Tuesday, March 4, 2014 12:39PM - 12:51PM |
G42.00006: Bi$_{2}$-Bi$_{2}$Se$_{3}$ Superlattice Materials Studied by Photoemission Spectroscopy Andrew Weber, Ivo Pletikosic, Quinn Gibson, Huiwen Ji, Leslie Schoop, Jurek Sadowski, Anthony Caruso, Elio Vescovo, Alexei Fedorov, Robert Cava, Tonica Valla Although searches are carried out independently for new 2D quantum spin Hall or 3D topological materials for their robust, spin-polarized edge or surface conduction states, little has been done to experimentally show that both phases can coexist in a single material or explore how they may interact. The superlattice series Bi$_{\mathrm{2m}}$(Bi$_{2}$Se$_{3})_{\mathrm{n}}$, featuring stacked layers of Bi$_{2}$ and Bi$_{2}$Se$_{3}$, may contain systems where a combination of 2D and 3D topological phenomena should be at play, the latter of which can be identified by combined computational and spin-and angle-resolved photoemission spectroscopy studies. We find that several members of the series, (m$=$0, n$=$1), (m$=$1, n$=$1) and (m$=$2, n$=$1) have spin-chiral surface states at the center of the surface Brillouin zone, a trait of strong topological insulators. The characterization of the topological surface states will be discussed for these series members. [Preview Abstract] |
Tuesday, March 4, 2014 12:51PM - 1:03PM |
G42.00007: Nontrivial topological electronic structures in a single Bi(111) bilayer on different substrates Feng-Chuan Chuang, Zhi-Quan Huang, Chia-Hsiu Hsu, Yu-Tzu Liu, Hua-Rong Chang, Hsin Lin, Arun Bansil Electronic structures, minimum energy configurations, and band topology of strained Bi(111) single bilayers placed on a variety of semiconducting and insulating substrates are investigated using first-principles calculations [1]. A topological phase diagram of a free-standing Bi bilayer is presented to help guide the selection of suitable substrates. Numerous substrates were studied to determine whether they are able to support 2D TIs. The insulating hexagonal-BN is identified as the best candidate substrate material for supporting nontrivial topological insulating phase of Bi bilayer thin films. A planar hexagonal Bi layer is predicted under tensile strain, which we show could be realized on a SiC substrate. The Bi bilayer becomes metallic under the compressive strain induced by Si and Ge substrates. [1] Zhi-Quan Huang, Feng-Chuan Chuang, Chia-Hsiu Hsu, Yu-Tzu Liu, Hua-Rong Chang, Hsin Lin, and Arun Bansil, Phys. Rev. B 88, 165301 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 1:03PM - 1:15PM |
G42.00008: Quantum Interference Control of Currents in Bi$_{2}$Se$_{3}$ Topological Insulators Derek Bas, Kevin Vargas, Sercan Babakiray, Trent Johnson, Yuri Glinka, Mikel Holcomb, David Lederman, Alan Bristow Quantum interference control of bulk and surface currents are investigated in Bi$_{2}$Se$_{3}$ films ranging from 6 to 40 quintuple layers in thickness. The samples are grown with a two-step method on sapphire substrates and protected with an MgF$_{2}$ capping layer that prevents oxidation. Co-polarized harmonically related pulses excite a population of carriers through interference of single- and two-photon absorption pathways. Dependences of the relative phase between the two pulses and intensity of each pulse show the correct signatures confirming the third-order nonlinear optical process. We observe an increase in the strength of the injected currents with decreasing thickness and a peak at 10 quintuple layers. It is believed that the peak coincides with the onset of hybridization of the Dirac cone on opposite surfaces of the sample. The increase in signal strength is related to an increase in the expected spin-polarized surface currents, which begin to dominate over the bulk pure charge currents. [Preview Abstract] |
Tuesday, March 4, 2014 1:15PM - 1:27PM |
G42.00009: Investigations of Topological Surface States in Sb (111) Ultrathin Films by STM/STS Experiments and DFT Calculations Ziyu Luo, Guanggeng Yao, Wentao Xu, Yuanping Feng, Xue-sen Wang Bulk Sb was regarded as a semimetal with a nontrivial topological order. It is worth exploring whether the Sb ultrathin film has the potential to be an elementary topological insulator [1]. In the presence of quantum confinement effect, we investigated the evolution of topological surface states in Sb (111) ultrathin films with different thickness by the scanning tunneling microscopy/ spectroscopy (STM/STS) experiments and density functional theory (DFT) calculations [2]. By comparing the quasiparticle interference (QPI) patterns obtained from Fourier-transform scanning tunneling spectroscopy (FT-STS) and from DFT calculations, we successfully derive the spin properties of topological surface states on Sb (111) ultrathin films. In addition, based on the DFT calculations, the 8BL Sb (111) ultrathin film was proved to possess up to 30{\%} spinseparated topological surface states within the bandgap. Therefore, the highquality 8BL Sb (111) ultrathin film could be regarded as an elementary topological insulator. [1] F.C. Chuang et al., App. Phy. Lett.102, 022424 (2013) [2] G Yao et al., Sci. Rep. 3, 2010 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 1:27PM - 1:39PM |
G42.00010: Tuning the Surface States of Ultra-thin Topological Insulator Films Jianxin Zhong In this talk, I will introduce our recent progress on tuning the surface states of ultra-thin topological insulator films. Using first-principles methods, we explain the puzzling band-topology difference between Sb$_{2}$Se$_{3}$ and Bi$_{2}$Se$_{3}$ and propose an approach to tuning the topological phase by strain [1]. We demonstrate that Sb$_{2}$Se$_{3}$ can be converted into a topological insulator by applying compressive strain while the tensile strain can turn Bi$_{2}$Se$_{3}$ into a normal insulator. I will also show that the separation distance between quintuple layers (QL) in ultra-thin Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ films have a large increase after relaxation, leading to gap-opening at the surface Dirac cone, in good agreement with the experimental observation [2]. I will further show that Pb adlayers on Bi$_{2}$Se$_{3}$ result in splitting of the Dirac cones and large Rashba spin splitting of the quantum well states [3]. Most importantly, the quantum size effect of Pb adlayers leads to an oscillatory behavior of the Rashba splitting.\\[4pt] [1] W. L. Liu et al., Phys. Rev. B 84, 245105 (2011);\\[0pt] [2] W. L. Liu et al., Phys. Rev. B 87, 205315 (2013);\\[0pt] [3] H. Yang et al., Phys. Rev. B 86, 155317 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 1:39PM - 1:51PM |
G42.00011: Electrical and structural properties of elemental Sb quantum wells grown by molecular beam epitaxy Kaushini Wickramasinghe, Chomani Gaspe, Shayne Cairns, Lin Lei, Nolan Teasdale, Tetsuya Mishima, Joel Keay, Sheena Murphy, Michael Santos Elemental Sb has gained attention recently because calculations indicate that the inherently large spin-orbit coupling enables topological insulator behavior. Because the band structure of bulk elemental Sb is semi-metallic, transport measurements will be dominated by bulk conduction. Our goal is to suppress the bulk conductivity by quantum confinement in thin Sb layers, to enable transport measurements of topological surface states. A growth procedure was developed to realize ultra-thin layers of Sb with thickness of $\sim$ 1nm to 10nm. Field-emission scanning electron microscopy and transmission electron microscopy measurements of ultra-thin Sb QWs show good crystalline quality with a suppression of the bulk conductivity at 20K by as much as 400x. We will discuss the epitaxial growth procedure for Sb quantum wells with GaSb barriers grown on GaAs(111)A and GaSb(111)A substrates. We will also discuss characterization of the structural and electrical properties of the ultra-thin films. [Preview Abstract] |
Tuesday, March 4, 2014 1:51PM - 2:03PM |
G42.00012: Current-Voltage Characteristics Along Terraces in MBE-Grown Bi$_{2}$Te$_{3}$ Rita Macedo, Sara Harrison, Tatiana Dorofeeva, James Harris, Richard Kiehl Capturing the novel, but elusive, physics of topological insulators for electronic devices will require a baseline characterization of their surface electronic properties in an ambient environment. We report on the current-voltage (I-V) characteristics observed along terraces in MBE-grown Bi$_{2}$Te$_{3}$ by room-temperature conductive atomic force microscopy (C-AFM). The films were grown on sapphire by a two-step MBE process, leading to large-area films with micrometer-sized domains formed by wide concentric atomically flat terraces with a typical width of 170 nm and a step height of 1 nm. Control samples comprised of spherical nanoparticles and HOPG terraces were also examined for comparison. The Bi$_{2}$Te$_{3}$ C-AFM measurements consistently showed well-behaved, nearly symmetric exponential I-V characteristics with similar ideality factors on terraces and in the transition regions between terraces. Notably, current in the 25-nm transition regions was 10X higher than on the terraces. Negligible current increase was observed for the controls, indicating that this behavior is not an artifact of the tip-sample contact, but rather is due to a difference in the conductance along the Bi$_{2}$Te$_{3}$ terrace edges. Possible mechanisms including oxide, doping, defect, strain and topological effects will be discussed. These results motivate further edge conduction studies in these materials as essential background for studying topological insulator physics and devices. [Preview Abstract] |
Tuesday, March 4, 2014 2:03PM - 2:15PM |
G42.00013: Growth of topological insulator Bi$_{2}$Se$_{3}$ thin films on amorphous for multi-channel structure Sahng-Kyoon Jerng, Yong Seung Kim, Jae Hong Lee, Youngwook Kim, Jun Sung Kim, Kisu Joo, Euijoon Yoon, Sang-Moon Yoon, Miyoung Kim, Seung-Hyun Chun A topological insulator exhibits the topologically protected gapless Dirac surface states in bulk band gap which was predicted in Bi$_{2}$Se$_{3}$. Thin layered films of Bi$_{2}$Se$_{3}$ have been heteroepitaxially grown on the crystalline substrate by molecular beam epitaxy (MBE). Here, we show the growth of Bi$_{2}$Se$_{3}$ thin films on amorphous SiO$_{2}$ substrate by MBE. In order to achieve the growth on amorphous surface, van der Waals epitaxy method with the selenium passivation was adopted. Bi$_{2}$Se$_{3}$ films are grown along [001] direction with periodical structure in spite of lattice mismatched amorphous substrate. Low-temperature transport measurement revealed the weak anti-localization effect with electrical gating, which suggest that surface transport properties can be comparable to those of epitaxially grown Bi$_{2}$Se$_{3}$ films on crystalline substrate. In addition, we demonstrate the growth of multi-channel Bi$_{2}$Se$_{3}$ films separated by amorphous insulating layer. These results provide a potential of growth of layered topological insulator films on amorphous materials and junctions. [Preview Abstract] |
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