Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session T43: Nanoscale Effects in Topological Insulators |
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Sponsoring Units: DCMP Chair: Jason Zhu, Harvard University Room: Mile High Ballroom 4B |
Thursday, March 6, 2014 11:15AM - 11:27AM |
T43.00001: The Aharonov-Bohm Effect in a 3D topological insulator nanowire Sungjae Cho, Brian Dellabetta, Alina Yang, John Schneeloch, Zhijun Xu, Genda Gu, Matthew Gilbert, Nadya Mason The three dimensional topological insulator (3D TI) is a new class of material having metallic surface states characterized by gapless Dirac dispersions and novel properties such as momentum-spin locking. A TI nanowire with an insulating bulk can be described as a hollow metallic cylinder, showing Aharonov-Bohm oscillations when a magnetic flux is threaded through the axis. The magneto-conductance of a TI nanowire near the Dirac point is expected to have a minimum at zero magnetic field and an oscillation period of one magnetic flux quantum, $\Phi $ (due to a Berry phase of $\pi $ acquired by electron waves upon 2$\pi $ rotation of electron spin around the surface of the nanowire) [1]. In this talk, we discuss magneto-conductance measurements of TI (Bi$_{2}$Se$_{3})$ nanowires, measured as the gate voltage is tuned through the Dirac point. The Aharonov-Bohm oscillations switch from a conductance maximum to a minimum at zero field as the Dirac point is approached, consistent with the existence of a Berry phase in the nanowire. \\[4pt] [1] J.H. Bardarson, P.W. Brouwer, and J. E. Moore, Phys. Rev. Lett. 105, 156803 (2010). [Preview Abstract] |
Thursday, March 6, 2014 11:27AM - 11:39AM |
T43.00002: Experimental Evidence of the Nontrivial Topological Order in the Semiconducting Bi(111) Thin Films Dong Qian, Mengyu Yao Bismuth is one of the most extensively studied elements in solid state physics because of its unique electronic properties. In the last five year, Bi becomes the key element that provides the spin-orbital interaction in the field of topological insulators (TIs) that, as a new quantum phase of matter, have attracted a great deal of attentions due to its many exotic properties and potential applications. Bulk Bi is a semi-metal with novel surface states. The topological order of bulk Bi is thought to be trivial though there are still some debates. Very interestingly, Bi(111) thin films of certain thickness ($>$ 19nm) were recently found to be an semiconductor with robust metallic conduction channel on the surface. In this work, using state-of-art angle resolved photoemission spectroscopy, in the first time we directly identified the nontrivial topological order of the semiconducting Bi(111) thin films. Fermi energy inside the bulk gap is found to intersect the surface states an odd number of times, which reveals that the semiconducting Bi(111) is a three dimensional topological insulator. [Preview Abstract] |
Thursday, March 6, 2014 11:39AM - 11:51AM |
T43.00003: \textit{Ab initio} studies of the electronic and transport properties of topological insulator-metal contacts Catalin Spataru, Francois Leonard Topological insulators (TI) hold great promise for novel applications in electronics and optoelectronics. For such device applications, TIs need to be contacted with a metal for electron injection. Depending on the character and strength of the interaction, a metal contact can modify the properties of TI surface states and induce new states at the interface. In this work, we study via \textit{ab initio} Density Functional Theory the electronic and transport properties of realistic interfaces between a thin film TI and several magnetic and non-magnetic metal surfaces. We will discuss how band topology, band bending and hybridization effects affect charge injection and the contact properties (Schotkky versus ohmic) of the interface. [Preview Abstract] |
Thursday, March 6, 2014 11:51AM - 12:03PM |
T43.00004: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 12:03PM - 12:15PM |
T43.00005: Directly observing edge electronic states of 1D and 2D topological insulators Han Woong Yeom, Sung-Hwan Kim, Tae-Hwan Kim We observed directly the edge states of~1D and 2D topological insulators formed on solid surfaces with scanning tunneling microscopy and spectroscopy. The charge density wave (CDW) state of In atomic wires self-assembled on Si(111) was used to observe solitons, the edge state of 1D topological insulator, in real space. The unusual four-fold degeneracy of the In/Si(111) CDW state introduces two distinct kinds of solitons, related to the physics of coupled CDW wires. The edge electronic states were also clearly resolved for the Bi single bilayer film grown on Bi$_{2}$Te$_{2}$Se. The strong interaction between the Bi bilayer and the 3D topological insulator substrate is discussed in detail, which is important to understand the complex topological nature of the supported Bi bilayer and the Bi-terminated Bi$_{2}$Te$_{2}$Se. [Preview Abstract] |
Thursday, March 6, 2014 12:15PM - 12:27PM |
T43.00006: Surface-Dominated Transport on a Bulk Topological Insulator Philip Hofmann, Lucas Barreto, Lisa K\"uhnemund, Frederik Edler, Christoph Tegenkamp, Jianli Mi, Martin Bremholm, Bo Brummerstedt Iversen, Christian Frydendahl, Marco Bianchi Topological insulators are guaranteed to support metallic surface states on an insulating bulk, and one should thus expect that the electronic transport in these materials is dominated by the surfaces states. Alas, due to the high remaining bulk conductivity, surface contributions to transport have so-far only been singled out indirectly via quantum oscillation, or for devices based on gated and doped topological insulator thin films, a situation in which the surface carrier mobility could be limited by defect and interface scattering. Here we present a direct measurement of surface-dominated conduction on an atomically clean surface of Bi$_2$Te$_2$Se. Using nano-scale four point setups with variable contact distance, we show that the transport at 30~K is two-dimensional rather than three-dimensional and by combining these measurements with angle-resolved photoemission results from the same crystals, we find a surface state mobility of 390(30)~cm$^{2}$V$^{-1}$s$^{-1}$ at 30~K at a carrier concentration of 8.71(7)$\times 10^{12}$~cm$^{-2}$. [Preview Abstract] |
Thursday, March 6, 2014 12:27PM - 12:39PM |
T43.00007: Direct observation of distributed topological surface current flow in Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ single crystals Janghee Lee, Jae-Hyeong Lee, Joonbum Park, Jun Sung Kim, Hu-Jong Lee Topological insulators (TIs) reveal new quantum states of matter, with the topological surface conducting state (TSS) on the insulating bulk. Accurate transport measurements on a TI surface offer crucial information on the topological nature of the TSS. But, with dominant surface conduction, current flow on the top surface of a TI is not confined between the current-biasing electrodes but are widely distributed over the entire surfaces of a TI, including the sides. This distributed current flow makes the estimation of the surface conductance erroneous, leading to difficulties with characterizing the topological nature of the TSS. In this study, we overcome the problem, by concurrent measurements of the local and nonlocal conductance of Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ TI crystalline flakes, in combination with the comprehensive numerical simulation, which yields highly relevant backgate-voltage, temperature, and magnetic-field dependences of the conductance on the top and bottom surfaces. Our study provides a reliable means of accurately characterizing the TSS with inherent nonlocal surface-dominant conducting channels in a TI. [Preview Abstract] |
Thursday, March 6, 2014 12:39PM - 12:51PM |
T43.00008: Ion Implantation in Topological Insulator Bismuth Selenide Peter Sharma, V. Stavila, D.L. Medlin, A.L. Lima Sharma, M. Hekmaty, K. Hattar, R.S. Goeke, K.J. Erickson The coupling between bulk and surface conductivity remains a major problem for understanding the transport properties of topological insulator surface states. For instance, topological insulator bismuth selenide must be doped in order to reduce bulk conductivity. Existing methods utilize equilibrium bulk doping in the melt or non equilibrium doping during thin film growth. We introduce a new method of doping using ion implantation of Ca in prototypical topological insulator bismuth selenide. Ion implantation is potentially suitable for a wide range of dopants and compatible with existing semiconductor fabrication processes. Ca doping is known to yield p-type material, while native bismuth selenide is n-type. Therefore, implantation of Ca should decrease bulk conductivity. We evaluate microstructural damage and dopant activation associated with ion implantation in order to assess the feasibility of this doping method. [Preview Abstract] |
Thursday, March 6, 2014 12:51PM - 1:03PM |
T43.00009: Cyclotron resonance of single valley Dirac fermions in a gapless HgTe quantum well Jonathan Ludwig, Yu. B. Vasilyev, N.N. Mikhailov, J.M. Poumirol, Z. Jiang, O. Vafek, D. Smirnov We report on the Landau level spectroscopy study of two HgTe quantum wells (QW) at and near the critical thickness, where the band gap vanishes. In magnetic fields up to $B=16$~T, oriented perpendicular to the QW plane, we observe a $\sqrt{B}$ dependence for the energy of the dominant cyclotron resonance (CR) transition characteristic of 2D Dirac fermions. The dominant CR line exhibits either a single or double absorption line shape for the gapless or gapped QW, respectively. We will show that the CR transitions can be quantitatively described by an effective Dirac model. Using this model, we extract a band velocity $v_F=6.4 \times10^5$~m/s in both QWs and interpret the double absorption of the gapped QW as arising from the addition of a small relativistic mass. [Preview Abstract] |
Thursday, March 6, 2014 1:03PM - 1:15PM |
T43.00010: Strain-induced topological phase transitions in HgTe Nirpendra Singh, Ravindra Pandey, Ambesh Dixit Mercury telluride is a known semi-metal in its bulk zinc-blende structure with electronic bandgap Eg $\sim$ -0.3 eV and has been predicted to be a topological insulator under strain. In this study, we carried out ab intio electronic structure calculations to investigate the transition of HgTe system from semi-metal into the topological insulating phase under compressive strain along [001], [110] and [111] directions. The compressive strains along these directions close and reopen a gap at the $\Gamma$ point and topological phase is emerged. We will discuss the evolution of topological insulator phase in the context of semimetal nature of bulk HgTe system. [Preview Abstract] |
Thursday, March 6, 2014 1:15PM - 1:27PM |
T43.00011: Probing the combined effects of Dirac dispersion and spin-orbit coupling in HgTe-based 2DEGs Mehdi Pakmehr, C. Bruene, H. Buhmann, Lauren Molenkamp, Bruce McCombe HgTe-based QWs show interesting behavior vs. well width (Dirac dispersion and a topological insulator state) due to the so-called ``inverted'' band structure ($\Gamma_{8}$ conduction band and $\Gamma _{6}$ valence band) of the bulk material. We have studied symmetric HgCdTe/HgTe-based QWs with ``normal'' band structure close to the Dirac point (width 6nm; critical width 6.3--6.6 nm) by magneto-transport and THz magneto-photoresponse (PR) measurements at low temperatures and in fields up to 10 T. Due to the relatively high carrier density (n$_{\mathrm{e}}=$1.5x10$^{12}$ cm$^{-2})$ the Fermi energy is well above the Dirac point, where one expects to observe effects of linear dispersion most clearly. We discuss how this situation leads to the measured values for the cyclotron resonance (CR) effective mass m* and g-factor. The CR mass was obtained both from separate transmission measurements and from fitting the envelope of the PR, while g-factors were obtained from fitting the splitting of the quantum oscillations. Additional interesting phenomena observed by THz photoresponse, e.g. beating patterns in quantum oscillations in the photoconductivity signal due to effective Rashba field, will also be discussed. [Preview Abstract] |
Thursday, March 6, 2014 1:27PM - 1:39PM |
T43.00012: Evidence for a positron bound state on the surface of a topological insulator K. Shastry, P.V. Joglekar, Z.H. Lim, B. Barbiellini, A.H. Weiss We describe experiments aimed at probing the sticking of positrons to the surface of a topological insulator. A magnetically guided positron beam was 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 the positron irradiation were measured. The spectra showed features that can be identified with Positron Annihilation induced Auger transitions from Bi, Te, and Se providing evidence that the incident positrons were trapped into a surface localized bound state at the time of annihilation. The evidence for a positron bound surface state suggests that positron annihilation can be used to selectively probe the critically important top most layer of topological insulator system. . [Preview Abstract] |
Thursday, March 6, 2014 1:39PM - 1:51PM |
T43.00013: Time Reversal Invariant Topologically Insulating Circuit Ningyuan Jia, Ariel Sommer, David Schuster, Jonathan Simon With the discovery of the quantum hall effect and topological insulators, there has been an outpouring of ideas to harness topologically knotted band-structures in the design of state-of-the art, disorder-insensitive materials. From studies of exotic quantum many- body phenomena to applications in spintronics and quantum information processing, topological materials are poised to revolutionize the condensed matter frontier. Here we demonstrate, for the first time, a circuit that behaves as a time-reversal invariant topological insulator for RF photons. In this meta-material, composed of capacitively coupled high-Q inductors, we observe a gapped density of states consistent with a modified Hofstadter spectrum at a flux per plaquette of phi=pi/2. In-situ probes further reveal time-resolved, spin-dependent edge-transport. We leverage the unique flexibility of our materials to investigate, for the first time, features of topological insulators on manifolds such as the Mobius strip. This new approach elucidates the fundamental ingredients essential to topologically active materials, whilst providing a powerful laboratory to study topological physics and a promising route to topological quantum science. [Preview Abstract] |
Thursday, March 6, 2014 1:51PM - 2:03PM |
T43.00014: Structural and Magnetotransport Studies of MBE-grown Pn(Sn)Te films and PbTe:Bi/CdTe Quantum Wells Xiang Li, Xinyu Liu, Jacek Furdyna, Malgorzata Dobrowolska, Tomasz Wojtowicz Recent studies confirmed the existence of topological crystalline insulators (TCIs), in which crystalline symmetry replaces the role of time-reversal symmetry in ensuring topological protection. In the narrow-gap semiconductor TCIs, chemical potential can be tuned by modifications of crystal growth and/or annealing to yield n-type or p-type conductivity, which makes them especially well-suited for magnetotransport measurements. In this work, we have grown a series of Pb$_{\mathrm{1-x}}$Sn$_{\mathrm{x}}$Te films and PbTe:Bi/CdTe QWs on CdTe/GaAs(100) substrates using MBE. Structural studies of these thin films were carried out using XRD and SEM techniques. XRD results shows satisfactory crystal quality of Pb(Sn)Te films grown on CdTe. SEM studies show the presence of inclusions in the films, indicating that the crystal quality still requires improvement. Magnetostransport studies of PbTe:Bi/CdTe QWs suggests that Bi acts as a donor in PbTe, and the electron mobility in the 2D electron gas in the QW depends on the growth conditions, such as substrate temperature. The study of Pb$_{\mathrm{1-x}}$Sn$_{\mathrm{x}}$Te QWs is currently underway, and will also be discussed in this talk. [Preview Abstract] |
Thursday, March 6, 2014 2:03PM - 2:15PM |
T43.00015: Resonant tunneling via Dirac electron states in a topological-insulator / semiconductor junction Ryutaro Yoshimi, Atsushi Tsukazaki, Ko Kikutake, Joseph Checkelsky, Kei Takahashi, Masashi Kawasaki, Yoshinori Tokura A defining characteristic of the topological classification of solids is the existence of gapless modes at the interface of materials with unequal topological invariants. In the context of the $Z_{2}$ topological invariant, this has been verified by the spectroscopic observation of spin-polarized Dirac electron states at the interface of three-dimensional topological insulators (3D TIs) and the vacuum. By performing tunneling spectroscopy in heterojunction devices based on the TI (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ and band insulator InP, we report the observation of such states at the interface between a 3D TI and a topologically trivial solid. In an applied magnetic field, the tunneling conductance through these heterojunctions resonates due to the formation of Landau levels at the interface; the observed energy and angular dependence indicates these carriers are two-dimensional surface electrons obeying a Dirac-like energy dispersion. Furthermore, the composition $x$ dependence of the deduced Fermi velocity and Dirac point energy agree with previous photoemission observations for the surface states of (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$ with a vacuum interface. This study gives strong evidence for the existence of interface topological states in solid heterojunction, which will provide new functional devices based on TI. [Preview Abstract] |
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