Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Y12: Spectroscopy on Topological Insulators |
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Sponsoring Units: DCMP Chair: Andrew Wray, New York University Room: 007C |
Friday, March 6, 2015 8:00AM - 8:12AM |
Y12.00001: Surface Plasmons in 3D Topological Insulators Anshul Kogar, Sean Vig, Gil Cho, Alexander Thaler, Yiran Xiao, Taylor Hughes, Man-Hong Wong, Tai-Chang Chiang, Greg MacDougall, Peter Abbamonte Most studies of three-dimensional (3D) topological insulators have concentrated on their one-electron properties as exhibited by angle-resolved photoemission spectroscopy (ARPES) or by scanning tunneling microscopy (STM). Many-body interactions are often neglected in the treatment of models of topological insulators, such as in the Kane-Mele and Bernevig-Hughes-Zhang models. Using angle-resolved inelastic electron scattering from the surface, I will present data on the collective mode that owes its existence to the presence of many-body interactions, the surface plasmon (SP), in two known 3D topological insulators, Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ and Bi$_{\mathrm{0.5}}$Sb$_{\mathrm{1.5}}$Se$_{\mathrm{1.5+x}}$Te$_{\mathrm{1.5-x}}$. Surprisingly, the SP was prominent even after depressing the Fermi energy into the bulk band gap. Having studied the SP as a function of doping, momentum transfer and its aging properties, I will present evidence to suggest that bulk-surface coupling is crucial in explaining many of its properties. A simple model with dynamic bulk screening will be presented showing qualitative agreement with the observations. Lastly, the relation of the observed surface plasmon to the predicted spin-plasmon mode and to the kinks seen in the electronic dispersion as measured by ARPES will be discussed. [Preview Abstract] |
Friday, March 6, 2015 8:12AM - 8:24AM |
Y12.00002: Scanning tunneling spectroscopy investigation of the topological phase transition in (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}})_{2}$Se$_{3}$ Daniel Walkup, Wenwen Zhou, Ilija Zeljkovic, Yoshinori Okada, Zhensong Ren, Kane Scipioni, Stephen Wilson, Vidya Madhavan The three-dimensional topological insulator (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}})_{2}$Se$_{3}$ undergoes a phase transition to a trivial insulator as Bi atoms are replaced with In. This chemical substitution is expected to reduce the spin-orbit coupling, lift the bulk band inversion and thus destroy the Dirac surface states present in the end-member Bi$_{2}$Se$_{3}$. Although photoemission and transport measurements have provided evidence for this phase transition in thin films, the nature of the surface state transformation across the critical point remains unclear, especially near the transition point where the surface state penetration depth becomes comparable to film thicknesses. Here, we present scanning tunneling microscopy experiments on single crystals of (Bi$_{\mathrm{1-x}}$In$_{\mathrm{x}})_{2}$Se$_{3}$ for x $\sim$ 0-10{\%}. Using Landau Level spectroscopy, we map the surface state dispersion across the phase transition. Additionally, we use local density of states mapping to reveal the local influence of the In dopants near the critical point. [Preview Abstract] |
Friday, March 6, 2015 8:24AM - 8:36AM |
Y12.00003: Andreev Reflection Spectroscopy of Nb-doped Bi$_2$Se$_3$ Topological Insulator C. Kurter, A.D.K. Finck, Y. Qiu, E. Huemiller, A. Weis, J. Atkinson, J. Medvedeva, Y.S. Hor, D.J. Van Harlingen Doped topological insulators are speculated to realize p-wave superconductivity with unusual low energy quasiparticles, such as surface Andreev bound states. We present point contact spectroscopy of thin exfoliated flakes of Nb-doped Bi$_2$Se$_3$ where superconductivity persists up to $\sim$ 1 K, compared to 3.2 K in bulk crystals. The critical magnetic field is strongly anisotropic, consistent with quasi-2D behavior. Andreev reflection measurements of devices with low resistance contacts result in prominent BTK-like behavior with an enhanced conductance plateau at low bias. For high resistance contacts, we observe a split zero bias conductance anomaly and additional features at the superconducting gap. Our results suggest that this material is a promising platform for studying topological superconductivity. [Preview Abstract] |
Friday, March 6, 2015 8:36AM - 8:48AM |
Y12.00004: Andreev Reflection Spectroscopy on Bismuth- Chalcogenide Topological Insulators C.R. Granstrom, I. Fridman, J.Y.T. Wei, H. Lei, C. Petrovic, R.X. Liang Andreev reflection (AR) is the basic mechanism underlying the superconducting proximity effect which, at the interface between a topological insulator (TI) and a spin-singlet superconductor, can give rise to Majorana-like states. Despite this basic importance, little is known about how AR is affected by the unique attributes of a three-dimensional TI, namely the linear dispersion and spin-momentum locking of its surface states. In this study, we use both s-wave and d-wave superconducting tips [1] to perform AR spectroscopy on variously flux-grown Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ single crystals. The AR measurements are complemented by in-situ scanning tunneling spectroscopy, down to 300 mK and up to 9 T, in order to determine the doping level and characterize both the sample surface and tip condition. Our data are analyzed in terms of the characteristic band structure of Bi-chalcogenides, to elucidate how it affects the AR process. \\[4pt] [1] C. S. Turel et al., Appl. Phys. Lett. 99, 192508 (2011) [Preview Abstract] |
Friday, March 6, 2015 8:48AM - 9:00AM |
Y12.00005: STM/S studies of BiSbTeSe2 alloys: Intrinsic topological insulators with robust Fermi level in the bulk band gap Hyoungdo Nam, Yang Xu, Ireneusz Miotkowski, Jifa Tian, Yong Chen, Chih-Kang Shih Topological insulators (TI) have been attracting a lot of interest in spin chiral topological surface state (TSS). One of the major material challenges has been the difficulty to create a topological insulator with true insulating bulk so that the topological surface states dominate the transport properties. There has been effort in creating the quaternary compounds, Bi$_{\mathrm{2-x}}$Sb$_{\mathrm{x}}$Te$_{\mathrm{3-y}}$Se$_{\mathrm{y}}$ (BSTS) with intrinsic bulk states. Namely the Fermi level is inside the bulk band gap with Dirac point also in the bulk band gap. Angle resolved photoemission has been used to show that 1112 compound, Bi$_{\mathrm{1}}$Sb$_{\mathrm{1}}$Te$_{\mathrm{1}}$Se$_{\mathrm{2}}$, possess this desirable property. Recent observation of topological surface state quantum Hall effect in this compound marks another important milestone. This work focuses on investigations of the electronic structure of BiSbTeSe$_{\mathrm{2}}$ using scanning tunneling microscopy and spectroscopy (STM/S). With the second derivative of tunneling current, we accurately observed the locations of Dirac point (DP), valence band maximum point, and conduction band minimum point, which consist with previous ARPES studies. The investigation confirms the intrinsic bulk states with Fermi level is very close to the DP. We will further discuss the potential correlation of the DP fluctuation with respect to the local compositional fluctuations. [Preview Abstract] |
Friday, March 6, 2015 9:00AM - 9:12AM |
Y12.00006: Visualizing the native atomic defects in Bi$_2$Se$_3$ with scanning tunneling microscopy Jixia Dai, Damien West, Xueyun Wang, Yazhong Wang, Daniel Kwok, Sang Wook Cheong, Shengbai Zhang, Weida Wu In topological insulators such as Bi$_2$Se$_3$ the existence of native atomic defects is one of the major bottlenecks for potential applications utilizing the topologically protected surface states. Native defects such as vacancies or antisites are believed to be responsible for the metallic transport observed in Bi$_2$Se$_3$. In this study, we examined a series of Bi$_2$Se$_3$ samples that were grown with different conditions using atomically resolving scanning tunneling microscopy. We have successfully identified several types of intrinsic defects, including Se vacancies and Bi-Se antisites. The individual defect images are corroborated by first principle calculations. The densities of these defects across different samples are correlated with their growth conditions. Preliminary results suggest the defect densities can account for the charge carrier density estimated from tunneling spectroscopy. [Preview Abstract] |
Friday, March 6, 2015 9:12AM - 9:24AM |
Y12.00007: Observation of Fermi Arcs in a Doped Pseudospin-1/2 Heisenberg Antiferromagnet Strontium Iridate Y. K. Kim, O. Krupin, J. D. Denlinger, A. Bostwick, E. Rotenberg, Q. Zhao, J. F. Mitchell, J. W. Allen, B. J. Kim Emergent properties of two microscopically different systems can be similar. Despite manifestly different underlying microscopic electronic structures, the effective low-energy physics of Sr2IrO4 has been shown to be remarkably similar to that of the parent insulators of superconducting cuprates. However, whether the parallel with the cuprates continues to hold for a metallic phase induced by carrier doping remains unclear, which holds the key to the realization of a new high temperature superconductor. In this presentation, we will report that the evolution of the fermiology of Sr2IrO4 with doping and temperature reproduces that observed for the cuprates. Upon surface electron doping through in situ deposition of alkali-metal atoms, angle-resolved photoemission spectra of Sr2IrO4 display disconnected segments of zero-energy states, known as `Fermi arcs', and a gap as large as 80 meV. The Fermi arc smoothly evolves to a closed Fermi surface at higher surface coverage and at higher temperature. [Preview Abstract] |
Friday, March 6, 2015 9:24AM - 9:36AM |
Y12.00008: A Scanning Tunnelling Microscopy Study on an Alloyed Topological Insulator, Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ Wonhee Ko, Insu Jeon, Hyo Won Kim, Hyeokshin Kwon, Youngtek Oh, Se-Jong Kahng, Joonbum Park, Jun Sung Kim, Sung Woo Hwang, Hwansoo Suh Efficient doping of topological insulators while protecting its topological nature is key ingredient to realize topological devices. Engineering the chemical potential in the alloyed compound Bi$_{2-x}$Sb$_{x}$Te$_{3-y}$Se$_{y}$ has been achieved by tuning its chemical composition. However, the effect of alloying in microscopic scale has not yet been fully investigated with local probes. Here we report on the atomic and electronic structures of Bi$_{1.5}$Sb$_{0.5}$Te$_{1.7}$Se$_{1.3}$ studied using scanning tunnelling microscopy/spectroscopy (STM/STS). Although there is significant surface disorder due to the alloying of constituent atoms, cleaved surfaces of the crystals present a well-ordered hexagonal lattice in STM topographs with 1 nm high quintuple layer steps. STS results reflect the band structure and indicate that the surface state and Fermi energy are both located inside the energy gap. The surface states do not show any electron back-scattering; due to their topological nature they are extremely robust. Landau levels generated by perpendicular magnetic field follow the massless Dirac fermions. This finding demonstrates that alloying is a promising route for efficient doping of topological insulators whilst keeping the topological surface state intact. [Preview Abstract] |
Friday, March 6, 2015 9:36AM - 9:48AM |
Y12.00009: Circular photocurrent response of a topological insulator thin film probed by scanning photocurrent microscopy Dong-Xia Qu, Xufeng Kou, Murong Lang, Jonathan Crowhurst, Michael Armstrong, Joseph Zaug, Kang L. Wang, George Chapline The remarkable nature of surface states in topological insulators is expected to have a unique photocurrent response to electromagnetic radiation. However, the surface and bulk photo-excited charge transport mechanisms, in relation to the band bending at the electrode-topological insulator interface, have not been well understood. Here, we present scanning photocurrent microscopy measurements on a gated topological insulator microdevice and show that the spin-polarized photocurrent displays direction reversal near the electrical contact interfaces. We discuss two possible mechanisms, which alternatively play dominant roles in the helicity-dependent photocurrent map. Our analysis determines the magnitude of each contribution, and reveals the governing process under different gate conditions. [Preview Abstract] |
Friday, March 6, 2015 9:48AM - 10:00AM |
Y12.00010: Rapid high-resolution spin- and time-resolved ARPES Chiu-Yun Lin, Kenneth Gotlieb, Chris Jozwiak, Zahid Hussain, Aaron Bostwick, Alessandra Lanzara A high-efficiency spin- and angle-resolved photoemission spectroscopy (spin-ARPES) spectrometer, coupled with a lab-based~6 eV laser, will be presented in this talk. Combining time-of-flight(TOF) energy measurements with~low-energy exchange scattering spin polarimetry, spin-TOF ARPES~achieves unprecedented measurements of near-EF physics~rapidly. In addition, the successful integration of the spectrometer with the pulsed laser system demonstrates its potential for simultaneous spin- and time-resolved ARPES with pump-probe based measurements. [Preview Abstract] |
Friday, March 6, 2015 10:00AM - 10:12AM |
Y12.00011: Photoelectron Spin Control in Spin-Orbit Systems Kenneth Gotlieb, Christopher Jozwiak, Jonathan Sobota, Zhi-Xun Shen, Zahid Hussain, Alessandra Lanzara Significant experimental and theoretical interest has followed the observation that the spin polarization of photoelectrons from a topological surface state can be controlled by choice of probing photons.~ We expand these findings by investigating other systems with strong spin-orbit coupling. Spin- and angle-resolved photoemission measurements from a uniquely efficient and high resolution spectrometer using various photon polarizations reveal the extent and limits of this spin-dependent photoemission process.~ [Preview Abstract] |
Friday, March 6, 2015 10:12AM - 10:24AM |
Y12.00012: Direct Observation of the Fermi Arc Surface State in the Three-Dimensional Dirac Semimetal Na$_{3}$Bi Aiji Liang, Zhijun Wang, Chaoyu Chen, Youguo Shi, Hemian Yi, Ya Feng, Zhuojin Xie, Shaolong He, Junfeng He, Yingying Peng, Xu Liu, Yan Liu, Lin Zhao, Guodong Liu, Jun Zhang, M. Nakatake, M. Arita, K. Shimada, H. Namatame, M. Taniguchi, Zuyan Xu, Chuangtian Chen, Xi Dai, Zhong Fang, Xingjiang Zhou The three dimensional (3D) Dirac semimetals have linearly dispersive 3D Dirac nodes where the conduction and valence bands connect to each other. Here we report the direct observation of the linearly dispersive 3D bulk Dirac points at the natural (001) cleaving surface of Na$_{3}$Bi single crystal by high resolution ARPES. In addition, we have directly observed two separated 3D bulk Dirac nodes by elaborately cleaving Na$_{3}$Bi samples at a non-natural-cleavage (100) crystalline surface. We further unveil the unusual Fermi-arc surface states connecting the two 3D Dirac nodes. At this unique (100) crystalline surface, the identification of the 3D Dirac semimetal state in Na$_{3}$Bi paves the way for systematically exploring rich exotic topological physics such as topological insulator and Weyl semimetal state. [Preview Abstract] |
Friday, March 6, 2015 10:24AM - 10:36AM |
Y12.00013: Magnetism-induced massive Dirac fermions and topological defects in the surface state (SS) of binary and ternary topological insulators (TIs) Chien-Chang Chen, M. L. Teague, N. Woodward, W. Fan, N.-C. Yeh, L. He, X. Kou, M. Lang, K. L. Wang Magnetic doping effects on the SS of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ and (Bi$_{\mathrm{1-y}}$Sb$_{\mathrm{y}})_{\mathrm{2}}$Te$_{\mathrm{3\thinspace }}$are studied by scanning tunneling spectroscopy (STS) on MBE-grown bilayers of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/(Bi$_{\mathrm{1-x}}$Cr$_{\mathrm{x}})_{\mathrm{2}}$Se$_{\mathrm{3}}$ and (Bi$_{\mathrm{0.5}}$Sb$_{\mathrm{0.5}})_{\mathrm{2}}$Te$_{\mathrm{3}}$/((Bi$_{\mathrm{0.5}}$Sb$_{\mathrm{0.5}})_{\mathrm{1-x}}$Cr$_{\mathrm{x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ on InP (111) as a function of the undoped layer thickness ($d)$ and the Cr-doping level ($x)$. For $x=$ 5{\%} and 10{\%}, magnetic proximity effect is induced in the SS of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ for $d$ \textless 4-QL, leading to gapped Dirac spectra for temperatures ($T)$ below a 2D Curie temperature $T_{c}=$ (210$\pm $10) K and (240$\pm $10) K for $x=$5{\%} and 10{\%}, respectively, which are much higher than the bulk $T_{c}\sim $25 K where anomalous Hall effect occurs. The gap ($\Delta )$ is spatially inhomogeneous in zero magnetic field ($H =$ 0), and reaching a maximum $\Delta =$ (0.4$\pm $0.1) eV at $T$ \textless \textless $T_{c}$. The gap inhomogeneity decreases with increasing $x$, $H$ and decreasing $T$. Moreover, localized sharp impurity resonances are found occasionally near the boundaries of gapped and gapless regions, which are attributed to isolated Cr impurities that couple with the spins of Dirac fermions and form long-lived topological defects. With increasing interlayer $H$, the resonance peaks diminish as the spatial gap homogenizes. These findings in the bilayer binary TIs are compared with those in the bilayer ternary TIs of more uniform bulk ferromagnetism. [Preview Abstract] |
Friday, March 6, 2015 10:36AM - 10:48AM |
Y12.00014: Exotic surface Dirac fermions in Cerium-based compounds Nasser Alidoust, Satya Kushwaha, Aris Alexandradinata, Minggang Zeng, Suyang Xu, Madhab Neupane, Chang Liu, Ilya Belopolski, Guang Bian, Daniel Sanchez, Pavel Shibayev, Hsin Lin, B. Andrei Bernevig, Robert J. Cava, M. Zahid Hasan Cerium-based compounds have received considerable attention due to their complicated antiferromagnetic (AFM) phase transitions at low temperatures as well as their large Kerr rotation angles. Here, we use angle-resolved photoemission spectroscopy (ARPES) to study the detailed electronic structure of some of these compounds, and find unusual intercalating topological Dirac surface states in them. These exotic surface Dirac fermions appear to be very anisotropic, with protected degeneracy at the high symmetry Kramers' points. We also present a theoretical model to account for the observed topological surface states. Our finding of these unusual Dirac surface states in this class of strongly correlated materials calls for more in-depth investigations of topological properties of strongly correlated systems, and more elaborate theoretical work on the origin of such phases. [Preview Abstract] |
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