Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session R13: Focus Session: Topological Materials - Surface Microscopy and Spectroscopy |
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Sponsoring Units: DMP Chair: Alexander Brinkman, University of Twente Room: 315 |
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R13.00001: Imaging the Impact of Impurities on Topological Surface States Invited Speaker: Jennifer Hoffman Harnessing the technological potential of the spin-polarized surface states on topological insulators requires a detailed understanding of the impact of nanoscale disorder on those surface states. We employ spectroscopic scanning tunneling microscopy (STM) in the presence of a magnetic field to visualize the impact of intrinsic impurities on topological surface states in Sb and Bi$_2$Se$_3$. We find a variety of impurities with different energy profiles that elastically scatter surface states through dispersive quasiparticle interference (QPI), that inelastically scatter surface states into the bulk, that locally destroy the extended surface state Landau level wavefunctions, or that form local resonant states interacting with the Dirac quasiparticles. By identifying impurities that strongly interact with and limit the mobility of the topological surface states, our impurity studies can directly advise the growth and development of future topological materials.\\[4pt] Measurements carried out by Anjan Soumyanarayanan, Michael Yee, Yang He. Samples grown by Dillon Gardner \& Young Lee; Zahir Salman \& Amit Kanigel; Zhi Ren \& Kouji Segawa \& Yoichi Ando.\\[4pt] Experiments supported by the National Science Foundation, under grant DMR-1106023. [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R13.00002: Scanning tunneling microscopy studies of topological crystalline insulators Andras Gyenis, Jungpil Seo, Oliver Jeong, Ilya Drozdov, Stevan Nadj-Perge, Quinn Gibson, Genda Gu, Robert Cava, Ali Yazdani Recent theoretical studies and experimental findings suggest the existence of a new topological phase: topological crystalline insulators (TCI). In contrast to the Z$_{2}$ topological insulators, where the time-reversal symmetry warrants the topological protection of the gapless surface states, in the TCI phase the protection is based on the crystal symmetry. Pb$_{1-x}$Sn$_{x}$Se and Pb$_{1-x}$Sn$_{x}$Te alloys are promising candidates for the TCI state: both of them have the rock-salt crystal structure (at certain doping values) with spatial mirror symmetry, and as a function of doping level the band structure can be changed from normal to inverted bandgap state. We present scanning tunneling microscopy/spectroscopy measurements on these alloys as a function of doping. Similar to previous experiments on spin-orbit coupled topological insulators [1], spectroscopic mapping with the STM can be used to establish the presence of topological properties through examining allowed and disallowed scattering transitions.\\[4pt][1] P. Roushan et al, Nature \textbf{460} 1106 (2009). [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R13.00003: LT-STM study of Nb islands on Bi2Se3 with W and Nb Tips Rami Dana, Anita Roychowdhury, Ireneusz Miotkowski, Yong P. Chen, Michael Dreyer Proximity effect between an s-wave superconductor (SC) and a topological insulator (TI) are expected to induce px $+$ ipy superconductor like state at the SC-TI interface. The vortex cores of that state are predicted to host Majorana fermions. In this work we study the TI Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ using W and Nb tips at 4.2 K with and without Nb islands. The W tip shows no SC gap on top and around the islands. The Nb tip shows variable SC gaps and a verity of zero bias conductance peaks. The possible sources for these observations and the part of the TI, Nb islands and/or Nb tip will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R13.00004: Scanning Tunneling Microscopy of the Topological Crystalline Insulator SnTe Duming Zhang, Tong Zhang, Jeonghoon Ha, Hongwoo Baek, Young Kuk, Fred Sharifi, Joseph Stroscio Topological insulators are a new state of matter characterized by a bulk insulating gap and gapless surface states protected by time reversal symmetry. This is realized by spin orbit coupling induced band inversion with an odd number of Dirac cones. Recently, the topological classification of states has been extended to a new class of matter called topological crystalline insulators. In contrast to topological insulators, topological crystalline insulators arise from crystal symmetry and are characterized by surface states with an even number of Dirac cones. Here, we report molecular beam epitaxy growth of SnTe thin films, a material recently predicted and experimentally confirmed as a topological crystalline insulator. The film morphology and SnTe (001) surface states were characterized \textit{in-situ} by low temperature scanning tunneling microscopy and spectroscopy will be discussed in relation to the predicated topological properties of this material. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R13.00005: Scanning tunneling microscopy of gate tunable topological insulator Sb2Te3 Tong Zhang, Niv Levy, Jeonghoon Ha, Young Kuk, Joseph Stroscio We achieved gate tunable topological insulator (TI) Sb2Te3 thin films which are suitable for low temperature scanning tunneling microscopy (STM) studies. The film is epitaxially grown on pre-patterned SrTiO3 substrates which are mounted on specially designed sample holders. This allows us to do in-situ gating on epitaxial films without any ex-situ processing of the sample. The tunneling conductance as well as film resistance is investigated as a function of gate voltage (Vg). In a 3 nm thick Sb2Te3 film, a gap opening at the Dirac point due to the coupling of the top and bottom surfaces is observed. More importantly, the gap size is found to be tunable by Vg, a result of the combination of coupling of the surface state bands and electric field effect. We show that our observation can be well described by an effective model of TI thin films and first principle calculations. The reduced surface states gap versus Vg indicates it is possible to create a topological phase transition by apply a strong enough electric field through the film. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R13.00006: Landau quantization and quasiparticle interference of Dirac fermions on a topologically protected Fermi surface Anjan Soumyanarayanan, Michael Yee, Yang He, Dillon Gardner, Young Lee, Jennifer Hoffman The discovery of topological materials hosting spin-polarized Dirac fermion surface states has been driven by the use of surface-sensitive spectroscopic tools. Scanning tunneling microscopy and spectroscopy (STM/STS) can, in principle, access the surface state band structure across a range of energies on the nanometer length scale through a combination of one particle (Landau quantization) and two-particle (quasiparticle scattering) techniques. However, the equivalence of these two STS techniques has yet to be established. Here we report the surprising simultaneous observation of Landau quantization and quasiparticle interference on the Fermi surface of the topological metal Sb(111). We establish the equivalence of the two momentum-resolved STS techniques, and use them to quantitatively reconstruct the multi-component surface state band structure, which would be inaccessible via either of these techniques alone. We further use these techniques to probe the local effects of single atom impurities on the surface states. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R13.00007: First principles transport calculations on topological surface states scattering Ivan Rungger, Awadhesh Narayan, Stefano Sanvito We study the scattering properties of topologically protected states on the Sb(111) and Bi$_2$Se$_3$(111) surfaces by using the ab initio transport code SMEAGOL \footnote{A. R. Rocha, V. M. Garcia-Suarez, S. Bailey, C. Lambert, J. Ferrer, and S. Sanvito, Phys. Rev. B \textbf{73}, 085414 (2006).}. We consider different types of defects, such as adatoms and extended barriers. In the presence of a strong surface perturbation in the form of a step separating surface terraces we obtain standing-wave states resulting from the superposition of spin-polarized surface states. By Fourier analysis, we identify the underlying two dimensional scattering processes and the spin texture \footnote{A. Narayan, I. Rungger, and S. Sanvito, Phys. Rev. B \textbf{86}, 201402(R) (2012).}. We find evidence of resonant transmission across the surface barrier at quantum well state energies and evaluate their lifetimes. Our results for the Sb surface are in agreement with experimental findings \footnote{J. Seo, P. Roushan, H. Beidenkopf, Y. S. Hor, R. J. Cava, and A. Yazdani, Nature (London) 466, 343 (2010).}. We also show that despite the presence of a step edge along a different direction, the surface states exhibit unperturbed transmission around the Fermi energy for states with near to normal incidence [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R13.00008: Electronic structure studies on p-type Pb$_{1-x}$Sn$_x$Te system above and below the band inversion topological transition Nasser Alidoust, Su-Yang Xu, M. Neupane, C. Liu, I. Belopolski, D. Qian, J.D. Denlinger, Y.J. Wang, H. Lin, L.A. Wray, Q. Gibson, R. Sankar, F.C. Chou, R.J. Cava, A. Bansil, M.Z. Hasan We present systematic ARPES studies on p-type Pb$_{1-x}$Sn$_x$Te samples at three different compositions with x = 0.26, 0.5, and 1.0. This material has been predicted as a topological crystalline insulator (TCI) upon band inversion at $x\simeq0.3$. We show that the observed bulk valence band is a single hole-like band in the vicinity of the $\mathrm{\overline{\textit{X}}}$ points of the surface Brillouin zone, and reveal the 3D dispersive nature of the valence band with a clear $k_z$ dispersion. We further show that despite the predicted band inversion and topological phase transition, the observed valence band electronic structure does not exhibit dramatic difference between these samples, demonstrating the critical importance of preparing in-gap or n-type samples for the realization of the TCI phase. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R13.00009: Observing electronic structures on \textit{ex-situ} topological insulator thin films Bo Zhou, S.H. Yao, M.H. Lu, Z.K. Liu, Y.B. Chen, J.G. Analytis, C. Brune, W.H. Dang, S.-K. Mo, Z.-X. Shen, I.R. Fisher, L.W. Molenkamp, H.L. Peng, Z. Hussain, Y.L. Chen Topological insulators represent a new state of quantum matter with insulating bulk but conducting surface states formed by an odd number of Dirac fermions. We present our progress on the study of electronic structures of \textit{ex-situ} grown topological insulator thin films by angle resolved photoemission spectroscopy (ARPES). We successfully obtained the topological band structures, after proper surface cleaning procedures, from HgTe films grown by molecular beam epitaxy and Bi$_2$Te$_3$ nanoplates synthesized by vapor-solid method. This new development will not only enable us to study more topological insulators that cannot be measured by conventional \textit{in-situ} ARPES technique, but also open the door to directly characterize the electronic properties of topological insulators used in functional devices. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 5:18PM |
R13.00010: Spatial fluctuations of helical Dirac fermions on the surface of topological insulators Invited Speaker: Haim Beidenkopf Strong topological insulators are materials that host exotic states on their surfaces due to a topological band inversion in their bulk band structure. These surface states have Dirac dispersion as if they were massless relativistic particles, and are assured to remain metallic by time reversal symmetry. The helical spin texture associated with the Dirac dispersion prohibits backscattering, which we have imaged using scanning tunneling microscopy (STM) and spectroscopic mappings [1,2]. This topological protection can be lifted by time-reversal breaking perturbations that induce a gap at the Dirac point and cant the helical spin texture. Massive Dirac electrons had been visualized by angular resolved photo emission spectroscopy in magnetically doped topological insulators. While we do not identify a gapped spectrum in our STM measurements of similar compounds, we do find a dominating electrostatic response to the charged content of those dopants [3]. In their presence the Dirac spectrum exhibits strong spatial fluctuations. As a result translational invariance is broken over a characteristic length scale and the Dirac-point energy is only locally defined. Possible global manifestations of these local fluctuations will be discussed, as well as alternative avenues for breaking time reversal symmetry while maintaining the integrity of the Dirac spectrum.\\[4pt] [1] P. Roushan, J. Seo, C. V. Parker, Y. S. Hor, D. Hsieh, D. Qian, A. Richardella, M. Z. Hasan, R. J. Cava, A. Yazdani, Nature 460, 1106 (2009).\\[0pt] [2] J. Seo, P. Roushan, H. Beidenkopf, Y. S. Hor, R. J. Cava, A. Yazdani, Nature 466, 343 (2010).\\[0pt] [3] H. Beidenkopf, P. Roushan, J. Seo, L. Gorman, I. Drozdov, Y. S. Hor, R. J. Cava, A. Yazdani, Nat. Phys. 7, 939 (2011). [Preview Abstract] |
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