Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session K45: Topological Materials: Synthesis and Characterization  Other MaterialsFocus

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Sponsoring Units: DMP Chair: Andrew Yeats, Univ of Chicago Room: 392 
Wednesday, March 15, 2017 8:00AM  8:36AM 
K45.00001: Breakdown of the Kondo insulating state in SmB$_6$ by introducing Sm vacancies Invited Speaker: Natalia Drichko We explore the stability of the hybridization gap in SmB$_6$ to presence of a small number of Sm vacancies typical for this material, and demonstrate the extreme fragility of the Kondo insulating state. For the most stoichiometric sample we detect the hybridization gap which appears below 50 K as a depressed electronic Raman intensity below about 30 meV. The spectral weight that shifts to higher frequencies on the opening of the hybridization gap, forms two electronic maxima at 100 and 41 meV. We assign these maxima to the excitations between hybridized 4$f$5$d$ bands using recent band structure calculations. Below 30 K, ingap exciton modes with long lifetimes protected by hybridization gap develop at 1618 meV. With the increase of the number of Sm vacancies the exciton features broaden, evidencing a decrease in the lifetime due to a presence of electronic states in the gap. When we reach a concentration of Sm vacancies of only about 1 \% the ingap exciton is completely quenched, and the hybridization gap is not fully opened. We suggest that only the most stoichiometric SmB$_6$ samples possess a bulk gap necessary for the topological Kondo insulator state. [Preview Abstract] 
Wednesday, March 15, 2017 8:36AM  8:48AM 
K45.00002: Drive the Dirac Electrons into Cooper Pairs in Sr$_{\mathbf{x}}$\textbf{Bi}$_{\mathbf{2}}$\textbf{Se}$_{\mathbf{3}}$ Guan Du, Jifeng Shao, Xiong Yang, Zengyi Du, Delong Fang, Jinghui Wang, Kejing Ran, Jinsheng Wen, Changjin Zhang, Huan Yang, Yuheng Zhang, HaiHu Wen Topological superconductor is a very interesting and frontier topic in condensed matter physics. Despite~the~tremendous efforts~in exploring~topological superconductivity, its presence is however still under heavy debates. The Dirac electrons have been proven to exist on surface of a topological insulator. It remains unclear whether and how the Dirac electrons fall into Cooper pairing in an intrinsic superconductor with the topological surface states. Here we present the systematic study of scanning tunneling microscope/spectroscopy on the possible topological superconductor Sr$_{x}$Bi$_{2}$Se$_{3}$. We first show that only the intercalated~Sr~atoms~can induce superconductivity. Then we show the full superconducting gaps without any abnormal ingap density of states as expected theoretically for a bulk topological superconductor. Finally, we find that the surface Dirac electrons will simultaneously condense into the superconducting state within the superconducting gap. This vividly demonstrates how the surface Dirac electrons are driven into Cooper pairs. [Preview Abstract] 
Wednesday, March 15, 2017 8:48AM  9:00AM 
K45.00003: Lowtemperature thermal transport in the Kondo insulator SmB$_6$ MarieEve Boulanger, F. Lalibert\'e, S. Badoux, N. DoironLeyraud, L. Taillefer, W.A. Phelan, S.M. Koopayeh, T.M. McQueen The striking observation of quantum oscillations in the Kondo insulator SmB$_6$ suggests that there may be chargeless fermionic excitations at low temperature in the bulk of this material [1]. One way to detect such putative excitations is through their ability to carry entropy, which a measurement of thermal transport should in principle detect as a nonzero residual linear term in the $T=0$ limit, {\it i.e.} $\kappa_0/T > 0$. Here we report lowtemperature measurements of the thermal conductivity $\kappa$ in SmB$_6$, down to 50~mK, performed on various single crystals in magnetic fields up to 15~T. By extrapolating, we obtain $\kappa_0/T$ at each field. We observe no residual linear term at any field, {\it i.e.} $\kappa_0/T = 0$ at all $H$, in agreement with a previous study [2]. In other words, we do not detect mobile fermionic excitations. However, unlike in the prior study [2], we observe a large enhancement of $\kappa(T)$ with increasing field. We discuss possible interpretations of this field dependence. \\ \\ $[1]$ B.~S.~Tan~{\it et al.}, Science~{\bf 349},~287~(2015). \\ $[2]$ Y.~Xu~{\it et al.}, Physical Review Letters~{\bf 116},~246403~(2016). [Preview Abstract] 
Wednesday, March 15, 2017 9:00AM  9:12AM 
K45.00004: Unconventional large magnetoresistance in Cu$_{2}_{x}$Te Joseph H. Ross, Ali A. Sirusi, Alexander Page, Ctirad Uher, Lucia Steinke, M. C. Aronson Copperbased chalcogenides have been shown to have interesting physical properties which make them promising for thermoelectric, solar energy, and magnetic sensor devices. Here we report magnetotransport measurements on Cu$_{2}_{x}$Te, for several polycrystalline samples with $x$ in the range 0.13 to 0.22. The results demonstrate that the magnetoresistance becomes linear above a relatively low applied field. The magnitude of the linear magnetoresistance reaches 250{\%} at 2 K in a 9 T field, comparable to the effect observed in Ag$_{2}$Se and Ag$_{2}$Te. We discuss the results in terms of recent evidence for topological band inversion in Cu$_{2}$Te, and the possibility of high mobility surface states. Evidence from NMR and transport vs. composition indicates the normal carriers to occupy simple hole pockets, precluding compensation effects. Furthermore, the magnitude of maximum magnetoresistance scales as the mobility, as does the crossover field, while the magnetoresistance amplitudes drop to lower values as the vacancy density ($x)$ increases. The results are discussed in terms of a ParishLittlewood model and high mobility carriers. [Preview Abstract] 
Wednesday, March 15, 2017 9:12AM  9:24AM 
K45.00005: Observation of unusual topological surface states in halfHeusler compounds LnPtBi (Ln$=$Lu, Y) Y.L. Chen, Z.K. Liu, L.X. Yang, S.C. Wu, C. Shekhar, J. Jian, H.F. Yang, Y. Zhang, S.K. Mo, Z. Hussain, B. Yan, C. Felser Topological quantum materials represent a new class of matter with both exotic physical phenomena and novel application potentials. Many Heusler compounds, which exhibit rich emergent properties such as unusual magnetism, superconductivity and heavy fermion behaviour, have been predicted to host nontrivial topological electronic structures. The coexistence of topological order and other unusual properties makes Heusler materials ideal platform to search for new topological quantum phases (such as quantum anomalous Hall insulator and topological superconductor). By carrying out angleresolved photoemission spectroscopy and ab initio calculations on rareearth halfHeusler compounds LnPtBi (Ln$=$Lu, Y), we directly observe the unusual topological surface states on these materials, establishing them as first members with nontrivial topological electronic structure in this class of materials. Moreover, as LnPtBi compounds are noncentrosymmetric superconductors, our discovery further highlights them as promising candidates of topological superconductors. [Preview Abstract] 
Wednesday, March 15, 2017 9:24AM  9:36AM 
K45.00006: Design and investigation of potential SnTeP and ZrTeP class of Dirac materials Prashant Sarswat, Sayan Sarkar, Michael Free A motivation of new Dirac materials design and synthesis by perturbing the symmetry, was explored by substitution of a Sn vacancy by P that maintains the intrinsic band inversion at the L point but also the direct bandgap shrinkage upon the incorporation of spinorbit coupling. In a similar line of investigation, ZrTeP was also systematically studied. The synthesis of both SnTeP and ZrTeP system of compounds resulted in the formation of long needles type crystals and the bulk porous deposits. The exotic morphology of the Pdoped SnTe needles possesses the pierced surface throughout its extension. First principle based calculations were also carried out for these sets of compounds using General Gradient Approximation (GGA) with PerdewBurkeErnzerhof (PBE) exchangecorrelation functional. In order to ensure structural optimization, a limited memory BroydenFletcherGoldfarbShanno (LBFGS) algorithm was employed and the total energy in PBE exchangecorrelation functional was considered for the calculation of the formation energy per atom. The new modifications have a potential to establish the new class of Dirac materials ushering upon new frontiers of interest. [Preview Abstract] 
Wednesday, March 15, 2017 9:36AM  9:48AM 
K45.00007: Anisotropic Dirac Fermions in BaMnBi$_{2}$ and BaZnBi$_{2}$ Hyejin Ryu, Se Young Park, Lijun Li, Weijun Ren, Cedomir Petrovic, Choonkyu Hwang, SungKwan Mo We report electronic structures of BaMnBi$_{2}$ and BaZnBi$_{2}$ sharing similar structural properties but having different valence configuration of the Mn/ZnBi complex. Our angleresolved photoemission measurements found a strong anisotropic Dirac dispersion in BaMnBi$_{2}$ and a complete departure from the Dirac dispersion in BaZnBi$_{2}$. Our findings, substantiated by the first principle calculations, allow us to understand role of Mn/ZnBi tetrahedra in the changes of the electronic structures as well as the effect of varying band filling of Bisquare net. [Preview Abstract] 
Wednesday, March 15, 2017 9:48AM  10:00AM 
K45.00008: Observation of the spinpolarized surface state in a noncentrosymmetric superconductor BiPd Madhab Neupane, N. Alidoust, M. M. Hosen, J.X Zhu, K. Dimitri, S.Y. Xu, N. Dhakal, R. Sankar, I. Belopolski, D.S. Sanchez, T.R. Chang, H.T. Jeng, K. Miyamoto, T. Okuda, H. Lin, A. Bansil, D. Kaczorowski, F.C. Chou, M. Z. Hasan, T. Durakiewicz Recently, noncentrosymmetric superconductor BiPd has attracted considerable research interest due to the possibility of hosting topological superconductivity. Here we report a systematic highresolution angleresolved photoemission spectroscopy (ARPES) and spinresolved ARPES study of the normal state electronic and spin properties of BiPd. Our experimental results show the presence of a surface state at higherbinding energy with the location of Dirac point at around 700 meV below the Fermi level. The detailed photon energy, temperaturedependent and spinresolved ARPES measurements complemented by our firstprinciples calculations demonstrate the existence of the spinpolarized surface states at highbinding energy. The absence of such spinpolarized surface states near the Fermi level negates the possibility of a topological superconducting behaviour on the surface. [Preview Abstract] 
Wednesday, March 15, 2017 10:00AM  10:12AM 
K45.00009: Band structure of ZrTe$_5$ measured by highmomentumresolution photoemission spectroscopy Hongyu Xiong, Jonathan Sobota, Shuolong Yang, Hadas Soifer, Alexandre Gauthier, MingHui Lu, YangYang Lv, Shuhua Yao, Donghui Lu, Makoto Hashimoto, Patrick Kirchmann, YanFeng Chen, ZhiXun Shen We have performed a systematic highmomentumresolution photoemission study on ZrTe$_5$ using a photon energy of $6$ eV. We have measured the band structure near $\Gamma$, and quantified the gap between conduction and valence band as $25\pm5$ meV. We have also observed photonenergydependent spectral changes, and attributed these changes to finalstate effects and the 3D nature of the material's band structure. Our observation suggests that ZrTe$_5$ is neither a 3D strong topological insulator, nor a Dirac semimetal. We further propose a model which promises to reconcile discrepancies on the existence of surface state in different literature studies of this material. [Preview Abstract] 
Wednesday, March 15, 2017 10:12AM  10:24AM 
K45.00010: Exploring the Local Electronic Structure of Monolayer 1T'WTe$_{2}$ via Scanning Tunneling Spectroscopy Zahra Pedramrazi, Shujie Tang, Chaofan Zhang, Dillon Wong, HsinZon Tsai, Salman Kahn, Chunjing Jia, Brian Moritz, Hao Yan, Robert Moore, Hyejin Ryu, Juan Jiang, Makoto Hashimoto, Donghui Lu, Chancuk Hwang, Choongyu Hwang, Zahid Hussain, Yulin Chen, Miguel Ugeda, Zhi Liu, Xiaoming Xie, Thomas Devereaux, SungKwan Mo, ZhiXun Shen, Michael Crommie The transition metal dichalcogenides host many novel electronic states of matter, and still others have been theoretically predicted. For example, strong spinorbit coupling is known to cause typeII Weyl semimetal behavior in the threedimensional T$_{d}$ phase of WTe$_{2}$, and has also been predicted to lead to twodimensional topological insulator behavior in the singlelayer 1T' phase of WTe$_{2}$. We have used scanning tunneling spectroscopy to help test this latter prediction by measuring the local electronic structure of singlelayer WTe$_{2}$ in the 1T' phase. Our scanning tunneling spectroscopy measurements reveal spatial variations in the local electronic structure between the edge of WTe$_{2}$ single layers and their interior bulk regions. [Preview Abstract] 
Wednesday, March 15, 2017 10:24AM  10:36AM 
K45.00011: From Normal Insulator to Topological Insulator in Plumbene XiangLong Yu, Jiansheng Wu Plumbene, similar to silicene, has a buckled honeycomb structure with a larger band gap ($\sim 400$ meV). However, all previous studies have shown that it is a normal insulator. Here, we perform firstprinciples calculations and employ a sixteenband tightbinding model with nearestneighbor and nextnearestneighbor hopping terms to investigate electronic structures and topological properties of plumbene monolayer. We find that it can become a topological insulator with a large bulk gap ($\sim 200$ meV) through electron doping, and the nontrivial state is very robust with respect to external strain. Plumbene can be an ideal candidate for realizing quantum spin Hall effect at room temperature. By investigating effects of external electric and magnetic fields on electronic structures and transport properties of plumbene, we present two rich phase diagrams with and without electron doping, and propose a theoretical design for a fourstate spinvalley filter. [Preview Abstract] 
Wednesday, March 15, 2017 10:36AM  10:48AM 
K45.00012: Antimonene: Experiments and theory of surface conductivity Juan Jose Palacios, Pablo Ares, Sahar Pakdel, Wendel Paz, Felix Zamora, Julio GomezHerrero Very recently antimony has been demonstrated to be amenable to standard exfoliation procedures opening the possibility of studying the electronic properties of isolated fewlayers flakes of this material, a.k.a. antimonene [1]. Antimony is a topological semimetal, meaning that its electronic structure presents spinsplit helical states (or Dirac cones) on the surface, but it is still trivially metallic in bulk. Antimonene, on the other hand, may present a much reduced electronic bulk contribution for a small number of layers. A novel technique to make electrical contacts on the surface of individual thin flakes (510 monolayers) has allowed us to measure the (surface) conductivity of these in ambient conditions. Our measurements show a high conductivity in the range of $12 \, e^2/h$, which we attribute to the surface Dirac electrons. We have also carried out theoretical work to address the origin of this value, in particular, the importance of scattering between the Dirac electrons and the bulk bands. Our calculations are based on density functional theory for the electronic structure and Kubo formalism for the conductivity, the latter considering random disorder and the presence of water. [1] P. Ares et al., Advanced Materials 28, 6515 (2016) [Preview Abstract] 
Wednesday, March 15, 2017 10:48AM  11:00AM 
K45.00013: Structural Characterization of Stanene Grown on Bi$_2$Te$_3$ by Anomalous XRay Scattering Stephen D. Albright, Ke Zou, Rui Peng, Claudia Lau, Hawoong Hong, Charles H. Ahn, Fred J. Walker Twodimensional topological insulators are promising materials for use in spintronics and the realization of dissipationfree conduction. Stanene, a hexagonal monolayer of Sn, is a promising candidate for these applications because it supports onedimensional edge states and is predicted to have a large enough bandgap ($>0.1$ eV) for devices to operate at elevated temperature. Here we report the growth of stanene on Bi$_2$Te$_3$ using molecular beam epitaxy. By growing stanene \textit{in situ} at a synchrotron light source, we measure energy dependent scattering along Bi$_2$Te$_3$ crystal truncation rods (CTRs). The scattered intensity along Bi$_2$Te$_3$ CTRs undergoes a strong modulation as the incident xray energy is tuned across the Sn Kedge, indicating coherent growth of the stanene to the Bi$_2$Te$_3$ substrate. Analysis of these measurements reveals the particular polymorph of stanene on Bi$_2$Te$_3$. [Preview Abstract] 
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