Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session F44: Dirac and Weyl Semimetals: STMFocus

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Sponsoring Units: DMP Chair: Jiunhaw Chu, University of Washington Room: 391 
Tuesday, March 14, 2017 11:15AM  11:51AM 
F44.00001: Spectroscopic Visualization of Inversion and TimeReversal Symmetry Breaking Weyl Semimetals Invited Speaker: haim beidenkopf A defining property of a topological material is the existence of surface bands that cannot be realized but as the termination of a topological bulk. In a Weyl semimetal these surface states are in the form of Fermiarcs. Their opencontour Fermisurface curves between pairs of surface projections of bulk Weyl cones. Such Diraclike bulk bands, as opposed to the gapped bulk of topological insulators, land a unique opportunity to examine the deep notion of bulk to surface correspondence. We study the intricate properties both of inversion symmetry broken and of timereversal symmetry broken Weyl semimetals using scanning tunneling spectroscopy. We visualize the Fermi arc states on the surface of the noncentrosymmetric Weyl semimetal TaAs [R. Batabyal et al., Science Advances 2, e1600709 (2016)]. Using the distinct structure and spatial distribution of the wavefunctions associated with the different topological and trivial bands we detect the scattering processes that involve Fermi arcs. Each of these imaged scattering processes entails information on the unique nature of Fermi arcs and their correspondence to the topological bulk. We further visualize the magnetic response of the candidate magnetic Weyl semimetal GdPtBi in which the magnetic order parameter is coupled to the topological classification. [Preview Abstract] 
Tuesday, March 14, 2017 11:51AM  12:03PM 
F44.00002: Unexpected Zero Bias Conductance Peak on the Topological Semimetal Sb(111) Yau Chuen Yam, Shiang Fang, Pengcheng Chen, Mohammad Hamidian, Yang He, Dillon Gardner, Young Lee, Bertrand Halperin, Jennifer Hoffman The expected signature of the longsought Majorana fermion in a heterostructure of a superconductor and a topological material is a zero bias conductance peak (ZBCP). We use scanning tunneling microscopy (STM) to image the cleaved surface of the topological semimetal antimony (Sb), whose long surfacestate lifetime and small critical thickness make it a good candidate for building such heterostructures. Its bilayer crystal structure is expected to cleave between bilayers, however we observed step heights corresponding to the intrabilayer distance, indicating the presence of a broken bilayer on some terraces. The dI/dV spectra observed on these abnormal terraces are quite different from the usual Sb spectra and there is a pronounced ZBCP. Using quasiparticle interference imaging, Landau level spectroscopy and density functional theory, we found that the ZBCP originates from a van Hove singularity in the band structure due to the broken layer. We acknowledge funding from the National Science Foundation grant DMR1410480 and the Canada Excellence Research Chair program. [Preview Abstract] 
Tuesday, March 14, 2017 12:03PM  12:15PM 
F44.00003: Temperaturedriven Topological Phase Transition in MoTe$_{\mathrm{2}}$ Ayelet Notis Berger, Erick Andrade, Alex Kerelsky, SangWook Cheong, Jian Li, B. Andrei Bernevig, Abhay Pasupathy The discovery of several candidates predicted to be weyl semimetals has made it possible to experimentally study weyl fermions and their exotic properties. One example is MoTe$_{\mathrm{2}}$, a transition metal dichalcogenide. At temperatures below 240 K it is predicted to be a type II Weyl semimetal with four Weyl points close to the fermi level. As with most weyl semimetals, the complicated band structure causes difficulty in distinguishing features related to bulk states and those related to topological fermi arc surface states characteristic of weyl semimetals. MoTe$_{\mathrm{2}}$ is unique because of its temperaturedriven phase change. At high temperatures, MoTe$_{\mathrm{2}}$ is monoclinic, with trivial surface states. When cooled below 240K, it undergoes a first order phase transition to become an orthorhombic weyl semimetal with topologically protected fermi arc surface states. We present STM and STS measurements on MoTe$_{\mathrm{2}}$ crystals in both states. In the orthorhombic phase, we observe scattering that is consistent with the presence of the Fermiarc surface states. Upon warming into the monoclinic phase, these features disappear in the observed interference patterns, providing direct evidence of the topological nature of the fermi arcs in the Weyl phase [Preview Abstract] 
Tuesday, March 14, 2017 12:15PM  12:27PM 
F44.00004: Electronic properties of two inequivalent surfaces in MoTe$_2$ studied by quasiparticle interference Davide Iaia, Yan Shichao, Vidya Madhavan MoTe$_2$ has received renewed interest due to its topological properties. At a temperature below 250 K, MoTe$_2$ is a type II Weyl semimetal hosting threedimensional (3D) linearly dispersing states with well defined chirality. Nodes in this 3D dispersion are called Weyl points. Weyl points of opposite chirality are expected to be connected by topologically protected Fermi arcs. In this talk we discuss low temperature scanning tunneling microscopy studies of the electronic structure of MoTe$_2$. The electronic properties are studied using quasiparticle interference technique which allows us to resolve Fermi arcs features and to clearly distinguish between two inequivalent MoTe$_2$ surfaces. Our results provide important contributions to further our understanding of the electronic properties of this new and exotic class of materials. [Preview Abstract] 
Tuesday, March 14, 2017 12:27PM  12:39PM 
F44.00005: Scanning Tunneling Microscopic investigation of a typeII Weyl semimetal surface Hao Zheng, Shuang Jia, Hsin Lin, M Zahid Hasan Recent discovery of typeI Weyl fermions has generated a flurry of new research directions in topological materials. The crystal symmetry and spinorbitcoupling induced tilting of the Weyl cone can lead to strong violation (typeII) of Lorentz invariance in a condensed matter system. In this talk, we will present the atomic resolution scanning tunneling spectromicroscopic investigation on a singlecrystalline typeII Weyl semimetal Mo$_{\mathrm{x}}$W$_{\mathrm{1x}}$Te$_{\mathrm{2}}$ for the first time. Interesting typeII Weyl Fermion and topological Fermi arc related physics will be discussed [Preview Abstract] 
Tuesday, March 14, 2017 12:39PM  12:51PM 
F44.00006: Scanning Tunneling Microscopy Study on Dirac Nodalline Semimetal ZrSiS ChihChuan Su, SyuYou Guan, TzuCheng Wang, Raman Sankar, GuangYu Guo, Fangcheng Chou, ChiaSeng Chang, TienMing Chuang 
Tuesday, March 14, 2017 12:51PM  1:03PM 
F44.00007: Abstract Withdrawn 3D Dirac materials are an intensive area of current condensed matter research. The related Dirac line node materials have come into focus due to many shared properties such as unconventional magnetotransport and the potential to host topologically nontrivial phases. ZrSiS is one of the first discovered materials of this new family, hosting a nodal line and an unconventional surface state [1]. Spectroscopic imaging scanning tunneling microscopy (SISTM) detects quasiparticle interference and has been extensively used to study the scattering mechanism and the band structures of exotic materials with high energy resolution at the atomic scale. Here in this presentation, we report the investigation of ZrSiS by SISTM at the atomic scale, in combination with DFT calculations. We succeeded in visualizing the Dirac nodal line both in real and momentum space, adding key pieces of evidences confirming the existence of a nodal line in this material and highlighting its exceptional properties. The breaking of a nonsymmorphic symmetry at the surface induces an unusual surface state whose dispersion was mapped. In particular, we observed spectroscopic signatures of a typeII Dirac fermion hosted by the surface state. Our data as seen by SISTM has impact beyond ZrSiS providing crucial insights into the properties of Dirac line node materials in particular and nonsymmorphic crystals in general. [1] Leslie Schoop et al., Nature Commnunications DOI: 0.1038/ncomms11696 
Tuesday, March 14, 2017 1:03PM  1:15PM 
F44.00008: Quasiparticle interference mapping of ZrSiS Michael Lodge, Md Mofazzle Hosen, Madhab Neupane, Masa Ishigami, Guoqing Chang, Bahadur Singh, Hsin Lin, Bent Weber, Jack Hellerstedt, Mark Edmonds, Michael Fuhrer, Dariusz Kaczorowski The emergent class of 3D Dirac semimetals presents intriguing new systems in which to study the rich physics of the robust, topologicallyprotected quasiparticles hosted within their bulk. For example, in nodalline Dirac semimetals, the conductance and valence bands meet along a closed loop in momentum space and disperse linearly in the vicinity of the resultant line node. This results in novel scattering phenomena, owing to the unique Fermi surfaces and scattering selection rules of these systems. Here, we have performed scanning tunneling microscopy and spectroscopy of ZrSiS, one such nodalline Dirac semimetal,$_{\mathrm{\thinspace }}$at 4.5 K. We have visualized quasiparticle scattering using differential conductance mapping. In conjunction with numerical modeling, we identify at least six allowed scattering vectors in the material, which gives insight into the scattering selection rules of these novel materials. [Preview Abstract] 
Tuesday, March 14, 2017 1:15PM  1:27PM 
F44.00009: Identifying the Dirac line node in the 3D semimetal ZrSiS Bent Weber, Michael S Lodge, Guoqing Chang, Bahadur Singh, Jack Hellerstedt, Mark Edmonds, Dariusz Kaczorowski, Md Mofazzel Hosen, Madhab Neupane, Hsin Lin, Michael S Fuhrer, Masa Ishigami With the advent of novel topological phases of matter, 3D Dirac semimetals are emerging as classes of materials which promise topological protection of electronic states within their bulk. In linenodal Dirac semimetals in particular, the conductance and valence bands touch along a closed loop in momentum space, giving rise to predictions of exotic states at their surface such as Dirac line node arcs and spin vortex rings. However, in many compounds  including ZrSiS  the line node itself is located above the Fermi energy, which makes it inherently inaccessible to experimental techniques such as angleresolved photoemission spectroscopy (ARPES). Here we employ quasiparticle interference (QPI) spectroscopy at 4.5K in combination with numerical modelling as complementary techniques to ARPES, allowing us to identify the position of the Dirac line node and the Dirac dispersion hundreds of meV into the conduction band. [Preview Abstract] 
Tuesday, March 14, 2017 1:27PM  1:39PM 
F44.00010: Atomically Resolved STM Characterization of the 3D Dirac Semimetal Cd$_{3}$As$_{2}$ Christopher Butler, Yi Tseng, ChengRong Hsing, YuMi Wu, Raman Sankar, MeiFang Wang, ChingMing Wei, FangCheng Chou, MinnTsong Lin Dirac semimetals such as Cd$_{3}$As$_{2}$ are a recently discovered class of materials which host threedimensional linear dispersion around pointlike band crossings in the bulk Brillouin zone, and hence represent threedimensional analogues of graphene. This electronic phase is enabled by specific crystal symmetries: In the case of Cd$_{3}$As$_{2}$, a C$_{4}$ rotational symmetry associated with its peculiar corkscrew arrangement of systematic Cd vacancies. Although this arrangement underpins the current crystallographic understanding of Cd$_{3}$As$_{2}$, and all its theoretical implications, it is strangely absent in surface microscopic investigations reported previously. Here we use a combined approach of scanning tunneling microscopy and ab initio calculations to show that the currently held crystallographic model of Cd$_{3}$As$_{2}$ is indeed predictive of a periodic zigzag superstructure at the (112) surface, which we observe in scanning tunneling microscopy images. This helps to reconcile the current state of microscopic surface observations with the prevailing crystallographic and theoretical models. [Preview Abstract] 
Tuesday, March 14, 2017 1:39PM  1:51PM 
F44.00011: Visualizing Fermi arcs by their weakly bound wave function in the Weyl semimetal TaAs Noam Morali, Rajib Batabyal, Nurit Avraham, Yan Sun, Marcus Schmidt, Claudia Felser, Ady Stern, Binghai Yan, Haim Beidenkopf The topological nature of Weyl semimetals guarantees the existence of surface Fermi arc states. The surface of tantalum arsenide, similar to that of other members of the Weyl semimetal class, hosts nontopological bands that obscure the exploration of the Fermi arc states. We use the spatial structure of the surface statesâ€™ wave function visualized by scanning tunneling microscopy to distinguish and characterize the surface Fermi arc bands [1]. The trivial states have a complex structure within the unit cell, which further evolves in the presence of an external magnetic field. In contrast, the Fermi arc wave function is essentially planewave like. It is weakly affected by the surface potential and thus spreads rather uniformly within the unit cell. We obtain these results using an analysis technique, based on the role of the Bloch wave function in shaping quantum electronic interference patterns. It thus carries broader applicability to the study of other electronic systems and other physical processes.\\ [1] Batabyal, Rajib; Morali, Noam; Avraham, Nurit; Sun, Yan; Schmidt, Marcus; Felser, Claudia; Stern, Ady; Yan, Binghai; Beidenkopf, Haim (2016). Visualizing Weakly Bound Surface Fermi Arcs and Their Correspondence to Bulk Weyl Fermions. Science Advances. 2:e1600709. [Preview Abstract] 
Tuesday, March 14, 2017 1:51PM  2:03PM 
F44.00012: Superconducting Topological Surface States in Noncentrosymmetric Bulk Superconductor PbTaSe$_{\mathrm{2}}$ TienMing Chuang, SyuYou Guan, PengJen Chen, MingWen Chu, Raman Sankar, Fangcheng Chou, HorngTay Jeng, ChiaSeng Chang The search for topological superconductors (TSCs) is one of the most exciting problems in condensed matter systems. Within each vortex core of TSCs, there exist the zero energy Majorana bound states, which are predicted to exhibit nonAbelian statistics and to form the basis of the faulttolerant quantum computation. So far, no stoichiometric bulk material exhibits the required topological surface states (TSSs) at E$_{\mathrm{F}}$ combined with fully gapped bulk superconductivity. Here, we report atomic scale visualization of the TSSs of the noncentrosymmetric superconductor, PbTaSe2. Our quasiparticle scattering interference imaging shows two TSSs with a Dirac point at E\textasciitilde 1.0eV, of which the inner TSS and partial outer TSS cross E$_{\mathrm{F}}$, on the Pbsurface of this fully gapped superconductor. Our results reveal PbTaSe2 as a promising candidate as a TSC. [Preview Abstract] 
Tuesday, March 14, 2017 2:03PM  2:15PM 
F44.00013: Band alignment and bending in Dirac semimetal Na$_3$Bi thin films on Al$_2$O$_3$ substrates Kyungwha Park, John Villanova Dirac semimetals Na$_3$Bi and Cd$_3$As$_2$ are interesting due to topologically protected degenerate Weyl nodes with linear dispersions at the Fermi level and topological Fermiarc surface states. Recently, thin films of Na$_3$Bi have been epitaxially grown on Al$_2$O$_3$ substrates and their electron transport properties have been measured. However, the interfaces between the Dirac semimetal films and the substrates have not been characterized yet. Here we investigate electronic and topological properties of thin Na$_3$Bi films on Al$_2$O$_3$ substrates near the Fermi level, by using densityfunctional theory with spinorbit coupling. We also discuss effects of band alignment and band bending on the electronic and topological properties and compare with experimental data. [Preview Abstract] 
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