Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session A23: Novel Phenomena and Routes to Realizations of Weyl and Dirac SemimetalsInvited

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Sponsoring Units: DCMP Chair: Zahid Hasan, Harvard University Room: New Orleans Theater B 
Monday, March 13, 2017 8:00AM  8:36AM 
A23.00001: Quantum oscillations from surface Fermi arcs and bulk chiral modes in Weyl semimetals Invited Speaker: Yi Zhang I will discuss our results on the quantum oscillations in generic Weyl semimetals from exotic cyclotron orbits that consist of surface Fermi arcs and bulk chiral modes. The semiclassical quantization conditions are consistent with numerical implementation of a layered construction of Weyl semimetals. Interesting experimental implications will also be discussed, including ‘magic’ magneticfield angles where quantum oscillations become independent of the sample thickness and the persistence of these quantum oscillations to disorder as the sample thickness exceeds the quantum mean free path. [Preview Abstract] 
Monday, March 13, 2017 8:36AM  9:12AM 
A23.00002: Detecting surfacebulk connectivity in Weyl semimetal TaAs via scanning tunneling microscopy Invited Speaker: Hiroyuki Inoue Weyl semimetal exhibits a new gapless topological phase, which is characterized by crossings of two nondegenerate bulk bands, called Weyl nodes. The surfaces of these compounds are expected to harbor topologically protected surface states with disjoint Fermi surfaces, Fermi arcs, connecting surface projections of the Weyl nodes with opposing Chern numbers. A distinct feature of topological semimetals such as the Weyl semimetals is surfacebulk connectivity, which is absent from topological insulators, due to the gapless nature of the bulk. Namely, the electrons on the surface Fermi arcs may dive into the Fermi sea of bulk Weyl cones through the surfaceprojected Weyl cones. Following theoretical predictions of transition metal monopnictides e.g. TaAs as a candidate inversionsymmetrybroken Weyl semimetal, angleresolved photoemission spectroscopy studies indeed showed the presence of Weyl cones and corresponding Fermi arcs. However, such kinetic aspect of Weyl semimetals had not yet been experimentally verified. While a theoretical proposal suggested quantum oscillations of novel trajectories in thin films as a route to probe such connectivity, an unanticipated alternative, which we exemplified in our present study, turned out to be quasiparticle interference (QPI) of the surface Fermi arcs. In this talk we will describe our scanning tunneling microscopy experiment where we performed a spectroscopic mapping to visualize QPI on TaAs[001] surface. Thanks to the stoichiometric nature of the sample, measuring atomically flat and pristine terraces revealed a rich variety of scattering wave vectors, which can be reproduced with a DFT calculation considering not only the spin texture but also subsurface distribution of the Fermi arc surface states. Our observation demonstrates the momentumdependent penetration of the Fermi arcs into the bulk, namely,~surfacebulk connectivity in the Weyl semimetal TaAs. Reference: H. Inoue*, A. Gyneis*, Z. Wang, J. Li, S. W. Oh, S. Jiang, N. Ni, B. A. Bernevig and A. Yazdani, ``Quasiparticle interference of the Fermi arcs and surfacebulk connectivity of Weyl semimetals,'' Science~\textbf{351}, 1184 (2016). [Preview Abstract] 
Monday, March 13, 2017 9:12AM  9:48AM 
A23.00003: Symmetry Protected Topological Insulators and Semimetals Invited Speaker: Charles Kane We will discuss recent developments in topological band theory, in which the combination of time reversal symmetry and crystal symmetries lead to novel insulating and semimetallic states. After introducing the interplay between symmetry and topology in the electronic structure of crystalline materials, we will discuss several examples of protected metallic states that can occur on the surface and in the bulk. These include Dirac semimetals in two and three dimensions, double Dirac semimetals and line node semimetals as well as new classes of topological crystalline insulators with surface states that violate symmetry enhanced doubling theorems. We will contrast semimetallic behavior that arises due to band inversion with filling enforced semimetals that arise due to the existence of nonsymmorphic space group symmetries. [Preview Abstract] 
Monday, March 13, 2017 9:48AM  10:24AM 
A23.00004: Topological crystalline semimetal in Iridates with strong spinorbit coupling Invited Speaker: HaeYoung Kee Numerous efforts have been devoted to reveal exotic semimetallic phases with topologically nontrivial bulk and/or surface states in materials with strong spinorbit coupling. In particular, semimetals with nodal line Fermi surface (FS) exhibit novel properties, and searching for candidate materials becomes an interesting research direction. I will first present a generic condition for a fourfold degenerate nodal line FS in nonsymmorphic crystals with inversion and timereversal symmetry. Such a nodal ring FS and topological surface states emerge in SrIrO$_3$ with strong spinorbit coupling. Possible experimental signatures will be also discussed. [Preview Abstract] 
Monday, March 13, 2017 10:24AM  11:00AM 
A23.00005: Realization of nonsymmorphic Dirac cones in PbFCl materials Invited Speaker: Leslie Schoop While most 3D Dirac semimetals require two bands with different orbital character to be protected, there is also the possibility to find 3D Dirac semimetals that are guaranteed to exist in certain space groups. Those are resulting from the nonsymmoprhic symmetry of the space group, which forces the bands to degenerate at high symmetry points in the Brillouin zone. Nonsymmorphic space groups can force three four, six and eight fold degeneracies which led to the proposal to find 3D Dirac Semimetals as well as new quasiparticles in such space groups [1,2]. Problematic for realizing this types of Dirac materials is that they require and odd band filling in order to have the Fermi level located at or also near by the band crossing points. Therefore, although the first prediction for using nonsymmoprhic symmetry to create a Dirac material was made in 2012 [1], it took almost four years for an experimental verification of this type of Dirac crossing [3]. In this talk I will introduce the material ZrSiS that has, besides other Dirac features, a Dirac cone protected by nonsymmorphic symmetry at about 0.5 eV below the Fermi level and was the first material where this type of Dirac cone was imaged with ARPES [3]. I will then proceed to discuss ways to shift this crossing to the Fermi edge and finally show an experimental verification of a fourfold Dirac crossing, protected by nonsymmorphic symmetry, at the Fermi energy. [1] Young, S.M., Zaheer, S., Teo, J.C., Kane, C.L., Mele, E.J. and Rappe, A.M., 2012. Phys. Rev. Lett., 108(14), p.140405. [2] Bradlyn, B., Cano, J., Wang, Z., Vergniory, M.G., Felser, C., Cava, R.J. and Bernevig, B.A., 2016. Science, p.aaf5037 [3] Schoop, L.M., Ali, M.N., Stra{\ss}er, C., Topp, A., Varykhalov, A., Marchenko, D., Duppel, V., Parkin, S.S., Lotsch, B.V. and Ast, C.R., 2016. Nat. comm., 7. [Preview Abstract] 
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