Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session B44: Focus Session Dirac and Weyl Semimetals: ARPES, STM and TheoryFocus

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Sponsoring Units: DMP Chair: Andreas Rost, Max Planck Institute Room: 391 
Monday, March 13, 2017 11:15AM  11:51AM 
B44.00001: Nonsymmorphic nodal line and nodal point semimetals Invited Speaker: Chen Fang In this talk, I will introduce two types of three dimensional topological semimetals, where crossing between the conduction and the valence bands are protected by nonsymmorphic symmetries. The first type is a doublenodal line semimetal, where the fourfold degeneracy along the nodal line is protected by a twofold screw axis. The second type is a Dirac point semimetal, the point nodes of which are protected by a glide plane. The key difference between this and the previously proposed Dirac point semimetals is that here the surface arcs are topologically protected. An observation is made on the mapping between the surface dispersion of topological semimetals and noncompact Riemann surfaces of meromorphic functions. The materials realizations of these semimetals are predicted in perovskite iridates as well as in photonic crystals. [Preview Abstract] 
Monday, March 13, 2017 11:51AM  12:03PM 
B44.00002: Topological nodal loop semimetals and insulators in alkaline earth triarsenides Warren E. Pickett, Yundi Quan, Lianyang Dong, Theo Siegrist, Jennifer Trinh, Arthur J. Ramirez, Haoxiang Li, Xiaoqing Zhou, Daniel S. Dessau The $X$As$_3$ class of semimetals has remained unexplored since their discovery in the 1980s by Bauhofer and von Schnering. We have discovered that this class, with $X$=Ca, Sr, Ba, Eu, are nodal loop semimetals (NLS), having a single loop of accidental degeneracies crossing the Fermi energy and a gap elsewhere in the zone: the nodal loop region dominates the transport Large single crystals of monoclinic SrAs$_3$ have been synthesized, allowing studies of its magnetotransport properties andits ARPES spectrum. Density functional calculations reveal that monoclinic CaAs$3$, the sole insulating member of this class, would be a NLS in the absence of spinorbit coupling (SOC). The position and characters of the nodal Fermi surfaces and associated boundard states will be discussed. The stark low symmetry of monoclinic CaAs$_3$ (with only inversion symmetry) makes it the ``hydrogen atom'' of NLSs; unlike all other classes, space group symmetry does not provide any ``protection'' of its loop of accidental degeneracies. [Preview Abstract] 
Monday, March 13, 2017 12:03PM  12:15PM 
B44.00003: Tunability of the topological nodalline semimetal phase in ZrSiXtype materials Md Mofazzel Hosen, Klauss Dimitri, Ilya Belopolski, Pablo Maldonado, Raman Sankar, Nagendra Dhakal, Gyanendra Dhakal, Taiason Cole, Peter M. Oppeneer, Dariusz Kaczorowski, Fangcheng Chou, M. Zahid Hasan, Tomasz Durakiewicz, Madhab Neupane The discovery of a topological nodalline (TNL) semimetal phase in ZrSiS has invigorated the study of other members of this family. Here, we use angleresolved photoemission spectroscopy (ARPES) to study the comparative electronic structure of ZrSiX (where X $=$ S, Se, Te) materials. Our experimental data and firstprinciples calculations, for the first time, reveal the tunability of topological nodalline fermion phase in ZrSiX materials by changing the spinorbit coupling strength via Xanion. Our findings establish a new material system with an SOC tunable nodalline phase. [Preview Abstract] 
Monday, March 13, 2017 12:15PM  12:27PM 
B44.00004: Discovery of topological nodalline semimetals in PbTaSe2 and TlTaSe2. Guang Bian Topological nodalline semimetal is a new topological phase of condensed matter where the conduction and valence bands cross each other at closed lines instead of discrete points in Weyl semimetals. The nodalline fermion resembles a particle that moves in the speed of light in the radial and outofplane directions but has infinite mass in the tangential direction. Such a strange and exotic particle does not even exist in the theory of highenergy physics. The nontrivial band topology of nodalline semimetals guarantees the existence of a new type boundary states on the surfaces, ``drumhead'' surface states. The electrons on this drumhead surface can be extremely heavy, resulting in a strong correlation between electrons and, consequently, a potential hightemperature surface superconductivity. Here we report on the discovery of two nodalline semimetals PbTaSe2 and TlTaSe2 by using a combined method of firstprinciples calculations and photoemission measurements, and discuss the exotic properties of the two compounds. [Preview Abstract] 
Monday, March 13, 2017 12:27PM  12:39PM 
B44.00005: Spatial Charge Inhomogeneity and Defect States in Topological Dirac Semimetal Thin Films Mark Edmonds, James Collins, Jack Hellerstedt, Indra Yudhistira, Joao Nuno Barbosa Rodrigues, Lidia Carvalho Gomes, Shaffique Adam, Michael Fuhrer Dirac materials are characterized by a charge neutrality point, where the system breaks into electron/hole puddles. In graphene, substrate disorder drives fluctuations in $E_{F}$, necessitating ultraclean substrates to observe Dirac point physics. Threedimensional topological Dirac semimetals (TDS) obviate the substrate, and should show reduced $E_{F}$ fluctuations due to better metallic screening and higher dielectric constants. Yet, the local response of the charge carriers in a TDS to various perturbations has yet to be explored. Here we map the potential fluctuations in TDS 20nm Na$_{3}$Bi films grown via MBE using scanning tunneling microscopy/spectroscopy. The potential fluctuations are significantly smaller than room temperature ($\Delta E_{F\, }\approx $ 5 meV $=$ 60 K) and comparable to the highest quality graphene on hBN; far smaller than graphene on SiO$_{2}$,$^{\, }$or the Dirac surface state of a topological insulator. This observation bodes well for exploration of Dirac point physics in TDS materials. Furthermore, surface Na vacancies show a bound resonance state close to the Dirac point with large spatial extent, a possible analogue to resonant impurities in graphene. [Preview Abstract] 
Monday, March 13, 2017 12:39PM  12:51PM 
B44.00006: Topological quantum chemistry I: Global band topology and topological phases Barry Bradlyn, Jennifer Cano, Zhijun Wang, Maia Vergniory, Luis Elcoro, Mois Aroyo, Claudia Felser, B. Andrei Bernevig For the past century, chemists and physicists have advocated fundamentally complementary perspectives on materials: while chemists have adopted a ``local'' viewpoint, through the theory of chemical bonding and hybridization, physicists have thought about materials predominantly through bandstructures in a nonlocal, momentumspace picture. The contrast between these two descriptions has been highlighted by the advent of topological insulators, the understanding of which overwhelmingly used the momentumspace picture. In this talk, I will present our method for unifying these two descriptions. By exploiting the constraints of symmetry on the relation between Bloch and Wannier functions, I will show how simple chemical input can be used to constrain the global band topology for materials in all 230 space groups. From this I will derive a predictive classification of topological insulators and semimetals. [Preview Abstract] 
Monday, March 13, 2017 12:51PM  1:03PM 
B44.00007: Topological invariants for nonsymmorphic symmetries Zhijun Wang, A.. Alexandradinata, Barry Bradley, Jennifer Cano, Benjamin J. Wieder, B. Andrei Bernevig Topological insulators with time reversal symmetry are known to fall into a Z2 classification. With additional nonsymmorphic symmetry, topological phases can be extended into a more detailed classification, which can be characterized by a Z4 invariant first defined in K theory in Shiozaki's paper [PRB93,195413]. In our work, we reformulate the Z4xZ2 invariant with the nonAbelian Wilson loop for the insulating systems preserving time reversal and a glide symmetry. Furthermore, we also extend the classification to the timereversalinvariant systems with two glide symmetries. A lot of materials have been proposed to realize the distinct topological phases as well. [Preview Abstract] 
Monday, March 13, 2017 1:03PM  1:15PM 
B44.00008: Floquet semimetals protected by "spacetime" nonsymmorphic symmetry Shenglong Xu, Congjun Wu Floquet systems have attracted considerable research interests during the past decade. Periodic driving may lead to exotic topological properties that are absent in static systems. We investigate the general symmetry structures and their physical consequences in Floquet systems. Because of the periodicity in both spatial and temporal directions, the symmetry is described by a discrete subgroup of the Galilean group, dubbed as ``spacetime" group. As an extension of the magnetic groups, the ``spacetime" groups contain additional elements such as timescrew rotations and timeglide reflections, which are composed of the point group operations with fractional translations in the time direction. We prove that, similar to their counterparts in static systems, these temporal nonsymmorphic operations can enforce Floquet band degeneracies and crossing, leading to Floquet semimetals. We discuss their topological properties and provide general principles to design topological band structures in Floquet systems. [Preview Abstract] 
Monday, March 13, 2017 1:15PM  1:27PM 
B44.00009: Topological nonsymmorphic metals from band inversion Lukas Muechler, Aris Alexandradinata, Titus Neupert, Roberto Car We expand the phase diagram of twodimensional, nonsymmorphic crystals at integer fillings that do \emph{not} guarantee gaplessness. In addition to the trivial, gapped phase that is expected, we find that band inversion leads to a class of topological, gapless phases. These topological phases are exemplified by the monolayers of MTe$_2$ (M $=$ W, Mo) if spinorbit coupling is neglected. We characterize the Dirac band touching of these topological metals by the Wilson loop of the nonAbelian Berry gauge field. Furthermore, we develop a criterion for the proximity of these topological metals to 2D and 3D $\Z_2$ topological insulators when spinorbit coupling is included; our criterion is based on nonsymmorphic symmetry eigenvalues, and may be used to identify topological materials without inversion symmetry. Reference: arXiv:1604.01398 (to be published in Phys. Rev. X) [Preview Abstract] 
Monday, March 13, 2017 1:27PM  1:39PM 
B44.00010: Topological Dirac semimetallic phase in transition metal compounds PengJen Chen, WanJu Li, TingKuo Lee It is known that the transition metal monoarsenides/phosphides (TaAs, TaP, NbAs, and NbP) are Weyl semimetals that can host the longsought Majorana fermions. Motivated by these exciting findings, we find that some of their allotropes reveal threedimensional Dirac points. Based on the classification of threedimensional topological Dirac semimetals proposed by Yang and Nagaosa (Nat. Commun. \textbf{5}, 4898 (2014)), the nontrivial topology is confirmed by computing the corresponding topological invariant. That is, we propose a new family of topological Dirac semimetals other than $\mathrm{Na_3Bi}$ and $\mathrm{Cd_3As_2}$. Besides, our results also indicate that the crystal symmetry plays an important role in determining the properties of the Dirac states and topological phase. [Preview Abstract] 
Monday, March 13, 2017 1:39PM  1:51PM 
B44.00011: Hybrid Weyl Semimetal Xi Luo, FeiYe Li, Xi Dai, Yue Yu, Fan Zhang, Gang Chen We construct a tightbinding model realizing one pair of Weyl nodes and three distinct Weyl semimetals. In the typeI (typeII) Weyl semimetal, both nodes belong to typeI (typeII) Weyl nodes. In addition, there exists a novel type, dubbed "hybrid Weyl semimetal", in which one Weyl node is of typeI while the other is of typeII. For the hybrid Weyl semimetal, we further demonstrate the bulk Fermi surfaces and the topologically protected surface states, analyze the unique Landau level structure and quantum oscillation, and discuss the material realization. [Preview Abstract] 
Monday, March 13, 2017 1:51PM  2:03PM 
B44.00012: Topological nodalline semimetals arising from crystal symmetry Ryo Takahashi, Motoaki Hirayama, Shuichi Murakami Nodal line semimetals, one of the topological semimetals, has lineshaped degeneracy (nodal line) where the gap is closed. Usually, nodal lines appear accidentally, and it is considered to be impossible to determine whether nodal lines appear from crystal symmetry alone. In this presentation, we show that for spinless systems with certain space groups, presence of nodal lines results only from symmetry. The nodal lines appears on a glide plane, and their appearance is attributed to difference in glide eigenvalues on several axis on the glide plane. Using a model Hamiltonian, we demonstrate that the presence of the nodal line comes only from its spacegroup symmetry and timereversal symmetry. We also show various space groups, under which spinless systems always have nodal lines coming from symmetry and illustrate how the nodal lines are located. We introduce some candidate materials. [Preview Abstract] 
Monday, March 13, 2017 2:03PM  2:15PM 
B44.00013: Coupled Wire Model Construction of a Weyl Semimetal Moon Jip Park, Jeffrey Teo, Matthew Gilbert Weyl semimetals (WSM) realize robust gapless Weyl fermions in the low energy spectrum of the Hamiltonian. In this talk, we construct a WSM phase using coupled wire model in which each of the wire is realized from the edge of the integer quantum hall effect. On the top of the wire model, by inserting chiral bosonic topological insulator slabs, we examine the stability of the WSM phase under manybody interaction, and we find that 16 copies of the coupled wire model can trivially gap out the WSM. Additionally, we construct a 4D quantum anomalous Hall (QAH) phase from stacks of the 3D WSM to discuss about the relation between the 16fold periodic classification of the WSM and chiral anomaly of the 4D QAH phase. [Preview Abstract] 
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