Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A10: Dirac/Weyl Semimetals  Disorder and Novel PhenomenaFocus

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Sponsoring Units: DMP Chair: Jed Pixley, Rutgers University Room: LACC 301B 
Monday, March 5, 2018 8:00AM  8:36AM 
A10.00001: Inhomogeneous Weyl Semimetals Invited Speaker: Roni Ilan Topological Weyl semimetals have an energy spectrum that hosts nondegenerate Weyl nodes appearing in pairs of opposite chirality, with a finite separation in momentum space that protects them. Inhomogeneities such as lattice deformations or a nonuniform magnetization couple to the electronic degrees of freedom and can affect their response and energy spectrum. These can arise either naturally or by engineering and can result, for example, in a spacedependent nodal separation which can be interpreted as an emergent axial vector potential. As we show, the resulting pseudofields represent a powerful concept with several profound consequences, such as equilibrium bound currents, and unconventional anomaly terms that allow for pumping and redistribution of charge or chiral charge in real as well as in momentum space. 
Monday, March 5, 2018 8:36AM  8:48AM 
A10.00002: Quantum Multicriticality in Disordered Weyl Semimetal Xunlong Luo , Baolong Xu , Tomi Ohtsuki , Ryuichi Shindou Threedimensional Weyl semimetal such as layered Chern insulator exhibits a rich variety of quantum phase transitions induced by disorders. Based on numerical calculations of localization length, density of states and critical conductance distribution, we first show that a localizationdelocalization transition between a Chern insulator (CI) phase and a diffusive metal (DM) phase belongs to an ordinary 3D unitary class. Moreover, a mobility edge and a band edge of CIDM phase transition is distinct, which means there is a special area of the CI phase in phase diagram with a finite zeroenergy density of states (zDOS) sandwiched between the DM phase and the CI phase with zero zDOS. Meanwhile, a localizationdelocalization transition between the CI phase with zero zDOS and a renormalized Weyl semimetal (WSM) phase turns out to be a direct phase transition whose critical exponent ν = 0.80 ± 0.01. We interpret these numerical results by a renormalization group analysis on a quantum multicritical phase transition among CI, DM and WSM phases. 
Monday, March 5, 2018 8:48AM  9:00AM 
A10.00003: Do the Fermi arcs in Weyl systems survive disorder? Justin Wilson , Jed Pixley , David Huse , Gil Refael Weyl semimetals exhibit topological surface states due to nodes in their Brillouin zone, but the semimetallic phase does not survive nonperturbative disorder effects. We, therefore, study the effect of quenched disorder on the surface states, finding evidence of surfacebulk hybridization. But the key question here is the topological robustness of the surface chiral properties in the presence of shortranged disorder. We investigate this with numerically exact calculations on a lattice model exhibiting Weyl Fermi arcs on its surface. We find that the Fermi arc surface states, in addition to having a finite lifetime from disorder broadening, hybridize with nonperturbative bulk rare states giving them finite spectral weight in the bulk and making them no longer exponentially bound to the surface (i.e. they lose their purely surface spectral character). Thus, we provide strong numerical evidence that the Weyl Fermi arcs are not topologically protected from disorder. Nonetheless, the surface chiral velocity is robust and survives in the presence of strong disorder, persisting all the way to the Anderson localized phase by forming local current loops into the bulk that live within the localization length of the surface. 
Monday, March 5, 2018 9:00AM  9:12AM 
A10.00004: Interaction Corrections to Charge Transport in Disordered Weyl Semimetals Aydin Keser , Roberto Raimondi , Dimitrie Culcer Quantum corrections to the conductivity are known to be important in disordered electron systems, and are due both to weak localization and to the interplay of disorder and electronelectron interactions. The consequences of these phenomena in topological systems have been a subject of many studies, and weak localization/antilocalization has been shown to affect the bulk conductivity and magnetoresistance of topological insulators and Weyl semimetals. In this study, we investigate the effects of electronelectron interactions and disorder on the conductivity tensor of 3D Weyl semimetals. We formulate the problem in the time domain suited to nonequilibrium applications and consider the regime where interactions and disorder can be treated perturbatively in the manner introduced by Altshuler and Aronov. We compute the temperature dependence of transport coefficients and compare with the recent experimental findings. 
Monday, March 5, 2018 9:12AM  9:24AM 
A10.00005: Interplay between Weyl points and FuKaneMele indices in timereversal invariant Weyl semimetals Guo Chuan Thiang , Koji Sato , Kiyonori Gomi In Weyl semimetals with timereversal symmetry, the interplay between Weyl points and Fu–Kane–Mele topological indices results in coexisting surface Dirac cones and Fermi arcs that are transmutable without a topological phase transition. We show that Weyl points act as a new type of monopole, and that their connectivity is essential for capturing the full topology of semimetals and their role as intermediaries of topological insulator transitions. The history of the creation and annihilation of Weyl points provides a simple and mathematically equivalent way to classify semimetals, and directly prefigures the surface state topology. 
Monday, March 5, 2018 9:24AM  9:36AM 
A10.00006: Topological Nodal Charge Density Waves in Weyl Semimetals Eric Bobrow , Canon Sun , Yi Li We examine a Weyl semimetal system with perfect Fermi surface nesting and a charge density wave. The charge density wave establishes a pairing in the particlehole channel between Fermi surfaces surrounding Weyl nodes of opposite chirality, resulting in a nontrivial nodal structure in the charge density wave order parameter. This nodal structure requires a description in terms of monopole harmonics, by analogy with previous work on monopole harmonic superconductivity. 
Monday, March 5, 2018 9:36AM  9:48AM 
A10.00007: Ideal Weyl phonons in telluride Tiantian Zhang , Hongming Weng , Ling Lu , Chen Fang , Zhong Fang Weyl nodes in momentum space of periodic lattices can result in many intriguing phenomena and have been actively studied in various systems recently, including electronic band in solid, photonic band in photonic crystal, phonon and acoustic spectra. The wellknown case is the Weyl nodes of electronic band around Fermi level, which is called Weyl semimetal. The only available samples are TaAs family compounds. They are not ideal for characterizing the unique responses of Weyl nodes since they have other trivial bulk pockets at Fermi energy. Thus, searching for Weyl nodes with a clean isoenergic surface is highly demanded for studying the expected novel phenomena. In this work, we propose a phononic crystal, which have six pairs of Weyl nodes related with symmetrical operations and they are the only phonon at 3.22 terahertz. On the surface of this crystal, the isoenergic surface show very clean arc states. 
Monday, March 5, 2018 9:48AM  10:00AM 
A10.00008: Quantum Anomalies in Strained Weyl Semimetals Sthitadhi Roy , Jan Behrends , Adolfo Grushin , Michael Kolodrubetz , Nathan Goldman , Jens Bardarson , Roni Ilan The chiral anomaly gives the topological response of Weyl semimetals to an external perturbation, insensitive to local details of the Hamiltonian. The recent identification of strain as axial fields in these materials gives rise to other anomalies, known from highenergy theory. However, these anomalies driven by axial fields are not necessary gaugeinvariant or chargeconserving; in fact, two different formulations exist, realizing just one of these conditions. In this work, we propose a way to resolve this ambiguity, supported by a lattice model that provides an intuitive picture. We identify experimental signatures and argue about their stability in presence of disorder. 
Monday, March 5, 2018 10:00AM  10:12AM 
A10.00009: Emergence of Bulk Gap and Metallic Side Walls in the Zeroth Landau Levels in Dirac and Weyl Semimetals ChingKit Chan , Patrick Lee A recent transport experiment has revealed the activation of longitudinal magnetoresistance of Weyl semimetals in the quantum limit, suggesting the breakdown of chiral anomaly in a strong magnetic field. Inspired by the experiment, we provide a general mechanism for gapping the zeroth chiral Landau levels applicable for both Dirac and Weyl semimetals. The induced bulk gap increases rapidly beyond a threshold field in Weyl semimetals, but has no threshold and is nonmonotonic in Dirac systems. We will also show how the systems can host counterpropagating edge states between the zeroth Landau levels, leading to a quantum state with metallic side walls and zero Hall conductance. 
Monday, March 5, 2018 10:12AM  10:24AM 
A10.00010: Topological phases in a Weyl semimetal multilayer Kazuki Yokomizo , Shuichi Murakami We investigate how topological phases are realized in Weyl semimetal multilayers, and how they are different from a Weyl semimetal phase. In our previous work [1], we showed that the quantum anomalous Hall phases with different Chern numbers emerge in a Weyl semimetal multilayer with broken timereversal symmetry when the stacking direction is parallel to the displacement vector connecting between the two Weyl nodes. To extend our previous research, we consider two setups: (i) a Weyl semimetal multilayer with broken inversion symmetry, (ii) a nodalline semimetal multilayer with magnetization. In (i), we show that various topological insulator phases appear in the multilayer. In (ii), we discuss relationships with the results in the Weyl semimetal multilayer. [1] K. Yokomizo and S. Murakami, Phys. Rev. B 95, 155101 (2017) 
Monday, March 5, 2018 10:24AM  10:36AM 
A10.00011: Localization and chiral magnetic effect in quantum diffusive Weyl semimetals ChungYu Mou , YenTing Lin , LiangJun Zhai We investigate the effect of disorders on the chiral magnetic effect 
Monday, March 5, 2018 10:36AM  10:48AM 
A10.00012: Stability of symmetry required 2D topological insulators: abinito calculations of point defects in monolayer WTe_{2} Lukas Muechler , Wei Hu , Chao Yang , Lin Lin , Roberto Car The prediction of a 2D TI state in monolayer WTe_{2} has stimulated large excitement and an immense experimental effort. In what we call symmetry required TIs, such as graphene or WTe_{2}, defects can act as trivial mass terms that can destroy the 2D TI state at a low critical disorder strength or defect concentration. However little is known about the types and energetics of defects that can occur in monolayer WTe_{2} and their effects on the electronic structure.We therefore calculate the formation energies of point defects of monolayer WTe_{2} in the 1T' structure. Three defects, all of them adatoms, have negative formation energies, while all other defects have large positive formation energies. The small difference in electronegativity between W and Te leads to a covalent network of bonds with strong contributions of both Wd and Tep states around the Fermi energy. We show that the adatoms with negative formation energies are weak perturbations that do not influence the 2D TI state, while the other defects can induce a phase transition to a trivial, insulating state. Additionally, we present STM images of Teadatoms on mono, bi and trilayers of WTe_{2 } that agree well with STM images reported in the literature. 
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