Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session H10: Dirac/Weyl Semimetals  Transport and AnomaliesFocus

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Sponsoring Units: DMP Chair: Kenneth Burch, Boston College Room: LACC 301B 
Tuesday, March 6, 2018 2:30PM  2:42PM 
H10.00001: Extremely high conductivity in the unconventional triple point fermion material MoP Chandra Shekhar, Yan Sun, Nitesh Kumar, Johannes Gooth Gooth, Michael Nicklas, Sarah Watzman, Kaustuv Manna, Vicky Suess, Lukas Muechler, Tobias Förster, Walter Schnelle, Uli Zeitler, Binghai Yan, Stuart S Parkin, Claudia Felser Weyl and Dirac fermions have created much attention in condensed matter physics and materials science. Recently, several additional distinct types of fermions have been predicted. Depending on the inherent symmetry of a particular compound, the crossing points can be severalfold degenerate: two and fourfold degenerate points are classified as Weyl and Dirac types, respectively. New materials going beyond Weyl and Dirac with higher band degeneracies have been proposed. MoP, a triple point Fermion material, shows characteristic like a correlated metal with anomalously high conductivity. Consistently, the momentum relaxing time of the electrons is found to be ~ 100 times larger than the momentum conserving time. We also find a strong violation of the WiedemannFranz law below ~ 40 K, the same temperature where a large magnetoresistance develops. These peculiar observations strongly indicate the role of hydrodynamic flow of charge carriers for extremely high conductivity in MoP. Our study opens a new window for hydrodynamic flow of fermions in low resistive topological metals. 
Tuesday, March 6, 2018 2:42PM  2:54PM 
H10.00002: Negative magnetoresistance from the anomalous N=0 Landau level in topological materials Badih Assaf, Thanyanan Phuphachong, Kampert Erik, Valentine Volobuev, Partha Mandal, Jaime SanchezBarriga, Oliver Rader, Guenther Bauer, Gunther Springholz, LouisAnne de Vaulchier, Yves Guldner Negative longitudinal magnetoresistance (NLMR) is shown to occur in topological materials in the extreme quantum limit, when a magnetic field is applied parallel to the excitation current. We perform pulsed and dc field measurements on Pb1−xSnxSe epilayers where the topological state can be chemically tuned. The NLMR is observed in the topological state, but is suppressed and becomes positive when the system becomes trivial. In a topological material, the lowest N = 0 conduction Landau level disperses down in energy as a function of increasing magnetic field, while the N = 0 valence Landau level disperses upwards. This anomalous behavior is shown to be responsible for the observed NLMR. Our work provides an explanation of the outstanding question of NLMR in topological insulators and establishes this effect as a possible hallmark of bulk conduction in topological matter. 
Tuesday, March 6, 2018 2:54PM  3:06PM 
H10.00003: Magnetotransport in Weyl semimetal nanowires Mikito Koshino, Akira Igarashi We theoretically study the band structure and the electronic transport in the Weyl semimetal nanowires in magnetic fields, and demonstrate that the interplay of the Fermiarc surface states and the bulk Landau levels plays a crucial role in the magnetotransport. We show that a magnetic field perpendicular to the surface immediately hybridizes the counterpropagating surface modes into a series of dispersionless zeroth Landau levels, and it leads to a significant reduction of the traveling modes and a rapid decay of the conductance. On the contrary, a magnetic field parallel to the wire adds linearly dispersed zeroth Landau levels to the traveling modes and increases the conductance. 
Tuesday, March 6, 2018 3:06PM  3:18PM 
H10.00004: Origin of the Butterfly Magnetoresistance in a Dirac Nodalline System YuChe Chiu, KuanWen Chen, Rico Schoenemann, Qiong Zhou, David Graf, Erik Kampert, Tobias Förster, Gregory McCandless, Julia Chan, Ryan Baumbach, Michelle Johannes, Luis Balicas We report a study on the geometry of the Fermi surface (FS) of ZrSi(Se,Te) \emph{via} the de Haasvan Alphen effect (dHvA), whose electronic dispersion was reported to display a line of Dirac nodes within their Brillouin zone. For both compounds we find that their FSs agree well with Density Functional Theory (DFT) calculations. In contrast to ZrSiTe, DFT finds that the nodal Dirac line is close to the Fermi level in ZrSiSe which displays a very small residual resistivity and pronounced fourfold symmetric magnetoresistivity as a function of field orientation. The transport lifetime in ZrSiSe is found to be 10E210E3 times larger than its quasiparticle lifetime indicating that its charge carriers are protected against backscattering under zerofield, as previously reported for Cd3As2. The quasiparticle lifetime in ZrSiSe is found to be angledependent displaying a minimum for fields along the \emph{c}axis. Therefore, the ``butterfly" magnetoresistivity observed in ZrSiSe, but not in ZrSiTe, seems to result from the magnetic fieldinduced suppression of this protection against backscattering, with this effect becoming particularly effective as the field is aligned along the interplanar direction. 
Tuesday, March 6, 2018 3:18PM  3:30PM 
H10.00005: Anomalous transport properties of tilted Weyl semimetals Alexander Zyuzin The anomalous transport properties of timereversalbreaking Weyl semimetal with a tilted conical spectrum around the Weyl points will be discussed. In addition to the contribution coming from the relative location of the Weyl points, the tilting gives rise to anisotropic Fermi surface contribution to the observables. For example, the anomalous Hall conductivity is not universal and can change sign as a function of the parameters quantifying the tilts. 
Tuesday, March 6, 2018 3:30PM  3:42PM 
H10.00006: Angular dependence of negative magneto resistance in the Weyl semimetals Muhammad Imran, Selman Hershfield Three dimensional Weyl semimetals have recently been 
Tuesday, March 6, 2018 3:42PM  3:54PM 
H10.00007: Behavior of Weyl Semimetals in the Hydrodynamic Electron Transport Regime Steven Torrisi, Jennifer Coulter, Christopher Rycroft, Prineha Narang In solid state transport regimes where electronelectron scattering dominates over impurity scattering and other momentumrelaxing processes, electron flow is governed by the laws of hydrodynamics. This leads to a variety of surprising behaviors in different materials, such as breakdown of the WiedemannFranz law, appearance of electron vortices in graphene, and tunable viscosity via magnetic field in a Weyl semimetal. Here we report a new microscopic model of this behavior using a combination of ab initio scattering and transport methods and fluid dynamics techniques. Our work establishes a connection between the observed hydrodynamic phenomena in Weyl semi metals and their crystal structure and symmetry. 
Tuesday, March 6, 2018 3:54PM  4:06PM 
H10.00008: Berry curvatureinduced huge anomalous Nernst effect in the absence of magnetic field in the Weyl semimetal YbMnBi_{2} Sarah Watzman, Kaustuv Manna, Timothy McCormick, Satya Guin, Chandra Shekhar, Nandini Trivedi, Claudia Felser, Joseph P Heremans YbMnBi_{2}, a Type II timereversal symmetrybreaking Weyl semimetal, yields a huge anomalous Nernst effect (ANE). This ANE is proven to be driven by Berry curvature and is present even in the absence of a magnetic field. The Seebeck thermopower, in contrast, is near zero. By combining magnetoresistance, Hall, Seebeck, and Nernst effects, the actual integral over the Fermi surface of the projection of the Berry phase can be derived. The ANE also results in transverse energy conversion ZT (thermoelectric figure of merit) values that compare favorably to those of conventional thermoelectric materials in the cryogenic temperature range. 
Tuesday, March 6, 2018 4:06PM  4:18PM 
H10.00009: Chiral anomaly without Landau levels Junhyun Lee, Jed Pixley, Jay Sau We study the chiral anomaly in disordered Weyl semimetals, where the broken translational symmetry prevents the direct application of Nielson and Ninomiya’s mechanism. In the weak disorder regime, there exists rare regions of the random potential where the disorder strength is locally strong, this gives rise to quasilocalized resonances. These rare states are nonperturbative and thus their effect on the chiral anomaly is unknown. We numerically show that rare states do not affect the chiral anomaly only in the case of a single Weyl node, but weakens the anomaly in a quantized way when there are two nodes without internode scattering. When the disorder strength is strong and the system is deep in the diffusive regime, the chiral Landau level itself is not well defined. In this limit, we analytically use the supersymmetry method and find an additional ChernSimons (CS) term in the effective action which is not present in nontopological systems. This CS term results in a nonzero average level velocity: using numerical calculations we show that this as an indicator of the chiral anomaly in the presence of strong disorder when the system can no longer be described using band theory. 
Tuesday, March 6, 2018 4:18PM  4:54PM 
H10.00010: The chiral anomaly in the Dirac semimetal Na3Bi and the halfHeusler GdPtBi* Invited Speaker: NaiPhuan Ong The Dirac semimetal Na_{3}Bi has two protected 3D Dirac nodes. In a magnetic field B, each node splits into two Weyl nodes. The lowest Weyl Landau levels are chiral with velocity vB determined by the chirality χ. Alignment of the electric field E with B violates conservation of the axial charge density. The resulting “axial” current J^{5} leads to a large, negative, longitudinal magnetoresistance (MR) known as the “chiral anomaly.” This is the analog of the process π^{0}→2γ in the physics of pion decay. The negative MR has been observed in Na_{3}Bi and in GdPtBi (where fieldinduced band crossings create twin Weyl nodes). Here we report new test results that distinguish the chiral anomaly MR from spurious currentjetting effects. In highmobility semimetals, the "squeezing" of the current density into a narrow beam  B increases dissipation along the beam axis (“the spine”), but lowers it at the edge of the sample. In Bi, we indeed observe a voltage (difference) that increases rapidly with B along the spine, but decreases along the edge. By contrast, in both Na_{3}Bi and GdPtBi, both voltage differences decrease. The chiral anomaly causes the spine voltage to decrease despite the increased dissipation. In a second test based on the planar angular MR, we find that the parametric plots of the angular MR voltages V_{yx} vs. V_{xx} define concentric orbits in the chiral anomaly systems. In Bi, however, the orbits expand in a strongly skewed fashion. The difference brings out a crucial difference in the conductance anisotropy. The separation of the Weyl nodes in Na_{3}Bi also creates anomalies in the Hall signal in weak B. 
Tuesday, March 6, 2018 4:54PM  5:06PM 
H10.00011: Large, nonsaturating thermopower of nodal semimetals in a quantizing magnetic field Brian Skinner, Liang Fu The thermoelectric effect is the generation of an electrical voltage from a temperature gradient in a solid material. Identifying materials with large thermoelectric response is pivotal for the development of novel electric generators and coolers. In this talk we consider theoretically the thermopower of Dirac/Weyl semimetals subjected to a quantizing magnetic field. We show that, under a sufficiently large magnetic field, the thermopower grows linearly with the field strength without saturation and can reach extremely high values. Our results suggest an immediate pathway for achieving recordhigh thermopower and thermoelectric figure of merit, and compare well with recent experiments. 
Tuesday, March 6, 2018 5:06PM  5:18PM 
H10.00012: Electronic Interactions and Transport in Weyl Semimetals Jennifer Coulter, Steven Torrisi, Christopher Rycroft, Prineha Narang The viscous electron phase of graphene is a wellknown phenomenon which occurs at the charge neutrality point due to the dominance of electronelectron scattering processes. Recent experiments have shown evidence of an analogous state of electron viscosity in tungsten phosphide (WP2), a known Weyl semimetal (WSM). WSMs are materials where electrons effectively interact as massless relativistic particles (Weyl fermions) and in 3dimensions the conduction and valence bands touch at isolated points. Hydrodynamics in such systems has typically been studied via kinetic theory. This talk will show ab initio linked hydrodynamic simulations of electron flow in WP2 with a focus on the role of electronphonon interactions and the suppression of momentum nonconserving processes in the hydrodynamic phase of WP2. 
Tuesday, March 6, 2018 5:18PM  5:30PM 
H10.00013: High Surface Conductivity of Fermi Arc Electrons in Weyl semimetals Giacomo Resta, ShuTing Pi, Xiangang Wan, Sergey Savrasov Weyl semimetals (WSMs) have attracted great interest recently due to their unusual electronic states and intriguing transport properties. These systems are close cousins of topological insulators (TIs) which are known for their disorder tolerant surface states. Similarly, WSMs exhibit unique topologically protected Fermi arcs surface states. We have analyzed electronphonon scattering as a function of the shape of the Fermi arc and find that the impact on surface transport disappears in the limit of a straight arc. Likewise, we have determined the effect of strong surface disorder on the resistivity by simulating a tightbinding model with the presence of quenched surface vacancies using the Coherent Potential Approximation (CPA) and KuboGreenwood formalism. We find that the limit of a straight arc is remarkably disorder tolerant, producing a surface conductivity 50 times larger than a comparable setup with surface states of TI. Finally I will discuss our simulations of the effects of surface vacancies on TaAs using the DFT + CPA method, illustrating the disorder tolerance of the topological surface states in a recently discovered WSM material. 
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