Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session K04: Dirac/Weyl Semimetals  Transport IFocus

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Sponsoring Units: DMP Chair: Jin Hu, University of Arkansas Room: BCEC 107C 
Wednesday, March 6, 2019 8:00AM  8:12AM 
K04.00001: Transport Properties in Dirac semimetal ZrTe_{5} Qiang Li We report transport measurements of Dirac semimetal ZrTe_{5} single crystals. We found the anomalous peak in the temperature dependence of resistivity is the result of the Lifshitz transition, which also leads to a sign change in thermopower. Large negative longitudinal magnetoresistance is the result of the chiral magnetic effect. Magnetic field effectively transforms Dirac semimetals into Weyl semimetals by breaking time reversal symmetry. Large anomalous Hall effect observed in ZrTe_{5} is the result of the Berry curvature generated by the Weyl nodes. 
Wednesday, March 6, 2019 8:12AM  8:24AM 
K04.00002: Magnetic field induced Weyl points and anomalous transport in Bi_{89}Sb_{11} WENJUAN ZHANG, Dung Vu, Cuneyt Sahin, Michael Flatté, Nandini Trivedi, Joseph P C Heremans A significant enhancement of magneto thermal conductivity in semimetal Bi_{1x} Sb_{x }alloy for a particular Sb concentration x=11% was discovered. As shown in a previous theoretical work on Weyl semimetals [1], Fermi arcs provide efficient ``conveyor belt'' for magneto thermal transport. In this talk, we argue that Bi_{1x} Sb_{x} becomes a Weyl semimetal under magnetic field and the enhancement of magneto thermal conductivity is due to the resulting Fermi arcs, based on the tight binding parameterization calculated by a modified virtual crystal approximation. By calculating Chern numbers on twodimensional slices of the threedimensional Brillouin zone, we show that a pair of Weyl points appear near L points above a critical field strength, and that the seperation between those Weyl points increases with the field strength which explains the anomalous angular dependence of magneto thermal conductivity on magnetic field. 
Wednesday, March 6, 2019 8:24AM  8:36AM 
K04.00003: Experimental evidence for the presence of Fermiarc driven entropy transport in the fieldinduced Weyl semimetal Bi_{89}Sb_{11} Dung Vu, Cuneyt Sahin, WENJUAN ZHANG, Michael Flatté, Nandini Trivedi, Joseph P C Heremans Topologically protected Fermi arc surface states in Weyl semimetals have been predicted to produce a conveyerbelt entropy transport [1]. Related theory talks [2][3] show that Bi_{89}Sb_{11} alloys become Weyl semimetals in a magnetic field applied along the trigonal direction above a critical value H_{C}. Here, we report a strong fieldinduced increase in electronic thermal conductivity of Bi_{89}Sb_{11} single crystals along the trigonal direction in longitudinal magnetic fields H>H_{C}. The Lorenz ratio increases up to 80fold at H= 9T. We report the temperature, length, and angular dependence of the effect in 5 different samples of BiSb alloys with mobilities up to 2x10^{6} cm^{2}V^{1}s^{1}. The effect is absent in fields not oriented along the trigonal direction; when the samples are doped ntype and E_{F} s not at the Weyl point; and in the ordinary semimetal Bi_{95}Sb_{5}. We posit that the large positive magnetothermal conductivity is a unique signature of topologically protected surface states. 
Wednesday, March 6, 2019 8:36AM  8:48AM 
K04.00004: Supercurrent oscillations in Weyl Semimetal MoTe_{2} Wudi Wang, Stephan Kim, Minhao Liu, Robert Cava, NaiPhuan Ong MoTe_{2} is a typeII Weyl semimetal with intrinsic superconductivity and pressureinduced superconductivity. Our interest is in the superconducting state at ambient pressure, with Tc around 100mK. We fabricated microscale MoTe_{2} flakes with Au contacts by mechanical exfoliation and nanofabrication and measured their transport properties. We observed multiple peaks in the differential resistance (dV/dI) vs. bias current curves, and the positions of these peaks change periodically with magnetic field. Most samples show two kinds of oscillations, one is Fraunhofer Pattern (single slit diffraction) type with oscillation amplitude decreasing at high magnetic field, the other one is Young’s Interference Pattern (doubleslit interference) type with oscillation amplitude not decreasing. The latter one (Young’s type) always has period corresponding to about 60% of the physical area of samples. We conclude that the latter one come from the edge of the crystals and indicating the existence of superconducting edge state in MoTe_{2} 
Wednesday, March 6, 2019 8:48AM  9:00AM 
K04.00005: General aspects of the anomalyrelated magnetoconductance in Weyl semimetals Hiroaki Ishizuka, Naoto Nagaosa Magnetoconductance (MC) in Weyl semimetals is intensively studied as it is presumably related to the chiral magnetic effect. While the theory of anomaly contribution is widely accepted, its relation to the experimentallyobserved MC remains controversial, because there are many other mechanisms for MC. For this purpose, the understanding on the general property of the MC is favorable, e.g., the effects of tilting, warping, and other details. However, studies on such effects are still limited, and no general understanding is reached so far. One challenge here is the lack of general and concise formalism, which is convenient for intuitive discussions. 
Wednesday, March 6, 2019 9:00AM  9:12AM 
K04.00006: Ultrahigh surface conductivity in Weyl semimetal NbAs Cheng Zhang, Zhuoliang Ni, Jinglei Zhang, Xiang Yuan, Xiangang Wan, Sergey Y. Savrasov, Faxian Xiu In twodimensional (2D) systems, high mobility is typically achieved in lowcarrierdensity semiconductors and semimetals. Increasing the carrier density in these systems to be comparable to typical metals usually degrades the mobility value, owing to an enhanced scattering probability from the large Fermi wave vector and possible charged impurities from the doping process. Here, we discover that the surface state of Weyl semimetal NbAs can overcome such a limit and maintain a high mobility even in the presence of a high carrier density. To study its surface transport properties, we first develop a new growth scheme to synthesize highquality nanostructures of NbAs with tunable Fermi levels. Owing to the large surfacetobulk ratio, the 2D surface state exhibits dominant quantum oscillations with a high carrier density. Combined with the high mobility value, a recordhigh surface sheet conductance among nonsuperconducting 2D systems is achieved up to 5100 S. Corroborated by theory, we attribute the origin of the ultrahigh surface conductance to the disordertolerant nature of Fermi arcs. Our results present the first transport evidence for the lowdissipation property of NbAs surface state and establish it as an excellent 2D metal with supreme conductivity. 
Wednesday, March 6, 2019 9:12AM  9:24AM 
K04.00007: Current at a distance in the Dirac semimetal ZrTe_{5} Nicholas Quirk, Sihang Liang, NaiPhuan Ong Nonlocal electronic conductivity in Dirac and Weyl semimetals has been predicted to arise from charge pumping in the bulk due to the chiral anomaly, and from cyclotronlike orbits of surface Fermi arcs weaving together with bulk states. We used focused ion beam (FIB) milling to thin samples of the Dirac semimetal ZrTe_{5} to subμm thicknesses, and we created electrical devices with these samples to probe these two nonlocal conductivity mechanisms. We will discuss difficulties we encountered using FIB and methods we tried in order to overcome them, and we will describe experiments we undertook to detect nonlocal conduction. 
Wednesday, March 6, 2019 9:24AM  9:36AM 
K04.00008: Anisotropic Giant Planar Hall Effect in typeII Weyl Semimetals Peng Li, Chenhui Zhang, George Nichols, Long Chen, Guoxing Miao, Xixiang Zhang Recently, giant planar Hall effect originating from chiral anomaly, which is related to the chiral Landau levels, has been predicted and realized in nonmagnetic Dirac/Weyl semimetals. WTe_{2} is considered to be an intriguing typeII Weyl semimetal with its Weyl cone tilted due to its broken inversion symmetry. Here, we report the observation in WTe_{2} of the giant planar Hall resistivity that shows a B^{2} magneticfield dependences as predicted by theory. Most importantly, we observed a highly anisotropic planar Hall resistivity when the current flows along a and baxis of WTe_{2}. The planar Hall resistivity at low temperatures with current flowing along baxis is almost twice of that along aaxis, because the chiral Landau levels and chiral anomaly are welldefined (or absent) when the magnetic field is parallel (or perpendicular) to the tilting direction (baxis) of Weyl cone in WTe_{2}. 
Wednesday, March 6, 2019 9:36AM  9:48AM 
K04.00009: Fermi surface study of the Weyl typeII semimetal candidate NbIrTe_{4} Rico Schoenemann, Shouvik Sur, Victor Quito, YuChe Chiu, Wenkai Zheng, Gregory T. McCandless, Julia Y. Chan, Luis Balicas Here we present a study of the Shubnikovde Haas (SdH) effect in the typeII Weyl semimetal candidate NbIrTe_{4}. Quantum oscillations were obtained at temperatures down to 0.3 K and in high magnetic fields up to 35 T. The angular dependence of the SdH frequencies is in good agreement with results from DFT calculations, although we are not able to detect every orbit. We observe an unusual butterfly shaped magnetoresistance as a function of the sample orientation with respect to the magnetic field, which is likely to originate from the shape and anisotropy of the Fermi surface. Furthermore, we measured a large nonsaturating magnetoresistance up to fields 35 T which suggests charge carrier compensation. On the other hand Hall measurements indicate significantly different electron and hole densities, which implies that the nonsaturating magnetoresistance is not driven by charge carrier compensation. 
Wednesday, March 6, 2019 9:48AM  10:00AM 
K04.00010: Weyl fermionphonon coupling and magneticfieldinduced large thermopower in TaP Fei Han, Thanh Nguyen, Nina Andrejevic, Ricardo Pablo Pedro, Mingda Li Weyl semimetals (WSM) have massless chiral fermions, the socalled Weyl Fermions. The chiral anomaly in its magnetotransport is one of the signatures for the existence of the plus and minus Weyl nodes which always appear in a pair. Recent theoretical predictions discussed the possibility to detect chiral anomaly based on phonon spectra, where the exotic electronic degrees of freedom in a WSM are playing an increasingly important role to influence the phonon structure. 
Wednesday, March 6, 2019 10:00AM  10:12AM 
K04.00011: Observation of Magic Angle Effectin Angular Dependence of Magnetoresistance of ZrTe5 in the Quantum Limit Joshua Mutch, Minhao He, Xinghan Cai, Paul Malinowski, Xiaodong Xu, JiunHaw Chu We report the discovery of sharp, anomalous dips in the angular dependence of magnetoresistance in ZrTe5 single crystals. These dips occur at fixed “magic angles” once the magnetic field passes the quantum limit. The magic angles correspond to commensurate angles where the field direction connecting neighboring atoms in the lattice, reminiscent of “Lebed’s Magic Angles” found in the organic Bechgaard salts (TMTSF)_{2}X. Unlike the Bechgaard salts, a quasi1D open fermi surface is absent in ZrTe5. In contrast, it has a closed ellipsoidal Fermi surface well contained within a small fraction of the Brillouin zone. We discuss how the effect of commensurability may arise in a 3D Dirac system in high magnetic field. 
Wednesday, March 6, 2019 10:12AM  10:24AM 
K04.00012: Engineering Spin Hall Conductivity and Topological Character in GroupIV Tellurides and a Family of Weyl Semimetals Haihang Wang, Jagoda Slawinska, Silvia Picozzi, Marco Buongiorno Nardelli The discovery of topological insulators and topological semimetals has become one of the leading fields of research in condensed matter physics. The origin of these emerging quantum properties can be traced to the topology of the bulk band structure and the interplay with spinorbit coupling. In particular, the ability to engineer band topologies would open the way to the design of materials with novel functionalities. In this work, we focus on the spinHall current as the main physical property emerging from the topology of the bands: through the interplay of strain and ferroelectricity, we demonstrate the emergence of giant spinHall effect in GroupIV Tellurides. Finally, we will also discuss the results of a recent investigation on a family of Weyl semimetals that was isolated through a highthroughput screening of materials in the AFLOW database. All the results, including the searching and characterization of topological materials, have been obtained using the PAOFLOW package, which is integrated into the AFLOWπ framework. 
Wednesday, March 6, 2019 10:24AM  11:00AM 
K04.00013: Quantum limit properties of Weyl semimetals Invited Speaker: Brad Ramshaw Electrons confined to their 0^{th} Landau level by extreme magnetic fields—a regime known as the quantum limit—experience strong electronelectron interactions, making them unstable to the formation of new states of matter. The discovery of monopnictide Weyl semimetals has renewed interest in the highfield properties of 3D electrons, with the added twist of linear electronic dispersions. We use magnetic fields up to 95 Tesla to take the Weyl semimetals NbP and TaAs into their quantum limit. In electrical transport and torque magnetometry, we identify signatures of the 0^{th} Landau levels that are unique to Weyl fermions. In NbP, we show that Weyl fermions can be accessed in high fields, even when the zerofield chemical potential lies far from the nodes. In TaAs, we find that the left and right Weyl nodes are mixed by magnetic field, which opens a gap and suppresses the anomalous "ABJ"induced conductivity. At the very highest fields in TaAs, we observe a thermodynamic phase transition to an asyet unidentified state, indicating that Weyl semimetals are unstable to the formation of new states of matter. 
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