Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session W59: Weyl semimetal, Transport IFocus

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Sponsoring Units: DMP Room: Mile High Ballroom 3C 
Friday, March 6, 2020 8:00AM  8:12AM 
W59.00001: Topological Surface States of Cadmium Arsenide Thin Films Luca Galletti, David Kealhofer, Timo Schumann, Manik Goyal, Salva Salmani Rezaie, Susanne Stemmer Thin films of the topological semimetal cadmium arsenide (Cd_{3}As_{2}) can host different types of topological surface states, depending on their crystallographic orientation^{1}. Here we elucidate the nature of these surface states by comparing films grown in (112) and (001) orientations, respectively. For (112) thin films Weyl orbits^{2}, involving Fermi arcs on opposite surfaces, have been predicted, which should exhibit a thicknessdependent phase in quantum oscillations. We compare quantum transport in films of varying thickness at apparently identical gatetuned carrier concentrations and find no clear dependence of the relative phase of the quantum oscillations on the sample thickness^{3}. Small variations in carrier densities, difficult to detect in lowfield Hall measurements, lead to shifts in quantum oscillations that are commensurate with previously reported phase shifts. By contrast, measurements of the quantum Hall effect in (001)oriented thin films show evidence of topological insulatorlike states at bottom and top interfaces. Quantum oscillation measurements in different geometries are used to determine interactions between the surfaces. 
Friday, March 6, 2020 8:12AM  8:24AM 
W59.00002: Large Thermal Hall effect observed in Bi_{89}Sb_{12} topological insulator and Weyl Semimetal Dung Vu, Nandini Trivedi, Joseph P C Heremans Bi_{89}Sb_{11} alloy, a topological insulator, becomes a Weyl semimetal in a magnetic field above a critical value H_{C} applied along the trigonal direction (001). We measured a Bi_{89}Sb_{11} single crystal sample along the trigonal axis in a longitudinal magnetic field H>H_{C}. We observed large fieldinduced increases in the longitudinal electronic thermal conductivity along (001), which is mostly linear in field and maximal for 35K <T< 200K, attributed to the thermal chiral anomaly [1]. We report a large thermal Hall effect in the sample for the same temperature range: magnetic field applied along (001), heat flux along (010); transverse temperature gradient measured along (100), longitudinal gradient along (010). peaks at 0.07 in the temperature range 160K<T<200K. We will present the field dependence of the sample’s transport properties including thermopower, Nernst thermopower, Hall and electrical resistivity, and thermal conductivity, and discuss possible connections of this large thermal Hall effect to conveyorbelt entropy transport mediated by topologically protected Fermi arc surface states in Weyl semimetals [2]. 
Friday, March 6, 2020 8:24AM  8:36AM 
W59.00003: Spin Zero Effect in Dirac Semimetal ZrTe_{5} Jingyue Wang, Jingjing Niu, Baoming Yan, Xinqi Li, Ran Bi, Yuan Yao, Dapeng Yu, Xiaosong Wu One of the characteristics of topological materials is their nontrivial Berry phase. Experimental determination of this phase largely relies on a phase analysis of quantum oscillations. We study the angular dependence of the oscillations in a Dirac material ZrTe_{5} and observe a striking spin zero effect, i.e., vanishing oscillations accompanied with a phase inversion. This indicates that the Berry phase in ZrTe_{5} remains nontrivial for arbitrary field direction, in contrast with previous reports. The Zeeman splitting is found to be proportional to the magnetic field based on the condition for the spin zero effect in a Dirac band. Moreover, it is suggested that the Dirac band in ZrTe_{5} is likely transformed into a linenode other than Weyl points for the field directions at which the spin zero occurs. The results underline a largely overlooked spin factor when determining the Berry phase from quantum oscillations. 
Friday, March 6, 2020 8:36AM  8:48AM 
W59.00004: Electric modulation of the Fermi arc spin polarization of a Dirac semimetal nanowire Benchuan Lin, Shuo Wang, Anqi Wang, Ke Xia, Dapeng Yu, ZhiMin Liao The spin polarization of the exotic topological surface states of Dirac/Weyl semimetals, namely Fermi arcs, were predicted to exist but never accessed in a simple transport structure. Here we report the direct access of the Fermi arc spin polarization through the electromagnetic transport experiment. The net spin polarization was manifested as a voltage of the ferromagnetic contact, which was induced by applying a charge current via spin momentum locking property of the Fermi arc in the Cd_{3}As_{2} nanowire. Moreover, a gate voltage controlled topological phase transition of the surface states is demonstrated to be able to switch on/off the spin signals. Thus a topological fieldeffect transistor is proposed. 
Friday, March 6, 2020 8:48AM  9:00AM 
W59.00005: Large inplane Hall/Nernst effect generated by Berry curvature in Weyl semimetallic phase of indium doped Pb_{1x}Sn_{x}Te Chenglong Zhang, Tian Liang, Yoshinori Tokura The Hall/Nernst effects have led to interesting phenomena in modern physics such as Berry curvature. The conventional Hall/Nernst signals appear only when the magnetic field is applied perpendicular to the sample because the conventional Lorentz force vanishes when the magnetic field is applied in the plane. 
Friday, March 6, 2020 9:00AM  9:12AM 
W59.00006: Magnetic electron lensing in the 3D Dirac semimetal Cd_{3}As_{2} Xiangwei Huang, Carsten Putzke, Chunyu Guo, Jonas Diaz, Markus Koenig, Horst Borrmann, Nityan Nair, James Analytis, Philip Moll While electrons moving perpendicular to a magnetic field are confined to cyclotron orbits, they can move freely parallel to the field. This simple fact leads to complex current flow in clean, low carrier density semimetals, such as current jets along the magnetic field when currents pass through pointlike constrictions. Occurring accidentally at imperfect current injection contacts, "current jetting" plagues the research of longitudinal magnetoresistance. We demonstrate the controlled generation of tightly focused electron beams in a new class of microdevices machined from crystals of the Dirac semimetal Cd_{3}As_{2}. The current beams can be guided by tilting an inplane magnetic field and their range tuned by the field strength. Finite element simulations quantitatively capture the voltage induced at faraway contacts when the beams are steered towards them, supporting the picture of controlled electron jets. These experiments demonstrate the first direct control over the highly nonlocal signal propagation unique to 3D semimetals in the current jetting regime. 
Friday, March 6, 2020 9:12AM  9:24AM 
W59.00007: Higher harmonics of quantum oscillations uncover Dirac Fermons in LaRhIn_{5} Aris Alexandradinata, Chunyu Guo, Carsten Putzke, Fengren Fan, Shengnan Zhang, QuanSheng Wu, Oleg V. Yazyev, Kent Shirer, Maja Bachmann, Eric Bauer, Filip Ronning, Claudia Felser, Yan Sun, Philip Moll Quantum oscillations are commonly used to experimentally diagnose the band topology of a semimetal. When the cyclotron orbit encloses a topological defect, it is often presumed that the nontrivial Berry phase results in a πphase shift of the fundamental harmonic. However, this presumption neglects how spinorbit coupling renders the Berry phase a continuously varying quantity, and ignores the Zeeman interaction with the spinorbitinduced magnetic moment. Here, we overcome these shortcomings and demonstrate how to rigorously identify threedimensional Dirac fermions from the higher harmonics of quantum oscillations. Applying this method to the intermetallic LaRhIn_{5}, we unambiguously identify the nontrivial Berry phase of a topological Fermi pocket with a small frequency ≈ 7T, despite the presence of large, trivial Fermi pockets which dominate transport by orders of magnitude. Our analysis identifies LaRhIn_{5} as a 3D Diracpoint metal, revising a previous proposal of LaRhIn_{5} as a nodalline semimetal by Mikitik et. al. [Phys. Rev. Lett. 93, 106403 (2004)]. The electronic similarity of LaRhIn_{5} to the prototypical heavyfermion superconductors Ce(Co,Rh,Ir)In_{5} further suggests them as prime candidates for stronglycorrelated Dirac systems. 
Friday, March 6, 2020 9:24AM  9:36AM 
W59.00008: αSn Magnetotransport Devices Owen Vail, Yu hao Chang, Sean Harrington, Patrick Aubrey Folkes, Patrick Taylor, barbara nichols, George J De Coster, Chris J Palmstrom αSn, the diamondcubic phase of tin, is of significant scientific interest due to its topological band structure and singleelement nature. Consistent highquality growth on insulating CdTe opens the doors to unconventional electronics using a widely available material. After verifying epitaxial growth of our films, we perform transport measurements to characterize the electronic carriers in the material. We identify twochannel transport and attribute the ntype transport to a semimetallic channel that tentatively suggests Dirac behavior, while the ptype component corresponds to growth impurities resulting from the CdTe substrate. We apply a field effect gate voltage in order to map out the charge neutrality point and modulate the channel between majority ntype and majority ptype carriers. Careful preparation of the CdTe surface before growth is considered crucial to attain a low dopant density and accessible topological states on an insulating substrate. This work readily lends itself to the development of topologically enabled devices for fieldable applications such as low power electronics in order to achieve a high level of computation at the edge. 
Friday, March 6, 2020 9:36AM  9:48AM 
W59.00009: Electric field gating of cadmium arsenide films using hexagonal boron nitride Biswajit Datta, Luca Galletti, Yu Saito, David Kealhofer, Manik Goyal, Omor F Shoron, Andrea Young, Susanne Stemmer Cadmium arsenide (Cd_{3}As_{2}) is a threedimensional Dirac semimetal that also hosts exotic surface states connecting the projections of the Dirac nodes. Probing the electronic properties of thin films of Cd_{3}As_{2} requires tuning of the Fermi level close to the Dirac nodes. Conventional gate dielectrics, such as Al_{2}O_{3}, must be deposited at low temperatures that do not exceed the stability limit of Cd_{3}As_{2}, which introduces a high density of (near) interface trap states that limit the modulation of carrier densities that can be achieved [1]. To address this problem, here we report on the use of hexagonal boron nitride (hBN) towards highquality Cd_{3}As_{2} devices. In addition to protecting the Cd_{3}As_{2} channel from photoresist, the high breakdown voltage of hBN also allows us to tune the carrier density up to a larger range allowing us to explore new quantum states. 
Friday, March 6, 2020 9:48AM  10:00AM 
W59.00010: Observation of an Unconventional Anomalous Hall Effect in a New Ferromagnetic Weyl Semimetal CeAlSi HungYu Yang, Bahadur Singh, Jonathan Gaudet, Baozhu Lu, ChengYi Huang, WeiChi Chiu, ShinMing Huang, Baokai Wang, Faranak Bahrami, David E Graf, Guangyong Xu, Hsin Lin, Darius Torchinsky, Collin Leslie Broholm, Arun Bansil, Fazel Tafti In the new ferromagnetic Weyl semimetal (FM WSM) CeAlSi, we find both a conventional and an unconventional anomalous Hall effect (AHE), that we name the loop Hall effect (LHE). The conventional AHE features a concurrent step increase in both Hall resistivity and magnetization when the applied field is along the magnetic easyaxis, similar to the AHE in the existing FM WSMs hosting finite Berry curvature. However, when the field is along the hardaxis, we observe the LHE, which does not exhibit a parallel field dependence between Hall resistivity and magnetization, in sharp contrast to the conventional AHE. More interestingly, by combining DFT calculation and quantum oscillation experiment, we find that the LHE only appears in the samples where the Fermi level lies within a short distance of 1 meV of the Weyl nodes. CeAlSi is also special in a way that both inversion and timereversal symmetries are intrinsically broken which leads to a skewed distribution of Weyl nodes; this property may account for the unprecedentedly close connection between the LHE and Weyl nodes in CeAlSi. 
Friday, March 6, 2020 10:00AM  10:36AM 
W59.00011: Slidinglayer structural phase transitions in the topological semimetal MoTe_{2} Invited Speaker: John Schneeloch Neutron scattering has a proud history of elucidating certain kinds of structural phase transitions, but sliding layer transitions such as those in MoTe_{2} have been relatively neglected. On cooling, the monoclinic 1T’MoTe_{2} transitions into the orthorhombic T_{d}MoTe_{2}, which has received much attention since it was reported to be a Weyl semimetal and to exhibit extreme magnetoresistance. MoTe_{2} structures can, to a good approximation, be built from sequences of two symmetryequivalent stacking operations, with transitions occurring via layer sliding between different stackings. Thus, a wide variety of nearlydegenerate structures are conceivable, and our elastic neutron scattering studies show that changes in stacking with temperature in MoTe_{2} are, indeed, complex. Both ordertoorder and ordertodisorder transitions exist along the T_{d}1T' thermal hysteresis loop. A pseudoorthorhombic T_{d}* phase with a fourlayer unit cell appears only on warming. T_{d}* is centrosymmetric, and the ordertoorder transitions between T_{d} and T_{d}* may make a more convenient topological switch than the disordered transitions to and from 1T’. The kink in resistivity vs. temperature on warming is primarily due to the onset of T_{d}*, and the residual hysteresis in the resistivity toward the temperature extremes is likely related to changes in the presence of 1T’ or T_{d}phase twin domain boundaries. Changes in stacking have a subtle effect on lowenergy shear phonon modes, as seen from inelastic neutron scattering. A multitude of ways of influencing these transitions are known; we will discuss how both Wsubstitution and pressure drive the transition toward a simpler phase coexistence behavior, though with opposite effects on transition temperature or the 1T’ β angle. We will discuss changes in band structure with pressure and strain. Finally, we will discuss open questions concerning the cause of the transition. 
Friday, March 6, 2020 10:36AM  10:48AM 
W59.00012: Quantitative analysis of chiral anomaly induced negative magnetoresistance in nodalline semimetal SrAs_{3} Minhao Zhao, Faxian Xiu Recently, threedimensional topological nodalline semimetals has been reported frequently as a new class of quantum materials. In nodalline semimetals, the conduction and the valence bands contact with each other. The band crossing points around the Fermi level form a closed loop. Chiral anomaly is one of the property of nodalline semimetal. The phenomenon of chiral anomaly in transport measurement is negative magnetoresistance(NMR) when current is parallel with magnetic field. The electronic conductivity induced by chiral anomaly is: σ_{zz}=e^2/(4π^2 hbar c) v/c ((eB)^2 v^2)/μ^2 τ 

W59.00013: Observation of a Thermoelectric Hall Plateau in the Extreme Quantum Limit Wenjie Zhang, Peipei Wang, Brian Skinner, Ran Bi, Vladyslav Kozii, Changwoo Cho, Ruidan Zhong, John Schneeloch, Dapeng Yu, Genda Gu, Liang Fu, Xiaosong Wu, Liyuan Zhang The thermoelectric Hall effect is the generation of a transverse heat current upon applying an electric field in the presence of a magnetic field. Here we demonstrate that the thermoelectric Hall conductivity α_{xy} in the threedimensional Dirac semimetal ZrTe_{5} acquires a robust plateau in the extreme quantum limit of magnetic field. The plateau value is independent of the field strength, disorder strength, carrier concentration, or carrier sign. We explain this plateau theoretically and show that it is a unique signature of threedimensional Dirac or Weyl electrons in the extreme quantum limit. We further find that other thermoelectric coefficients, such as the thermopower and Nernst coefficient, are greatly enhanced over their zerofield values even at relatively low fields. 
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