Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session B10: Magnetotransport and Quantum Oscillations in Topological SemimetalsFocus

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Sponsoring Units: DMP Room: LACC 301B 
Monday, March 5, 2018 11:15AM  11:51AM 
B10.00001: Fermi Surface Topology and Transport in Weyl Semimetals Invited Speaker: Elena Hassinger Weyl Fermions are the solution of the massless Dirac equations and have been long sought after in high energy physics [1]. Weyl semimetals host quasiparticles that can be described as Weyl electrons. Recently the noncentrosymmetric monopnictides (Ta,Nb)(P,As) were predicted to be Weyl semimetals by ab initio DFT calculations [2]. The presence of Weyl nodes and Fermi arc surface states in these materials was later confirmed by ARPES [3]. Here, we present the precise Fermi surface topography of our TaP and TaAs single crystals as determined by quantum oscillation measurements and ab intio bandstructure calculations. It will be shown that chirality in TaP is illdefined [4]. In TaAs, on the other hand, welldefined Weyl pockets of opposite chirality exist [5]. Thus quantum phenomena due to chirality are only expected in TaAs. As a second point, we also show evidence that experimental results of the longitudinal magnetoresistance in these compounds can easily be dominated by effects of a fieldinduced resistance anisotropy. In that case, current inhomogeneities [6] can lead to an apparent “negative magnetoresistance” as expected for the chiral anomaly [7]. 
Monday, March 5, 2018 11:51AM  12:03PM 
B10.00002: Rules for Phase Shifts of Quantum Oscillations in Topological Nodalline Semimetals Cequn Li, Chunming Wang, Bo Wan, Xiangang Wan, Haizhou Lu, Xincheng Xie Nodalline semimetals are topological semimetals in which band touchings form nodal lines or rings. Around a nodal line, an electron can accumulate a nontrivial Pi Berry phase, so the phase shift in the Shubnikovde Haas (SdH) oscillation may give a transport signature for the nodalline semimetals. However, different experiments have reported contradictory phase shifts, in particular, in the WHM nodalline semimetals (W=Zr/Hf, H=Si/Ge, M=S/Se/Te). For a generic model of nodelline semimetal, we present a systematic calculation for the SdH oscillation of resistivity under a magnetic field normal to the nodalline plane. From the analytical result of the resistivity, we extract general rules to determine the phase shifts for arbitrary cases and apply them to ZrSiS and Cu3PdN systems. Depending on the magnetic field directions, carrier types, and cross sections of Fermi surface, the phase shift shows rich results, quite different from those for normal electrons and Weyl fermions. Our results may help exploring transport signatures of topological nodalline semimetals and can be generalized to other topological phases of matter. 
Monday, March 5, 2018 12:03PM  12:15PM 
B10.00003: Anomalous Shubnikovde Haas effect in Topological Semimetal YPtBi Hyunsoo Kim, Halyna Hodovanets, Kefeng Wang, Johnpierre Paglione Theory predicts the RPtBi (R=rare earth) halfHeusler compounds are topological semimetals, and recent experimental observations of the large anomalous Hall effect and the chiral anomaly in GdPtBi were both attributed to the nontrivial Berry phase. However, there has not been any direct measurements of the Berry phase in the RPtBi system. Here we study the angledependent Shubnikovde Haas (SdH) effect arising from the spinsplit bulk Fermi surface of a prototypical halfHeusler compound YPtBi. We will describe observations of an anomalous angle dependence of SdH oscillations, investigating the angledependent cyclotron mass, scattering rate, and Berry phase of two beating oscillations from the spinsplit Fermi surface. This unprecedentedly anomalous SdH effect sheds light on the topological nature of the halfHeusler family of compounds as well as offers an excellent model system for the SdH effect influenced by the strong spinorbit coupling. 
Monday, March 5, 2018 12:15PM  12:27PM 
B10.00004: Fermi surface topology of LaAs revealed by magnetotransport, ARPES and firstprinciples calculation Ni Ni, Bing Shen, xiaoming ma, Xiaoyu Deng, Eve Emmanouilidou, Gabriel Kotliar, Chang Liu Recently, the first principles calculation has predicted that rocksaltstructured lanthanum monopnictides LaX (X=N, P, As, Sb and Bi) show band inversion, resulting in nontrivial topology. Transport and ARPES studies have been extensively performed for LaBi and LaSb. While the study of LaBi has pointed to its nontrivial topology, the investigation of LaSb has led to highly controversial results, which may suggest the importance of the spinorbit coupling of the pnictide in affecting the band inversion. To shed light on this issue, we studied the lighter version of the system, LaAs. In this talk, we will present a combined study of quantum oscillation, ARPES and firstprinciples calculations on the cubic semimetal LaAs. And the Fermi surface topology of LaAs will be discussed in details. 
Monday, March 5, 2018 12:27PM  12:39PM 
B10.00005: Twodimensional transport and strong spin orbit interaction in SrMnSb_{2} Jiwei Ling, Faxian Xiu Breaking the timereversal symmetry (TRS) or inversion symmetry (IS) of a Dirac semimetal can realize Weyl state. IS breaking Weyl semimetal (WSM) has been experimentally realized in TaAs family, while the TRS breaking WSM is relatively rare. Recently, the nonstoichiometric SrMnSb2 is reported to be a longsought TRSbreaking Weyl semimetal with canted antiferromagnetic order. Here we report the first pulse high field (up to 60T) magneto transport study for this material. Clear Shubnikovde Haas Oscillations were resolved at relatively low magnetic field (around 4T), revealing a quasi 2D Fermi surface. We observed the development of quantized plateaus in R_{xy }which we interpreted as the bulk Quantum Hall. Moreover, angular dependent Zeeman splitting reveals an anisotropic g factor. The presence of the anisotropic g factor together with the formation of the beating pattern in oscillation reveal strong spinorbit interaction in the material. Our observation provides more insights of this material system and calls for further investigations into the composition related topological property evolution. 
Monday, March 5, 2018 12:39PM  12:51PM 
B10.00006: Quantum Hall States in an Anisotropic Bulk Weyl Semimetal TaAs Jianfeng Zhang, Xiaohu Zheng, Haiwen Liu, Jian Mi, Zhujun Yuan, Chenglong Zhang, Shuang Jia, Xincheng Xie, RuiRui Du, Chi Zhang Our study reports the twodimensional (2D)like Shubnikovde Hass (SdH) oscillations accompanied by quantized Hall resistivity in the high mobility (multiquantum layers) TaAs crystal with c (001) sample surface. On the other hand, there is no quantum Hall (QH) plateau with the single crystal sample of a (100) crystalline surface. Moreover, microwave photovoltage (PV) and photocurrent (I_{ph}) present distinct 2Dlike quantum oscillations and QH states with beautifully features. PV and I_{ph} minima appear at more Landau level fillings, because the QH degeneracy is broken further. Microwave enhances the conductivity and mobility within the skin depth of the electromagnetic field, thus more QH features are observed. Thus our work shows important implications for transport studies of 3D topological materials. 
Monday, March 5, 2018 12:51PM  1:03PM 
B10.00007: Origin of extremely large magnetoresistance in the semimetal YSb Jing Xu, Nirmal Ghimire, YongLei Wang, Zhili Xiao The discovery of nonsaturating and extremely large magnetoresistance (XMR) in the semimetal WTe2 in 2014 triggered extensive research to uncover the origin of XMR, which has also been observed in many nonmagnetic materials. Recently, XMR was observed in the rareearth monopnictides LnX (Ln = La, Y, Nd, Ce and X = Sb, Bi). Here we report on magnetoresistivity measurements to uncover the origin of the nonsaturating XMR in YSb. We measured angledependent Shubnikov–de Haas (SdH) oscillations to determine the shape and volume of the Fermi surface. Our results show that YSb has nearly compensated densities of electrons and holes. We can describe both transverse and hall magnetoresistivities with a semiclassic theory by including contributions from all the isotropic and anisotropic Fermi pockets. We demonstrate that the XMR in YSb originates from the high mobility and nearly prefect compensation of electrons and holes. 
Monday, March 5, 2018 1:03PM  1:15PM 
B10.00008: AngleResolved Photoemission Spectroscopy and Shubnikovde Haas Measurements of Ca_{3}Pb_{1x}Bi_{x}O Yukiko Obata, Ryu Yukawa, Koji Horiba, Hiroshi Kumigashira, Yoshimitsu Kohama, Yoshitake Toda, Satoru Matsuishi, Hideo Hosono Recently, there has been a growing interest in the cubic antiperovskite family as potential source of such novel topological phases of matter as threedimensional (3D) massive Dirac fermions, topological crystalline insulators, topological nodal line semimetals, and topological superconductors. In particular, recent theoretical calculations on Ca_{3}PbO predict the presence of a 3D gapped Diraclike cone at finite momentum along the ΓX direction, which results from a band inversion of the Ca 3d and Pb 6p bands at the Γ point. We employ soft xray angleresolved photoemission spectroscopy (ARPES) to experimentally observe the Diraclike dispersion along the ΓX direction in the Brillouin zone of bulk Ca_{3}PbO. By comparing these results with those for Bidoped Ca_{3}PbO, we show that Bi doping is effective in tuning the Fermi level of Ca_{3}PbO without fundamentally changing its Diraclike band structure [1]. In addition, we report magnetotransport and tunnel diode oscillation measurements in pulse magnet fields up to 55 T at temperatures between 2 K and 200 K. From the observed quantum oscillations, we confirm the bulk 3D Fermi surface with a nontrivial Berry phase shift and very light effective masses. [1] Y. Obata et al., Phys. Rev. B 96, 155109 (2017), doi: 10.1103/PhysRevB.96.155109. 
Monday, March 5, 2018 1:15PM  1:27PM 
B10.00009: Evolution of FermiArc Surface States in a MagneticField Induced Weyl Semimetal John Villanova, Kyungwha Park Weyl semimetals (WSMs) have a threedimensional (3D) bulk band structure in which the conduction and valence bands meet at discrete points, i.e. Weyl points. Projections of Weyl points with opposite chirality are connected by Fermi arcs at a surface. Topological Dirac semimetals (DSMs) have 3D Dirac points which can be viewed as two superimposed copies of Weyl points stabilized by rotational symmetry. When an external magnetic field is applied to a DSM, Dirac points can be separated into multiple Weyl points and so a WSM phase can be driven. DSMs and WSMs have received a lot of attention because they exhibit the chiral anomaly and novel magnetotransport signatures. We develop a tightbinding model based on Wannier functions directly from density functional theory (DFT) calculations for a topological DSM. We add spinorbit coupling and Zeeman splitting terms in the tightbinding model. We find that each Dirac node splits into two single Weyl points with linear dispersion and two double Weyl points with quadratic dispersion. Our calculations also reveal interesting evolution of Fermiarc surface states and other topological surface states as a function of chemical potential in the presence of the external magnetic field. 
Monday, March 5, 2018 1:27PM  1:39PM 
B10.00010: Quantum transport study of Fermi arc surface states in lowdimensional topological semimetals Cheng Zhang, Zhuoliang Ni, Awadhesh Narayan, Shiheng Lu, Huiqin Zhang, Xiang Yuan, Yanwen Liu, Stefano Sanvito, Faxian Xiu Owing to the coupling between open Fermi arcs on opposite surfaces, topological semimetals exhibit a new type of cyclotron orbit in the surface states known as Weyl orbit. It differs from conventional twodimensional electron states owing to the onedimensional bulk propagating process along the magnetic field direction through the chiral modes of Weyl fermions. Here we present a systematical quantum transport study of Weyl orbits in Cd_{3}As_{2} and NbAs nanoplates. In Cd_{3}As_{2}, the relationship between Fermi level and Fermi surface size of Weyl orbit is investigated. Interestingly, quantum Hall effect is observed under high magnetic field in lowFermi level samples. The thickness dependence of oscillation phase factor is studied to distinguish the Weyl orbits and possible topological insulator surface states from the finite size effect. In contrast, the Weyl orbits in NbAs exhibit largefrequency oscillations even when the Fermi level is near the Weyl nodes, resulting in exceptional high electrical conductivity far away from a quantized transport. The origin for such distinction between these two materials is discussed. 
Monday, March 5, 2018 1:39PM  1:51PM 
B10.00011: Observation of a two dimensional Fermi surface in TaAs Nityan Nair, Sanyum Channa, James Analytis Three dimensional Weyl semimetals are characterized by bulk quasiparticles that behave as massless, linearly dispersing Weyl fermions and have excited physicists with their unique topological properties and potential for applications. TaAs, one such Weyl material, has been shown to host exotic phenomena such as the chiral anomaly and Fermi arc surface states. Here, we report transport measurements indicating the presence of an additional, twodimensional Fermi surface in TaAs. High quality samples of TaAs show pronounced Shubnikov de Haas oscillations beginning at fields below 1T. The lowfield oscillations are in close agreement with what has been reported in the literature. At intermediate fields, however, an additional, highfrequency oscillation is observed. This new oscillation disperses in angle in a manner consistent with a twodimensional state, indicating the presence of a previously unobserved Fermi surface. 
Monday, March 5, 2018 1:51PM  2:03PM 
B10.00012: Nonsaturating large magnetoresistance in semimetals Ian Leahy, Peter Siegfried, Andrew Treglia, Minhyea Lee We investigate magnetotransport properties of semimetals that exhibit nonsaturating magnetoresistance (MR). We particularly focus on identifying three characteristic transport quantities to classify the quasilinear, large MR behavior: (1) the Hall angle, (2) the exponent of field dependence of MR, i.e. H^{α} and (3) the change of fractional MR as a function of temperature (T). We compare these parameters in four different semimetals – NbP, NbSb_{2}, TaP, and TaSb_{2}, and discuss their implications in connection to characteristics of electronic structures as well as the disorderinduced linear MR mechanisms. 
Monday, March 5, 2018 2:03PM  2:15PM 
B10.00013: Absence of Quantum Oscillations in Nodal Loop Semimetals Alberto Cortijo, Laszlo Oroszlany, Jozsef Cserti Nodal loop semimetals are three dimensional (semi)metallic systems where the valence and conduction bands closest to the Fermi level cross each other forming a loop in momentum space. In general, any metallic system placed under the effect of a magnetic field display quantum oscillations when electrons move along cyclotron orbits. The scape of Landau levels from the filled Fermi sea (the LifshitzKosevich theory) accounts for these oscillations in several observables, as the Shubnikov de Haas oscillations in the conductivity. 
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