Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
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 non-centrosymmetric mono-pnictides (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 ill-defined [4]. In TaAs, on the other hand, well-defined 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 field-induced 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 Nodal-line Semimetals Cequn Li, Chunming Wang, Bo Wan, Xiangang Wan, Haizhou Lu, Xincheng Xie Nodal-line 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 Shubnikov-de Haas (SdH) oscillation may give a transport signature for the nodal-line semimetals. However, different experiments have reported contradictory phase shifts, in particular, in the WHM nodal-line semimetals (W=Zr/Hf, H=Si/Ge, M=S/Se/Te). For a generic model of nodel-line semimetal, we present a systematic calculation for the SdH oscillation of resistivity under a magnetic field normal to the nodal-line 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 nodal-line semimetals and can be generalized to other topological phases of matter. |
Monday, March 5, 2018 12:03PM - 12:15PM |
B10.00003: Anomalous Shubnikov-de Haas effect in Topological Semimetal YPtBi Hyunsoo Kim, Halyna Hodovanets, Kefeng Wang, Johnpierre Paglione Theory predicts the RPtBi (R=rare earth) half-Heusler 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 non-trivial Berry phase. However, there has not been any direct measurements of the Berry phase in the RPtBi system. Here we study the angle-dependent Shubnikov-de Haas (SdH) effect arising from the spin-split bulk Fermi surface of a prototypical half-Heusler compound YPtBi. We will describe observations of an anomalous angle dependence of SdH oscillations, investigating the angle-dependent cyclotron mass, scattering rate, and Berry phase of two beating oscillations from the spin-split Fermi surface. This unprecedentedly anomalous SdH effect sheds light on the topological nature of the half-Heusler family of compounds as well as offers an excellent model system for the SdH effect influenced by the strong spin-orbit coupling. |
Monday, March 5, 2018 12:15PM - 12:27PM |
B10.00004: Fermi surface topology of LaAs revealed by magneto-transport, ARPES and first-principles calculation Ni Ni, Bing Shen, xiaoming ma, Xiaoyu Deng, Eve Emmanouilidou, Gabriel Kotliar, Chang Liu Recently, the first principles calculation has predicted that rock-salt-structured lanthanum monopnictides LaX (X=N, P, As, Sb and Bi) show band inversion, resulting in non-trivial topology. Transport and ARPES studies have been extensively performed for LaBi and LaSb. While the study of LaBi has pointed to its non-trivial topology, the investigation of LaSb has led to highly controversial results, which may suggest the importance of the spin-orbit 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 first-principles 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: Two-dimensional transport and strong spin orbit interaction in SrMnSb2 Jiwei Ling, Faxian Xiu Breaking the time-reversal 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 long-sought TRS-breaking Weyl semimetal with canted antiferromagnetic order. Here we report the first pulse high field (up to 60T) magneto transport study for this material. Clear Shubnikov-de 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 Rxy 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 spin-orbit 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, Rui-Rui Du, Chi Zhang Our study reports the two-dimensional (2D)-like Shubnikov-de Hass (SdH) oscillations accompanied by quantized Hall resistivity in the high mobility (multi-quantum 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 (Iph) present distinct 2D-like quantum oscillations and QH states with beautifully features. PV and Iph 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, Yong-Lei 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 rare-earth 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 angle-dependent 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: Angle-Resolved Photoemission Spectroscopy and Shubnikov-de Haas Measurements of Ca3Pb1-xBixO 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 three-dimensional (3D) massive Dirac fermions, topological crystalline insulators, topological nodal line semimetals, and topological superconductors. In particular, recent theoretical calculations on Ca3PbO predict the presence of a 3D gapped Dirac-like cone at finite momentum along the Γ-X direction, which results from a band inversion of the Ca 3d and Pb 6p bands at the Γ point. We employ soft x-ray angle-resolved photoemission spectroscopy (ARPES) to experimentally observe the Dirac-like dispersion along the Γ-X direction in the Brillouin zone of bulk Ca3PbO. By comparing these results with those for Bi-doped Ca3PbO, we show that Bi doping is effective in tuning the Fermi level of Ca3PbO without fundamentally changing its Dirac-like 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 Fermi-Arc Surface States in a Magnetic-Field Induced Weyl Semimetal John Villanova, Kyungwha Park Weyl semimetals (WSMs) have a three-dimensional (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 magneto-transport signatures. We develop a tight-binding model based on Wannier functions directly from density functional theory (DFT) calculations for a topological DSM. We add spin-orbit coupling and Zeeman splitting terms in the tight-binding 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 Fermi-arc 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 low-dimensional 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 two-dimensional electron states owing to the one-dimensional 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 Cd3As2 and NbAs nanoplates. In Cd3As2, 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 low-Fermi 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 large-frequency 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, two-dimensional Fermi surface in TaAs. High quality samples of TaAs show pronounced Shubnikov de Haas oscillations beginning at fields below 1T. The low-field oscillations are in close agreement with what has been reported in the literature. At intermediate fields, however, an additional, high-frequency oscillation is observed. This new oscillation disperses in angle in a manner consistent with a two-dimensional state, indicating the presence of a previously unobserved Fermi surface. |
Monday, March 5, 2018 1:51PM - 2:03PM |
B10.00012: Non-saturating large magnetoresistance in semimetals Ian Leahy, Peter Siegfried, Andrew Treglia, Minhyea Lee We investigate magnetotransport properties of semimetals that exhibit non-saturating magnetoresistance (MR). We particularly focus on identifying three characteristic transport quantities to classify the quasi-linear, large MR behavior: (1) the Hall angle, (2) the exponent of field dependence of MR, i.e. Hα and (3) the change of fractional MR as a function of temperature (T). We compare these parameters in four different semimetals – NbP, NbSb2, TaP, and TaSb2, and discuss their implications in connection to characteristics of electronic structures as well as the disorder-induced 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 Lifshitz-Kosevich theory) accounts for these oscillations in several observables, as the Shubnikov de Haas oscillations in the conductivity. |
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