Bulletin of the American Physical Society
APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session F10: Three Dimensional Dirac and Weyl MaterialsFocus

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Sponsoring Units: DMP Chair: Hyunsoo Kim, Univ of MarylandCollege Park Room: LACC 301B 
Tuesday, March 6, 2018 11:15AM  11:51AM 
F10.00001: Dirac dispersion and nontrivial Berry's phase in new 3D semimetals Invited Speaker: Kefeng Wang , Brad Ramshaw , David Graf , Limin Wang Following the discovery of topological insulators, new classes of topological materials such as Dirac semimetals, Weyl semimetals, nodal line and nodal chain semimetals, have received considerable interest. In Dirac systems, linearly dispersing valence and conduction bands touch at discrete points of fourfold degeneracy in the Brillouin zone, giving Dirac nodes protected against gap formation by crystal symmetry. Although there have extensive studies, the topological semimetals are still rare, making it imperative to explore more candidates and their physical properties. Here we will report the electronic structure analysis, transport properties and ARPES/quantum oscillation study of several new threedimensional semimetals including RhSb3, VAl3, etc. These semimetals exhibit Dirac dispersion and nontrivial Berry’s phase, as well as extremely large magnetoresistance, which suggests them as 3D Dirac semimetal candidates. 
Tuesday, March 6, 2018 11:51AM  12:03PM 
F10.00002: Electronic Structures of typeI Dirac semimetal BaAl_{4} Ryo Mori , Kefeng Wang , Jonathan Ma , Jonathan Denlinger , Johnpierre Paglione , Alessandra Lanzara Topological nontrivial quantum states, such as topological insulators and Dirac/Wyle semimetals, have been attracting the community because of their rich novel physical properties and the wide range of possibilities of applications. Recently, BaAl_{4} has been proposed as a Dirac semimetal with typeI Dirac point protected by the symmetry of C_{4v} with extremely large magnetoresistance. We will report a study of the electronic structure of this material from angleresolved photoemission spectroscopy (ARPES) measurements, with focus on both the surface and bulk states. The semimetal nature of these states will be discussed. 
Tuesday, March 6, 2018 12:03PM  12:15PM 
F10.00003: Magneto and Thermal Transport Properties of Weyl Semimetal BaMnSb_{2} Silu Huang , William Shelton , E Plummer , Rongying Jin Recently, magnetic BaMnSb_{2} has emerged as an attractive Wyle semimetal candidate with broken timereversal symmetry. BaMnSb_{2} crystalizes in a tetragonal structure, consisting of an Sb square net layer critical to form Weyl points. Both inplane and outofplane resistivities (ρ_{ab}, ρ_{c}) show metallic behavior and large anisotropy, indicating quasi2D electronic structure. By applying magnetic field (up to 35 Tesla) along the c direction, ρ_{ab}, ρ_{c} and Hall resistivity (ρ_{xy}) show Shubnikovde Haas (SdH) oscillations in the low temperature regime. The lowest Landau Level is observed at ~25 T with evidence of Landau level splitting ~20 T. However, there is no sign for SdH oscillations in thermal conductivity, likely due to predominant phonon contribution. The underlying physics will be discussed. 
Tuesday, March 6, 2018 12:15PM  12:27PM 
F10.00004: Coexistence of Dirac and Weyl points in hexagonal ABC crystals Heng Gao , Youngkuk Kim , J.W.F. Venderbos , Charles Kane , Eugene Mele , Andrew Rappe , Wei Ren We propose that the noncentrosymmetric hexagonal ABC crystal SrHgPb realizes a new type of topological semimetal that hosts both Dirac and Weyl points in momentum space. Six pairs of Weyl points and one pair of Dirac points coexist in the Brillouin zone. All twelve Weyl points are located in the k_{z} = 0 plane at about 0.25 eV above the Fermi energy and are related by the C_{6v} point group symmetry. The existence of the Weyl points is confirmed by calculations of the Berry flux and the surface Fermi arcs. The Dirac points, which can be considered as double Weyl points with zero Chern number, occur along the C_{6z }rotational axis and are protected by C_{6v} point group symmetry. The Dirac points in the z axis are about 0.11 eV below the Fermi energy and form two electron pockets. We study topological phase transitions as a function of the HgPb buckling in SrHgPb and other material realizations of related topological semimetals that occur in the family of hexagonal ABC crystals. Our finding should provide a realistic and promising platform for studying the interplay between Dirac points and Weyl points, and for understanding their effects on transport and optical properties. 
Tuesday, March 6, 2018 12:27PM  12:39PM 
F10.00005: 3D quantum Hall effect of Fermi arcs in topological semimetals Chunming Wang , HaiPeng Sun , Haizhou Lu , Xincheng Xie The quantum Hall effect is usually observed in 2D systems. We show that the Fermi arcs can give rise to a distinctive 3D quantum Hall effect in topological semimetals. Because of the topological constraint, the Fermi arc at a single surface has an open Fermi surface, which cannot host the quantum Hall effect. Via a "wormhole" tunneling assisted by the Weyl nodes, the Fermi arcs at opposite surfaces can form a complete Fermi loop and support the quantum Hall effect. The edge states of the Fermi arcs show a unique 3D distribution, giving an example of (d2)dimensional boundary states. This is distinctly different from the surfacestate quantum Hall effect from a single surface of topological insulator. As the Fermi energy sweeps through the Weyl nodes, the sheet Hall conductivity evolves from the 1/B dependence to quantized plateaus at the Weyl nodes. This behavior can be realized by tuning gate voltages in a slab of topological semimetal, such as the TaAs family, Cd_{3}As_{2}, or Na_{3}Bi. This work will be instructive not only for searching transport signatures of the Fermi arcs but also for exploring novel electron gases in other topological phases of matter. 
Tuesday, March 6, 2018 12:39PM  12:51PM 
F10.00006: Anomlous Hall Effect in Single Crystal Heusler Alloy Co_{2}MnAl Peigang Li , Yanglin Zhu , Yu Wang , Zhiqiang Mao Discoveries of Weyl semimetals have attracted much attention due to their unique properties. Weyl semimetal states in ferromagnetic (FM) materials are expected to be particularly useful for spintronic applications. However, despite numerous efforts, FM Weyl semimetals are still missing. Cobased Heusler alloys with the formula of Co_{2}XZ (X = IVB or VB; Z = IVA or IIIA) were recently predicted to be FM Weyl semimetals [1]. But there has been no reported experimental evidence up to date, partly because single crystal growth of these materials is challenging. In this presentation, we will show our recent studies on Co_{2}MnAl single crystals grown by floating zone methods. Our magnetic measurements verifies both its room temperature ferromagnetism and its magnetic anisotropy. Moreover, our magnetotransport measurements unveils a remarkable anomalous Hall effect with exceptionally high Hall conductivity [~1800 (W cm)^{1} at 2K], consistent with the theoretical prediction that large anomalous Hall conductivity of Co_{2}MnAl caused by nontrivial band topology [2]. 
Tuesday, March 6, 2018 12:51PM  1:03PM 
F10.00007: Surface properties of 3D Weyl semimetal BaMnSb_{2} Qiang Zou , Anthony Yoshimura , Yifan Yang , Kun Zhao , William Shelton , Vincent Meunier , E Plummer , Rongying Jin , Zheng Gai Recently BaMnSb2 was found to be a 3D Weyl semimetal with 2D electronic structure where the Dirac cone was predicted from 1^{st} principles electronic structure calculations to result from the Sb in the square lattice. One of fingerprints of Weyl fermions is the formation of Fermi Arc at the Fermi level, which can be measured by techniques such as ARPES and scanning tunneling microscopy (STM). However, these techniques require an indepth understanding of the surface properties. Freshly cleaved BaMnSb2 surface was studied using STM and Low energy electron diffraction (LEED). We found two cleavage terminations that are all at Ba layers, but one is above the Sb/Mn layer and the other is above the Sb square lattice. We found although both surfaces show similar 2x1 surface structure reconstructions, their electronic properties are drastically different. Both surfaces are semiconducting with no sign of the Diractype cones. The existence of defects can greatly modify the local density of state to create electronic phase separations on the surface. The defects behavior will be discussed based on surface characterization, theoretical calculations and global transport properties. 
Tuesday, March 6, 2018 1:03PM  1:15PM 
F10.00008: Bulk and Surface Electronic Structure of the WeylSemimetal TaP Tim Figgemeier , ChulHee Min , Jennifer Neu , Max Ünzelmann , Hendrik Bentmann , Theo Siegrist , Friedrich Reinert The transition metal monopnictide Tantalum Phosphide, TaP, is a prototypical Weyl semimetal. We examined the electronic band structure of TaP(001) using angleresolved photoemission spectroscopy (ARPES) within a broad range of excitation energies from the VUV regime to the soft Xray regime. Utilizing the different probing depths depending on photon energy, we deconvolve surface and bulk contributions to the electronic structure. Based on a comparison to firstprinciples calculations we identify genuine surface and bulk states, as well as surface resonance states, originating from the hybridization between surface and bulk. We show that the surface resonances display a considerably higher bulk penetration than the pure surface states. Based on our detailed analysis of the character of the electronic states, we discuss the topological properties of the surface band structure. 
Tuesday, March 6, 2018 1:15PM  1:27PM 
F10.00009: Anomalous Hall Effect in ZrTe_{5} Tian Liang , Jingjing Lin , Quinn Gibson , Satya Kushwaha , Minhao Liu , Wudi Wang , Hongyu Xiong , Jonathan Sobota , Makoto Hashimoto , Patrick Kirchmann , ZhiXun Shen , Robert Cava , NaiPhuan Ong ZrTe_{5} has been of recent interest as a potential Dirac/Weyl semimetal material. Here, we report the results of experiments performed via insitu 3D doubleaxis rotation to extract the full 4π solid angular dependence of the transport properties. A clear anomalous Hall effect (AHE) was detected for every sample, with no magnetic ordering observed in the system to the experimental sensitivity of torque magnetometry. Interestingly, the AHE takes large values when the magnetic field is rotated inplane, with the values vanishing above ~ 60 K where the negative longitudinal magnetoresistance (LMR) also disappears. This suggests a close relation in their origins, which we attribute to Berry curvature generated by the Weyl nodes. 
Tuesday, March 6, 2018 1:27PM  1:39PM 
F10.00010: ThreeDimensional Massive Dirac Fermions in the Topological Semimetal NbAs_{2} Yinming Shao , Ying Wang , Zhiyuan Sun , Chenchao Xu , R. Sankar , Alexander Breindel , M Brian Maple , Chao Cao , Michael Fogler , Zhiqiang Li , Fangcheng Chou , Dimitri Basov Topological nodalline semimetals (NLSM) are newly discovered quantum materials where the Dirac conical structure extends over lines in the Brillouin zone. Compared to Dirac/Weyl semimetal, NLSMs are difficult to identify in part due to insufficient bulk sensitivity of available probes. Using magnetooptical spectroscopy, we have demonstrated the first example of electrodynamics associated with NLSM in NbAs_{2}, where the spinorbit interaction triggers energy gaps along the nodal lines. We find sharp steps in the interband optical conductivity, which also features the linear scaling σ_{1}(ω)∼ω over an extended frequency range. Combined with the series of Landaulevel transitions across the gap, we demonstrate the massive Dirac nature of the gap. Optical data offer a natural explanation for the giant magnetoresistance and negative longitudinal magnetoresistance in NbAs_{2}. Intriguing weak ingap states also appear in magnetooptical spectra and may be related to the theoretically predicted surface states. Our findings pave the way for future explorations of the gapped NLSM phase in NbAs_{2}. 
Tuesday, March 6, 2018 1:39PM  1:51PM 
F10.00011: Quantum transport studies of the topological semimetal candidate SrZnSb_{2} Jinyu Liu , Ni Ni Recently, the relativistic fermions hosted by squarenet lattices of main group elements have attracted a lot of attentions as they display distinct properties in a large range of materials including AMnX_{2} (A=alkaline earth or rare earth elements, and X=Sb/Bi) family [1, 2] and the WHM (W=Zr, Hf, or La, H=Si, Ge, Sn or Sb, and M=O, S, Se or Te) family [3]. Bradlyn, et al. have identified many other topological phases with squarenet or distorted squarenet lattices in space group P4/nmm or Pnma that share a similar band topology [4]. Among the proposed candidates, we did quantum transport studies on SrZnSb_{2} with space group of Pnma, where the Sb layer is distorted forming zigzag chains. Its crystal structure is identical with that of SrMnSb_{2}. In SrMnSb_{2}, the MnSb4 tetrahedral layers develop antiferromagnetic order while the Sb zigzag chain layers can host relativistic fermions [2]. The comparison of the transport behaviors between SrZnSb_{2} and SrMnSb_{2 }allows us to examine how magnetic ordered layers could affect the transport nature of relativistic fermions. [1] Park, et al. PRL 107, 126402 (2011) [2] Liu, et al. Nat. Mater. 16, 905–910 (2017) [3] Xu, et al. PRB 92, 205310 (2015) [4] Bradlyn, et al. Nature 547, 298–305 (2017) 
Tuesday, March 6, 2018 1:51PM  2:03PM 
F10.00012: Triple Point Fermions and Topological Phase Transitions in NaCu_{3}Te_{2} Yunyouyou Xia , Gang Li Quasiparticle excitations of free electrons in condensedmatter physics, characterized by the dimensionality of the band crossing, can find their elementaryparticle analogs in highenergy physics, such as Majorana, Weyl, and Dirac fermions. While crystalline symmetry allows more quasiparticle excitations and exotic fermions to emerge. Using symmetry analysis and abinitio calculations, we propose that the threedimensional honeycomb crystal NaCu_{3}Te_{2} hosts triply degenerate nodal points (TDNPs) residing at the Fermi level. Furthermore, in this system we find a tunable phase transition between a trivial insulator, a TDNP phase and a weak topological insulator (TI) triggered by a symmetryallowed perturbation and the spinorbital coupling (SOC). Such topological nontrivial ternary compound not only serves as a perfect candidate for studying threecomponent fermions, but also provides a beautiful playground for understanding the topological phase transitions between TDNPs, TIs and trivial insulators, which distinguishes this system from other TDNP candidates. 
Tuesday, March 6, 2018 2:03PM  2:15PM 
F10.00013: Designing 3D Dirac materials for dark matter detectors using density functional theory Sinead Griffin , Zhenfei Liu , Kathryn Zurek , Jeffrey Neaton Light dark matter has been postulated to both scatter and excite electrons in a Dirac semimetal. However, the cross section depends strongly on the band gap, Fermi velocities and cone degeneracy in the material. Here we use firstprinciples calculations to propose materials that fulfil these criteria. We also discuss how these properties can be engineered with chemistry and pressure, thus optimizing the dark matter interaction criteria in a 3D Dirac material[1]. 
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