Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E10: Dirac/Weyl Semimetals  Thin Films, Surfaces and InterfacesFocus

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Sponsoring Units: DMP Chair: James Analytis, University of California, Berkeley Room: LACC 301B 
Tuesday, March 6, 2018 8:00AM  8:36AM 
E10.00001: Towards topological electronics: Epitaxial thin films of topological Dirac semimetal Na3Bi Invited Speaker: Michael Fuhrer I will discuss our experiments on epitaxial thin films of topological Dirac semimetal Na_{3}Bi as a route to novel topological electronic phenomena and devices, including the conventionaltotopological quantum phase transition (QPT) as a function of layer thickness and a topological transistor based on an electric fieldtuned QPT. Na_{3}Bi thin films are grown by molecular beam epitaxial and transferred in UHV to a lowtemperature STM capable of magnetotransport at 5 K. Thin films (20 nm) of Na_{3}Bi on αAl_{2}O_{3}(0001) substrates have low temperature charge carrier mobilities exceeding 6000 cm^{2}V^{1}s^{1} with ntype carrier densities below 1 x 10^{18} cm^{3}[1], comparable to single crystal values. Mapping the local Dirac point via scanning tunneling spectroscopy reveals a high degree of spatial uniformity, with rms variations in Dirac point energy less than 5 meV[2], comparable to the best graphene samples on hexagonal boron nitride. Chemical doping[3] and electrostatic gating[4] (using SiO_{2}/Si substrates) can be used to tune the carrier density, first steps toward a topological transistor. 
Tuesday, March 6, 2018 8:36AM  8:48AM 
E10.00002: Electrostatic Gating and Transport Studies of Thin Films of the 3D Dirac Semimetal Cd_{3}As_{2} Omor Shoron, David Kealhofer, Manik Goyal, Timo Schumann, Luca Galletti, Susanne Stemmer 3D Dirac semimetals are an unusual class of quantum materials with unique electronic states. Topological transitions have been predicted as a result of confinement, symmetry breaking, and by the application of external electric fields. Here, we present transport studies of Cd_{3}As_{2} films under external electric and magnetic fields. 
Tuesday, March 6, 2018 8:48AM  9:00AM 
E10.00003: Investigating Quantum Quasiparticle Interference on the surface of Weyl Semimetal by Scanning Tunneling Microscopy Xiaohu Zheng, Yiyuan Liu, Bingbing Tong, Jianfeng Zhang, Chi Zhang, Shuang Jia, RuiRui Du We report work in progress for measurements of atomic structure and quantum quasiparticle interference (QPI) on the surface of Weyl semimetal (TaAs) using a low temperature scanning tunneling microscopy/spectroscopy equipped with vector magnets. The sample surface that is being studied is prepared by UHV in situ cleavage. Ta and As terminated surfaces are observed with atomic scale. QPI patterns that arise from quasiparticle scattering of the surface states including the topological Fermi arcs in TaAs are observed on As and Ta terminated surfaces respectively and compared without/with external magnetic field. The electron scattering near the atomic steps are also investigated. The results should help us to understand the electron scattering behaviors of the topological Fermi arcs of the Weyl semimetal, more comprehensively. 
Tuesday, March 6, 2018 9:00AM  9:12AM 
E10.00004: Transport in Coherently Strained Thin Films of a 3D Dirac Semimetal Manik Goyal, David Kealhofer, Timo Schumann, Luca Galleti, Susanne Stemmer Cd_{3}As_{2} is a 3D Dirac semimetal with degenerate band crossings at isolated points in kspace 
Tuesday, March 6, 2018 9:12AM  9:24AM 
E10.00005: Interfaces between Weyl semimetals Vatsal Dwivedi We study junctions of two Weyl semimetals and derive an analytical closedform condition for the existence of localized modes at the interface. In certain cases, stable interface modes are obtained even when the two Weyl semimetals have identical low energy properties and differ only in their Fermi arc connectivities. This provides further support for the notion that a complete characterization of Weyl semimetals requires not only the nodal configuration in the bulk, but also the connectivity of the Fermi arcs on the surface. These interface modes also have experimental signatures in the quantum oscillation of the density of states. 
Tuesday, March 6, 2018 9:24AM  9:36AM 
E10.00006: Growth and characterization of Cd_{3}As_{2} epitaxial thin films on a IIVI substrate Yusuke Nakazawa, Masaki Uchida, Shinichi Nishihaya, Yusuke Kozuka, Masashi Kawasaki Cd_{3}As_{2} is a representative system of the Dirac semimetal, characterized by an inverted band structure with a linear Dirac dispersion. A lot of quantum transport phenomena originating from the unique band structure have been reported so far. Among them, magnetotransport measurements on Cd_{3}As_{2} thin films are highly limited, because there are many difficulties to fabricate thin films with high crystallinity and surface flatness. To challenge this problem, we have investigated the growth mechanisms of Cd_{3}As_{2}, especially during the initial process of the epitaxial growth. By optimizing the growth conditions, singlecrystalline Cd_{3}As_{2} films with step and terrace structures have been successfully synthesized insitu on a IIVI substrate. Detailed transport properties of the low carrier density films will be also presented. 
Tuesday, March 6, 2018 9:36AM  9:48AM 
E10.00007: Epitaxial growth and transport studies of [001]oriented cadmium arsenide, a 3D Dirac semimetal David Kealhofer, Luca Galletti, Manik Goyal, Honggyu Kim, Salva Rezaie, Timo Schumann, Susanne Stemmer In the 3D Dirac semimetal cadmium arsenide, the Dirac nodes fall along the k_{z} axis and are stabilized by the fourfold rotation symmetry of the tetragonal crystal structure. However, the easy growth direction is along [221], a lowersymmetry direction. In [221]oriented thin films, epitaxial strain and finite thickness reduce the symmetry of the k_{z} axis and can cause the Dirac nodes to vanish. Here, we report the first growth and electronic transport studies of [001]oriented thin films (i.e. those grown parallel to k_{z}), in which the fourfold axis is preserved by the sample geometry. These films are grown by molecular beam epitaxy on a III–V substrate with a tunable buffer layer. Lowtemperature mobilities in these films greater than 17,000 cm^{2}/Vs permit a comparison with the more widely studied [221]oriented films to elucidate the role of surface states in the electronic transport. 
Tuesday, March 6, 2018 9:48AM  10:00AM 
E10.00008: Topolectrical circuits Ronny Thomale, Ching Hua Lee First developed by Alessandro Volta and Felix Savary in the early 19th century, circuits consisting of resistor, inductor and capacitor (RLC) components are now omnipresent in modern technology. The behavior of an RLC circuit is governed by its circuit Laplacian, which is analogous to the Hamiltonian describing the energetics of a physical system. We show that “topolectrical” boundary resonances (TBRs) appear in the impedance readout of a circuit whenever its Laplacian bandstructure resembles that of topological semimetals  materials with extensive degenerate edge modes known as Fermi arcs that also harbor enigmatic transport properties. Due to the versatility of electronic circuits, our topological semimetal construction can be generalized to topolectrical phases with any desired lattice symmetry, spatial dimension, and even quasiperiodicity. Topolectrical circuits establish a bridge between electrical engineering and topological states of matter, where the accessibility, scalability, and operability of electronics promises to synergize with the intricate boundary properties of topological phases. 
Tuesday, March 6, 2018 10:00AM  10:12AM 
E10.00009: Magnetotransport study of topological phase transition in (Cd_{1x}Zn_{x})_{3}As_{2} thin films Shinichi Nishihaya, Masaki Uchida, Yusuke Nakazawa, Kazuto Akiba, Markus Kriener, Yusuke Kozuka, Atsushi Miyake, Yasujiro Taguchi, Masashi Tokunaga, Masashi Kawasaki Topological Dirac semimetal (DSM) Cd_{3}As_{2}, stabilized through the band inversion and the symmetry protection, provides a useful platform not only for investigating Weyl physics such as chiral anomaly, but also for pursuing topological phase transitions originating from DSM. Among others, chemical doping to control strength of the spinorbit interaction is one effective way to modulate the band structure. In the case of Cd_{3}As_{2}, chemical doping of Zn is expected to suppress the band inversion of Cd_{3}As_{2}, leading to a transition from the DSM phase to a trivial insulator phase. In this context, we discuss magnetotransport of high quality (Cd_{1x}Zn_{x})_{3}As_{2} thin films under high magnetic fields. Especially when the field is aligned to the current, the longitudinal magnetoresistance exhibits a clear Znconcentration dependence, changing from negative to positive. Along with analysis of transport properties such as Fermi velocity, effective mass, and scattering times, we have clarified details of the topological phase transition in (Cd_{1x}Zn_{x})_{3}As_{2}. 
Tuesday, March 6, 2018 10:12AM  10:24AM 
E10.00010: Metalinsulator Transition in Weyl Semimetal Thin Films Maryam Taherinejad, Nicola Spaldin In this talk we demonstrate metalinsulator transition in Weyl semimetal thin films using model Hamiltonians, and introduce special cases in which a quadratic crossing remains even in singlelayer limit. The band structure of Weyl semimetals is distinguished by pairs of Dirac points in the 3D Brillouin zone which are sink and source of the Berry curvature. These Weyl nodes act like magnetic monopoles with opposite charges in momentum space. In a slab configuration, this nontrivial topology leads to topological surface states that form unique Fermi arcs connecting two Weyl nodes. Here we show that as the thickness of the slab is reduced, the Fermi arcs shrink with the two magnetic monopoles moving toward each other, distinguishing the top of the Fermi arc as the most stable metallic point on the surface. In the absence of symmetries to protect the metallic crossing at the top of the arc, the metallic surface states disappear at a critical thickness. In special cases in which the metallic state at the top of the Fermi arc is protected by symmetries in the 2D plane, the pair of Fermi arcs on opposite surfaces reduces to a quadratic crossing in the thin film. Finally, we use density functional theory to explore the manifestations of this Fermi arc evolution in real material systems. 
Tuesday, March 6, 2018 10:24AM  10:36AM 
E10.00011: Gap opening and quantum Hall effect in thin films of the threedimensional Dirac semimetal Cd_{3}As_{2} Timo Schumann, Luca Galletti, David Kealhofer, Honggyu Kim, Manik Goyal, Susanne Stemmer Threedimensional Dirac semimetals, which are zero band gap materials with a linear dispersion relation at the Fermi energy, have attracted tremendous interest. While most studies are being performed on bulk crystals, fewer reports exist for epitaxial thin films. Moreover, predicted transitions into other topological phases induced, e.g., by confinement or symmetry breaking, have not yet been reported. 
Tuesday, March 6, 2018 10:36AM  10:48AM 
E10.00012: Localization effects in 3D Dirac antiperovskite thin films Hiroyuki Nakamura, Johannes Merz, Eslam Khalaf, Pavel Ostrovsky, Debakanta Samal, Hidenori Takagi We report localization effects in the low temperature magnetotransport of 3D Dirac materials Sr_{3}PbO and Sr_{3}SnO. Robust weak antilocalization was observed for Sr_{3}SnO, whereas weak localization was dominant for Sr_{3}PbO. The sign change of quantum interference part of the magnetoconductance is related to the position of the Fermi energy with respect to the Dirac nodes, pointing to the role of Berry curvature. Detailed properties of Dirac electrons derived by localization analysis will be discussed. 
Tuesday, March 6, 2018 10:48AM  11:00AM 
E10.00013: Weyl fermion on reconstructed surface of Bi/Si(110) with nonsymmorphic lattice symmetry HyunJung Kim, SeoungHun Kang, YoungWoo Son In twodimensional crystals, the nonsymmorphic lattice symmetry together with time reversal symmetry can stabilize Weyl fermionic states. With 4n+2 electron filling, the states can be tuned to locate near the charge neutral point. Here, using firstprinciples calculations, we propose a stable Bi adatom induced superstructure on Si(110) surface, characterized by wallpaper symmetry group pg, as a candidate system for the realization of Weyl fermion states with 4n+2 electron filling. Out of various adsorption geometries, the ground structure bears single glide line parallel to [110] direction, results in a twofold degenerate Bloch bands along glide lines. We will also discuss a way of controlling band dispersions where the Weyl fermion reside near the fermi level. 
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