Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A04: Dirac/Weyl Semimetals -- Thin Films and NanostructuresFocus
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Sponsoring Units: DMP Chair: Hugh Churchill, Univ of Arkansas-Fayetteville Room: BCEC 107C |
Monday, March 4, 2019 8:00AM - 8:12AM |
A04.00001: Weyl orbit quantum Hall states observed in Dirac semimetal Cd3As2 thin films Shinichi Nishihaya, Masaki Uchida, Yusuke Nakazawa, Ryosuke Kurihara, Kazuto Akiba, Markus Kriener, Atsushi Miyake, Yasujiro Taguchi, Masashi Tokunaga, Masashi Kawasaki Topologically protected surface states in topological materials provide access to various unconventional transport phenomena. One example in gapless topological semimetals is the unique interplay between bulk and surface Fermi-arc states resulting in an exotic magnetic orbit (Weyl orbit). The Weyl orbit weaves together two spatially-separated Fermi-arc states across the bulk state under the field, allowing the appearance of two-dimensional (2D) quantized conduction even in a 3D system. Here, we report the observation of quantum Hall (QH) states in Dirac semimetal Cd3As2 thin films. By controlling Fermi level and band topology of the 3D bulk state with electrostatic gating and chemical-doping-induced topological phase transition, we clarify that the quantized conduction emerges originating from the Weyl orbit. In particular, the successive scan from bulk-dominant conduction to Weyl orbit QH effect, reveals that the emergence of the QH states depends on the bulk Landau level occupation and the induced asymmetry between the film surfaces. |
Monday, March 4, 2019 8:12AM - 8:24AM |
A04.00002: Confinement Effects in Cd3As2 (001) Grown by Molecular Beam Epitaxy David Kealhofer, Luca Galletti, Manik Goyal, Honggyu Kim, Timo Schumann, Susanne Stemmer Cadmium arsenide (Cd3As2) is a 3D Dirac semimetal in which two Dirac nodes along the kz axis are stabilized by a symmetry of the crystal lattice. In a thin film, the fate of the Dirac nodes and associated surface states depends on the orientation of the confinement potential, which is effected by the growth direction. Here we report on improvements to the epitaxial growth of (001)-oriented cadmium arsenide thin films on a III–V compound semiconductor substrate. We show that smooth films can be achieved by altering the surface chemistry at the epilayer–buffer layer interface. We discuss the nature of the resulting clean, two-dimensional transport on the (001)-like surfaces with reference to previous work on the more widely studied (112)-like surfaces. |
Monday, March 4, 2019 8:24AM - 8:36AM |
A04.00003: Transport study in Coherently Strained Thin Films of a 3D Dirac Semimetal Manik Goyal, Timo Schumann, David Kealhofer, Salva Salmani-Rezaie, Luca galletti, Susanne Stemmer Cd3As2 is a 3D Dirac semimetal with doubly degenerate band crossings at isolated points in Brillouin zone. Here, we present a study of epitaxially strained thin films of Cd3As2, grown by molecular beam epitaxy on (111) GaAs with InxGa1-xSb buffer. The composition (x) was varied to obtain layers that are lattice matched with the Cd3As2, as well as to obtain tensile and compressively strained Cd3As2. Films grow coherently strained between 0.44% (tensile) and -0.46% (compressive) strain up to 85 nm thickness. Magneto transport measurements for the thin films (below 50 nm) reveal 2D quantum oscillations under tensile strain, whereas in compressive direction the quantum oscillations are from 3D Fermi surface. We discuss how strain engineering can be used to control the electronic states in thin films of Dirac Semimetals. |
Monday, March 4, 2019 8:36AM - 8:48AM |
A04.00004: Non-local Signatures of the Chiral Magnetic Effect in Dirac Semimetal Bi0.97Sb0.03 Daan Wielens, Jorrit C. De Boer, Joris A. Voerman, Bob de Ronde, Yingkai Huang, Mark Golden, Chuan Li, Alexander Brinkman Recently there has been a lot of interest in 3D Dirac semimetals (DSM), which exhibit robust Dirac phases in the bulk of the material. The chiral magnetic effect (CME), which originates from the Weyl cones, causes an E.B-dependent chiral charge polarization, which manifests itself as negative magnetoresistance. |
Monday, March 4, 2019 8:48AM - 9:00AM |
A04.00005: Quantum transport properties of Cd3As2 films with low carrier density Yusuke Nakazawa, Masaki Uchida, Shinichi Nishihaya, Shin Sato, Masashi Kawasaki Cd3As2 is a typical three-dimensional topological Dirac semimetal, characterized by a pair of Dirac points protected by rotational symmetry. While high-crystallinity and high-flatness Cd3As2 films have been obtained by the combination of pulsed laser deposition and subsequent high-temperature annealing [1,2], the carrier density is rather high compared to ones prepared by molecular beam epitaxy [3,4]. In this talk, we report quantum transport properties of Cd3As2 thin films epitaxially grown by molecular beam epitaxy. A typical film thicker than 100 nm shows a carrier density of 5×1016 cm-3 and an electron mobility exceeding 3×104 cm2/Vs. In this thickness regime corresponding to the three-dimensional electronic structure, the film shows plateau-like structures in the Hall resistance, indicating the emergence of a two-dimensional conduction state. This can be understood to originate from the Weyl orbit surface state of the topological Dirac semimetal. |
Monday, March 4, 2019 9:00AM - 9:12AM |
A04.00006: Negative longitudinal magnetoresistance in GaAs quantum wells Jing Xu, Meng Ma, Zhili Xiao, Yonglei Wang, Dafei Jin, Yangyang Lyu, Wei Zhang, Loren Pfeiffer, Kenneth West, K. W. Baldwin, Mansour Shayegan, Wai-Kwong Kwok, Maksim Sultanov
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Monday, March 4, 2019 9:12AM - 9:24AM |
A04.00007: Topological phase transitions by interface engineering Dong Zhang Novel phase transitions, espacially topological phase transitions, have attracted intense interests in condensed mattered physics in recent years. However, the reported materials possess novel phases are majorly composed of heavy elements with relatively low abundances. We switched investigate objectives into commonly-used semiconductors, and demonstrated that, utilizing giant electric fields generated by charge accumulation at GaAs/Ge/GaAs opposite semiconductor interfaces and band folding, the interface engineering can reduce the sizable gap in Ge, induce large spin-orbit interaction, and drive Ge into a topological insulating phase [1]. Natural topological semimetals hosting multiple topological states will be demonstrated through simple symmetry considerations [2,3], and the possibilities to realize “ideal” topological semimetals utilizing interface design[4] will also be disscussed. |
Monday, March 4, 2019 9:24AM - 9:36AM |
A04.00008: Charge transfer in a transition metal dichalcogenide semiconductor/Weyl semimetal van der Waals junctions Kyusup Lee, Jie Li, Liang Cheng, Junyong Wang, Dushyant Kumar, Qisheng Wang, Mengji Chen, Yang Wu, Goki Eda, Ee Min Chia, Haixin Chang, Hyunsoo Yang Transient metal dichalcogenides (TMDs) heterostructures have recently made a meteoric rise in quantum device engineering due to its van der Waals (vdW) layered nature. A Weyl semimetal WTe2, also a TMD compound, displays a wide range of exotic electronic and spintronic properties. A central approach in heterostructures is driving charge transfer across the interface mainly governing the carrier dynamics, which determines fundamental optoelectronic properties. Here, we report a new type of CVD grown TMD vdW junctions with a semiconducting 2H-MoTe2 and its sister compound of semimetallic Td-WTe2. Time-resolved terahertz spectroscopy reveals the ultrafast relaxation of the photo-excited carriers in the junctions, which attributes to the charge transfer and the interlayer exciton decay serving as a fast relaxation channel with a characteristic time of ~0.6 ps, faster than that of the each layer (~1.5 ps from Td-WTe2 and ~5.9 ps from 2H-MoTe2). Moreover, we observe the negligible band-filling and hot-phonon effects according to the optical fluence (< 10 mJ/cm2) due to such an ultrafast interfacial relaxation channel. This ultrafast photoresponse in sister-compound large-area TMDs vdW junctions provide a platform for high-speed optoelectronic devices. |
Monday, March 4, 2019 9:36AM - 9:48AM |
A04.00009: Exploring Transport Properties in the 2D–3D Dirac Semimetal Heterostructures Yanfei Wu, Liang Zhang, Li Caizhen, Zhi-Min Liao, Dapeng Yu, Song Liu, Zhen-Sheng Zhang Dirac semimetal is an emerging class of quantum matters, ranging from two-dimensional (2D) category, such as graphene and surface states of topological insulator to three-dimensional (3D) category, for instance, Cd3As2 and Na3Bi. Exotic surface states in 3D Dirac semimetals is attractive for the interface engineering and the research on 2D–3D Dirac van der Waals heterostructures. Here we fabricated graphene–Cd3As2 heterostructure through direct layer-by-layer stacking and investigated their electron transport properties. The electronic coupling results in a notable interlayer charge transfer, which modulates the Fermi level of graphene through Cd3As2. This heterostructure enable us to naturally fabricate graphene p–n–p junctions, showing the quantized conductance plateaus. Moreover, the nonlocal transport studies show large nonlocal signals near Dirac point in graphene–Cd3As2 device, due to the charge transfer from the spin-polarized surface states in Cd3As2. Our results enrich the family of van der Waals heterostructures and can inspire more studies on the application of Dirac/Weyl semimetals in spintronics. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A04.00010: Manipulating the Topological Surface States of Cd3As2by N* Plasma Exposure Timo Schumann, Luca Galletti, Thomas E Mates, Susanne Stemmer Cd3As2belongs to the family of three-dimensional Dirac semimetals, which are characterized by a linear dispersion of their bulk bands at the Fermi energy. For thin films, the electronic transport properties of Cd3As2are dominated by surface states, which, as we show in this presentation, are highly sensitive to the chemistry of the surface. Temperature-dependent magneto-transport measurements show that exposure of the surface to a low-energy nitrogen plasma improves carrier mobility and facilitates the observation of the quantum Hall effect in confined thin films. X-ray photoemission spectroscopy reveals changes in the surface chemistry of air-exposed and nitrogen-plasma surfaces, respectively. The results provide insights into the role of surface band bending and the relative contributions of surface and bulk states to the measured transport properties. |
Monday, March 4, 2019 10:00AM - 10:36AM |
A04.00011: Realization of an Elemental Topological Dirac Semimetal: α-Sn on InSb(111) Invited Speaker: Tai-Chang Chiang Three-dimensional (3D) topological Dirac semimetals (TDSs) are rare but important as a versatile platform for exploring exotic electronic properties and topological phase transitions. A quintessential feature of TDSs is 3D Dirac fermions associated with bulk electronic states near the Fermi level. Using angle-resolved photoemission spectroscopy, we have observed such bulk Dirac cones in epitaxially grown α-Sn films on InSb(111), the first such TDS system realized in an elemental form. First-principles calculations confirm that epitaxial strain caused by the in-plane lattice mismatch of 0.14% is key to the formation of the TDS phase. A phase diagram as a function of epitaxial strain is established that connects the 3D TDS phase through a singular point of a zero-gap semimetal phase to a topological insulator phase. The nature of the Dirac cone crosses over from 3D to 2D as the film thickness is reduced to a few layers. |
Monday, March 4, 2019 10:36AM - 10:48AM |
A04.00012: First principles study of WTe2 with Wannier model Amartyajyoti Saha, Turan Birol Two-dimensional transition metal dichalcogenides are a class of atomically thin materials which can host a wide range of physical properties such as strong spin-orbit coupling and nontrivial topological phases. Both monolayer and bulk tungsten ditelluride (WTe2) has been studied in great detail over the past years for their electronic properties. Here we present the results of our first principles study of the behaviour of finite thickness WTe2. To connect between the bulk and the monolayer material, we employ Wannier functions to build tight binding models for different numbers of WTe2 layers. This study provides insight into the effect of van der Waals interactions and spin-orbit coupling on the symmetry, crystal and band structures, and topology of WTe2. |
Monday, March 4, 2019 10:48AM - 11:00AM |
A04.00013: Direction dependent giant optical conductivity in 2D semi-Dirac materials Bhaskaran Muralidharan, Alestin Mawrie Using linear response theory, we evaluate the optical conductivity of 2D semi-Dirac materials for light polarized along two mutually orthogonal directions and predict a strong anisotropy of the optical response as a consequence of their unique dispersion spectrum. Our results also predict a large degree of sensitivity for inter-band optical conductivity to the polarization direction. While it reveals an abruptly large value for certain frequency for light along a particular polarization direction, it is significantly suppressed along the direction orthogonal to the former. The frequency corresponding to the predicted giant optical conductivity is found to be independent of the chemical potential, for a lightly doped system. This direction-dependency of the giant inter-band optical conductivity may, in turn, be used to uniquely predict the dispersive nature of the 2D \textit{semi}-Dirac materials, in addition to possible applications that arise from this direction dependent optical transparency. |
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