Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session A44: Dirac and Weyl Semimetals: Transport IFocus

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Sponsoring Units: DMP Chair: Philip Moll, Max Planck Institute Room: 391 
Monday, March 13, 2017 8:00AM  8:12AM 
A44.00001: Zeeman splitting and dynamical mass generation in Dirac semimetal ZrTe$_{\mathrm{5}}$ Yanwen Liu, Xiang Yuan, Cheng Zhang, Zhao Jin, Awadhesh Narayan, Chen Luo, Zhigang Chen, Lei Yang, Jin Zou, Xing Wu, Stefano Sanvito, Zhengcai Xia, Liang Li, Zhong Wang, Faxian Xiu Dirac semimetals have attracted extensive attentions in recent years. It has been theoretically suggested that manybody interactions may drive exotic phase transitions, spontaneously generating a Dirac mass for the nominally massless Dirac electrons. So far, signature of interactiondriven transition has been lacking. In this work, we report highmagneticfield transport measurements of the Dirac semimetal candidate ZrTe5. Owing to the large g factor in ZrTe5, the Zeeman splitting can be observed at magnetic field as low as 3 Tesla. Most prominently, high pulsed magnetic field up to 60 Tesla drives the system into the ultraquantum limit, where we observe abrupt changes in the magnetoresistance, indicating fieldinduced phase transitions. This is interpreted as an interactioninduced spontaneous mass generation of the Dirac fermions. Our work establishes Dirac semimetals as ideal platforms for investigating emerging correlation effects in topological matters. Reference: Yanwen Liu, et al. Nature Communications 7, 12516 (2016) [Preview Abstract] 
Monday, March 13, 2017 8:12AM  8:48AM 
A44.00002: Weyl Wiggles: exotic quantum oscillatory phenomena in Weyl and Dirac semimetals Invited Speaker: James Analytis Dirac semimetals show a linear electronic dispersion in three dimensions described by two copies of the Weyl equation, a theoretical description of massless relativistic fermions. At the surface of a crystal, the breakdown of fermion chirality is expected to produce topological surface states without any counterparts in highenergy physics nor conventional condensed matter systems, the socalled ``Fermi Arcs''. Here we present Shubnikovde Haas oscillations in Focused Ion Beam prepared microstructures of Cd3As2 that share characteristics of surface and bulk states as expected for ``Weyl orbits'', the theoretically predicted cyclotron path that weaves together Fermi arc and chiral bulk states. In contrast to conventional cyclotron orbits, these are governed by the chiral bulk dynamics rather than the common momentum transfer due to the Lorentz force. Our observations provide evidence for direct access to the topological properties of charge in a transport experiment, a first step towards their potential application. [Preview Abstract] 
Monday, March 13, 2017 8:48AM  9:00AM 
A44.00003: Magnetotransport study of Weyl orbit in Dirac semimetal Cheng Zhang, Shiheng Lu, Xiang Yuan, Faxian Xiu Formed by the periodic motion of electrons through closed orbits in the momentum space, cyclotron orbits have been known for decades and widely used as an effective way to probe the Fermi surface by detecting the resultant quantum oscillations. Recent studies in topological systems show that a new type of electron orbits with open loop, known as Fermi arcs, will emerge at the surface of Weyl semimetals as a result of broken translational symmetry. Nevertheless, a complete cyclotron orbit can still be developed within open Fermi arcs on both sides of the surface, if electrons can tunnel through the bulk and remain phase coherent. Here, we present a systematic study of such “Weyl orbit” in Cd3As2 nanoplates with different Fermi levels through the Shubnikovde Haas oscillations. We found that Weyl orbit persists as Fermi level approaches the Dirac nodes. In the meantime, a new set of Landau levels emerge at high field, indicating a crossover from inter to intra surface cyclotron due to the reduced symmetry under magnetic field. Our study clarifies recent debate whether the double Fermi arcs can form a stable Weyl orbit and highlights the importance of crystal symmetry as protection mechanism for double Fermi arcs. The strong interplay between crystal symmetry and surface Fermi arcs further enriches the underlying physics in topological semimetals. [Preview Abstract] 
Monday, March 13, 2017 9:00AM  9:12AM 
A44.00004: Landau level spectroscopy of twodimensional massive Dirac fermions in singlecrystal ZrTe$_{\mathrm{5}}$ thin flakes Yuxuan Jiang, Zhiling Dun, Haidong Zhou, KuanWen Chen, Seongphill Moon, Ryan Baumbach, Tiglet Besara, Dmitry Smirnov, Theo Siegrist, Zhigang Jiang ZrTe$_{\mathrm{5}}$ has recently attracted much interest due to the possibility of hosting a largegap quantum spin Hall insulator in its monolayer form. However, its electronic structure in the bulk is currently under heated debate, with interpretations ranging from weak/strong topological insulator to Dirac semimetal. Here, we report on a ``bulksensitive'' magnetoinfrared transmission study of ZrTe$_{\mathrm{5}}$ thin flakes. At zero magnetic field, our samples exhibit graphenelike optical absorption, which signifies their twodimensional (2D) nature. In a magnetic field, we observed a series of interband Landau level (LL) transitions that can be described by a massive Dirac fermion model with a mass of \textasciitilde 4.7meV. More interestingly, we observed a fourfold splitting of lowlying LL transitions in our samples, which we attributed to the effect from finite mass, large Zeeman effects and the electronhole asymmetry. Our results support a 2D Dirac semimetal interpretation, consistent with recent electronic transport studies. [Preview Abstract] 
Monday, March 13, 2017 9:12AM  9:24AM 
A44.00005: Quantum Hall effect in Cd$_3$As$_2$ films Masaki Uchida, Yusuke Nakazawa, Shinichi Nishihaya, Kazuto Akiba, Markus Kriener, Yusuke Kozuka, Atsushi Miyake, Yasujiro Taguchi, Masashi Tokunaga, Naoto Nagaosa, Yoshinori Tokura, Masashi Kawasaki A well known semiconductor Cd$_3$As$_2$ is reentering the spotlight due to its unique electronic structure of threedimensional Dirac semimetal. We have succeeded in fabricating high crystallinity and high mobility Cd$_3$As$_2$ thin films epitaxially grown on SrTiO$_3$ to observe quantum Hall effect at high magnetic fields up to 55 T. With a decrease in film thickness to 10 nm, the quantum Hall states exhibit various changes such as of degeneracy accompanied with topological phase transitions. Detailed electronic structures of subband splitting and gap opening are identified from the quantum transport depending on the confinement thickness. Our findings and techniques pave the way for further investigation of quantum transport originating from the topological electronic structures in Cd$_3$As$_2$. [Preview Abstract] 
Monday, March 13, 2017 9:24AM  9:36AM 
A44.00006: Ambipolar quantum transport in Cd$_{\mathrm{3}}$As$_{\mathrm{2}}$ field effect transistors Shinichi Nishihaya, Masaki Uchida, Yusuke Nakazawa, Markus Kriener, Yusuke Kozuka, Yasujiro Taguchi, Mashashi Kawasaki Cd$_{\mathrm{3}}$As$_{\mathrm{2}}$ with ultrahigh electron mobility, has recently been discovered to be a Dirac semimetal with a pair of doubly degenerate Weyl points, which can lead to emergent transport phenomena such as chiral anomaly. Experimental materialization of the Dirac semimetal has triggered intensive research on unveiling the new topological state. In many cases, however, defectinduced high carrier density of Cd$_{\mathrm{3}}$As$_{\mathrm{2}}$ makes Fermi energy far above Weyl points, hindering such attempts. Here we report systematic control of carrier density of Cd$_{\mathrm{3}}$As$_{\mathrm{2}}$ in a thin film form. Electric field effect with use of SrTiO$_{\mathrm{3}}$ substrate as backgating dielectric as well as chemical doping effect of Zn are employed to achieve carrier type inversion from $n$type to $p$type with Fermi energy crossing the charge neutral Weyl points. Quantum transport properties are examined with changing the Fermi energy position and band inversion energy for systematic understanding of the electronic structures. [Preview Abstract] 
Monday, March 13, 2017 9:36AM  9:48AM 
A44.00007: GateTunable Negative Longitudinal Magnetoresistance as Transport Evidence of a TypeII Weyl Semimetal WTe2. Yaojia Wang, Erfu Liu, Huimei Liu, Zhenlin Wang, Haizhou Lu, Baigeng Wang, Xiangang Wan, Feng Miao The progress in exploiting new electronic materials has been a major driving force in solidstate physics. As a new state of matter, a Weyl semimetal (WSM), particularly a typeII WSM, hosts Weyl fermions as emergent quasiparticles and may harbor novel electrical transport properties. In this work, by performing systematic magnetotransport studies on thin films of a predicted material candidate WTe2, we observe notable negative longitudinal magnetoresistance, which can be attributed to the chiral anomaly in WSM. This phenomenon also exhibits strong planar orientation dependence with the absence along the tungsten chains, consistent with the distinctive feature of a typeII WSM. By applying a gate voltage, we demonstrate that the Fermi energy can be insitu tuned through the Weyl points via the electric field effect. Our results may open opportunities for implementing new electronic applications, such as fieldeffect chiral devices. Reference: Y. J. Wang. et al., Nature Communications 7,13142 (2016). [Preview Abstract] 
Monday, March 13, 2017 9:48AM  10:00AM 
A44.00008: Controlling superlong relaxation of photoexcited Dirac fermions in Bi$_2$Te$_2$Se Haiming Deng, Zhiyi Chen, Shihua Zhao, Lia KrusinElbaum, Marcin Konczykowski, Evangelos Papalazarou Among the recently engineered topological insulators (TIs) ternary compound Bi$_2$Te$_2$Se (BTS) displays the highest todate bulk resistivity (16 $\Omega$ cm at 4 K). It is of particular interest in novel photonic and optoelectronic applications owing to the unusually long relaxation time ($\tau > 4~\mu$s) of photoexcited Dirac electrons. Yet, the origin of these longlived photoexcited carriers, observed by angle and femtosecond timeresolved photoelectron spectroscopy (trARPES)[1], has not been resolved. Here, we report on transport measurements of electrostatically gated BTS structures which, in conjunction with ARPES studies, demonstrate that $\tau$ is controlled by the subsurface band bending and trap states. As expected from the trap states, gate voltage dependence of BTS resistivity at low temperatures is hysteretic and asymmetric, suggesting that long relaxation times can be ultimately tuned by a charge transfer to the surface. Gating hysteresis can be manipulated by doping BTS with small amounts (1\%) of Sn and by cooling the system under applied gate voltage, pertinent to engineering a lifetime switch for photoexcited Dirac fermions. [1] E. Papalazarou \textit{et al}, preprint (2016). [Preview Abstract] 
Monday, March 13, 2017 10:00AM  10:12AM 
A44.00009: Probing the low energy electronic structure of ZrTe$_5$ using elastoresistance measurements JiunHaw Chu, Joshua Mutch ZrTe$_5$ has been known for its anomalous transport behavior for decades. Recent band structure calculations suggested that it is a quantum spin Hall insulator in the monolayer form, and the bulk electronic structure’s topology sensitively depends on the interlayer coupling. In this work, we show results of elastoresistance measurements which revealed features that cannot be detected by the conventional magnetoresistance measurements. We will discuss its significance in the context of the low energy electronic structure of ZrTe$_5$. [Preview Abstract] 
Monday, March 13, 2017 10:12AM  10:24AM 
A44.00010: Growth and Magnetotransport Properties of Dirac Semimetal Candidate Cu$_{\mathrm{3}}$PdN C. X. Quintela, N. Campbell, D. T. Harris, D. F. Shao, L. Xie, X. Q. Pan, E. Y. Tsymbal, M. S. Rzchowski, C. B. Eom Since the discovery of threedimensional Dirac semimetals (DSM) Cd$_{\mathrm{3}}$As$_{\mathrm{2}}$ and Na$_{\mathrm{3}}$Bi, many efforts have been made to identify new DSM materials. Recently, nitride antiperovskite Cu$_{\mathrm{3}}$PdN has been proposed by two different groups as a new DSM candidate. However, until now, the experimental realization of bulk Cu$_{\mathrm{3}}$PdN and the study of its electronic properties has been hindered due to the difficulty of synthesizing bulk single crystals of this material. Here, we report the first growth and magnetotransport characterization of epitaxial Cu$_{\mathrm{3}}$PdN thin films on (001) SrTiO$_{\mathrm{3}}$ substrates. Magnetotransport measurements reveal $p$type metallic conduction with very low temperature coefficient of the resistance and small nonlinear magnetoresistance at low temperatures. The successful growth of Cu$_{\mathrm{3}}$PdN thin films opens the path to investigating the unknown electronic properties of this material, and provides a template for further research on other antiperovskite DSM candidates such as Cu$_{\mathrm{3}}$ZnN. [Preview Abstract] 
Monday, March 13, 2017 10:24AM  10:36AM 
A44.00011: PhononInduced Topological Transition to a TypeII Weyl Semimetal LinLin Wang, Na Hyun Jo, Yun Wu, Adam Kaminski, Paul C. Canfield, Duane D. Johnson The emergence of topological quantum states requires certain combinations of crystalline symmetry with or without time reversal symmetry. Without restricting to searches for crystal structures with nonsymmorphic symmetry operations in the space groups, we have studied the interplay between crystal symmetry, atomic displacements (lattice vibration), band degeneracy and topology. For a system with a full gap opening between the two band manifolds near the Fermi energy, we show that small atomic displacements (accessible via optical phonons near room temperature) can lower the symmetry to induce typeII Weyl points at the boundary between a pair of closelylying electron and hole pockets. [Preview Abstract] 
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