Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session P28: Advances in Topological Materials IFocus
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Sponsoring Units: DMP Chair: Anton Burkov, University of Waterloo Room: 327 |
Wednesday, March 16, 2016 2:30PM - 3:06PM |
P28.00001: Quantum Transport of Spin-helical Dirac Fermion Topological Surface States in Topological Insulators Invited Speaker: Yong P. Chen Three-dimensional (3D) topological insulators (TI) are a novel class of electronic materials with topologically-nontrivial band structure such that the bulk is gapped and insulating yet the surface has topologically protected gapless conducting states. Such “topological surface states” (TSS) give helically spin polarized Dirac fermions, and offer a promising platform to realize various other novel physics such as topological magnetoelectric effects and Majorana fermions. However, it is often challenging to unambiguously access and study the transport properties of TSS in many practical TI materials due to non-negligible bulk conducting states. I will discuss our recent experiments on high-quality “intrinsic” TIs with insulating bulk and surface-dominated conduction that allow us to reveal a number of characteristic transport properties of spin-helical Dirac fermion topological surface states. We have observed, for example, a thickness-independent and surface-dominated conductance (even at room temperature) in exfoliated TI thin films [1] and well-developed “half-integer” Dirac fermion quantum Hall effect (QHE) arising from TSS (observed up to ~40K) [1]; fully-tunable “two-species” Dirac fermion QHE and other intriguing states in dual gated devices where both top and bottom surfaces can be independently controlled [2]; current-induced helical spin-polarization detected by spin sensitive transport measurements using magnetic electrodes [3]; and in TI nanoribbons, Shubnikov-de Hass (SdH) oscillations showing gate-tunable Berry phase and ultra-relativistic Dirac mass [4]; and a “half-integer” Aharonov-Bohm effect (ABE) unique to the circumferentially quantized spin helical Dirac fermion surface state modes (sub-bands), with a gate-tunable conductance oscillation and alternation between the “half-integer” ABE and regular ABE periodic in fermi momentum [5]. Such TIs and related devices may enable promising future applications in spintronics, thermoelectrics and various topological quantum devices. References: [1] Y. Xu et al., Nature Physics 10, 956 (2014); [2] Y. Xu et al., arXiv:1511.04597 (2015); [3] J.Tian et al., Scientific Reports 5, 14293 (2015); [4] L. A. Jauregui et al., Scientific Reports 5, 8452 (2015); [5] L. A. Jauregui et al., Nature Nanotechnology, in press, arxiv:1503.00685 (2015). [Preview Abstract] |
Wednesday, March 16, 2016 3:06PM - 3:18PM |
P28.00002: Large linear magnetoresistance and high carrier mobility in a new Dirac semimetal candidate Johnpierre Paglione, Kefeng Wang, David Graf, Limin Wang, F. Boschini, A. Damascelli We report experimental results on a new potential Dirac semimetal, the skutterudite material RhSb$_3$. Together with a very large magnetoresistance and carrier mobility, the linear dispersion of the electronic band structure suggests properties similar to other recently reported Dirac and Weyl semimetals that deserve further investigation. Together with angle-resolved photoemission data, we present high field transport and quantum oscillations measurements that point to very large Fermi velocity and aspects of Berry's curvature that warrant further investigation. [Preview Abstract] |
Wednesday, March 16, 2016 3:18PM - 3:30PM |
P28.00003: Temperature-Induced Lifshitz Transition in WTe$_{2}$ Na Hyun Jo, Yun Wu, Masayuki Ochi, Lunan Huang, Daixiang Mou, Sergey L. Bud'ko, P.C. Canfield, Nandini Trivedi, Ryotaro Arita, Adam Kaminski We use thermoeletric power (TEP), temperature- and field-dependent resistivity, and ultrahigh resolution, tunable, vacuum ultraviolet laser-based, angle-resolved photoemission spectroscopy (ARPES) measurements to study the electronic properties of WTe$_2$, a compound that manifests exceptionally large, temperature-dependent magnetoresistance. The Fermi surface consists of two pairs of electron and two pairs of hole pockets along the $X-\Gamma-X$ direction. We find a rare example of a temperature-induced Lifshitz transition at $T \approx 160$ K. Temperature dependent TEP shows a change of slope at $T \approx$ 175 K and Kohler's rule was breakdown in the 70-140 K range. ARPES temperature scans confirm that the hole pockets completely disappear around 160 K. Our electronic structure calculations show a clear and substantial shift of the chemical potential $\mu(T)$ due to the semimetal nature of this material driven by modest changes in temperature. [PRL 115, 166602 (2015)] [Preview Abstract] |
Wednesday, March 16, 2016 3:30PM - 3:42PM |
P28.00004: High-field magnetoconductivity of topological semimetals Hai-Zhou Lu, Song-Bo Zhang, Shun-Qing Shen The chiral anomaly has been widely believed to give a positive magnetoconductivity or negative magnetoresistivity in strong and parallel fields in topological semimetals. However, several recent experiments on both Weyl and Dirac topological semimetals show a negative magnetoconductivity in high fields. Here, we study the magnetoconductivity of Weyl and Dirac semimetals in a strong magnetic field applied along the direction that connects the Weyl nodes, we find that the conductivity along the field direction is not only determined by the Landau degeneracy, but also depends on the Fermi velocity and scattering potentials. We identify several scenarios in which the high-field magnetoconductivity is negative. It shows that the high-field positive magnetoconductivity may not be a compelling signature of the chiral anomaly. The quantum linear magnetoresistance will also be discussed. [Preview Abstract] |
Wednesday, March 16, 2016 3:42PM - 3:54PM |
P28.00005: Orbital and anisotropy effects on the itinerant exchange interaction in 3D Dirac semimetals Sergio Ulloa, Diego Mastrogiuseppe, Nancy Sandler Dirac semimetals are new materials that can be considered analogues of graphene in three dimensions. Their band structure exhibits robust Dirac points that are protected by crystalline symmetry, and strong spin-orbit interaction. These unusual properties suggest that magnetic impurities may reveal exotic behavior with potential technological importance. In metallic hosts, magnetic impurities interact through the electron gas via the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction that depends strongly on the band structure of the material. We report on the RKKY interaction in 3D Dirac semimetals, such as Na$_3$Bi and Cd$_3$As$_2$ [1, 2]. We discuss asymptotic expressions for the interaction corresponding to settings with magnetic impurities at different distances and relative angle with respect to high symmetry directions on the lattice. We show that the Fermi velocity anisotropy produces a strong renormalization of the magnitude of the interaction, and a correction to the frequency of oscillation in real space. Hybridization of the impurities to different conduction electron orbitals results in interesting anisotropic interactions which can generate spiral spin structures in doped samples. [1] Z. Liu et al., Science 343, 864 (2014); [2] Z. Liu et al., Nat. Mater. 13, 677 (2014) [Preview Abstract] |
Wednesday, March 16, 2016 3:54PM - 4:06PM |
P28.00006: Study of angle dependent magnetoresistance in half-Heusler YPtBi Hyunsoo Kim, Kefeng Wang, Halyna Hodovanets, Johnpierre Paglione Semimetallic half-Heusler compounds (RTBi, R = rare earth, T = Pd, Pt) have been attracting much attention because multiple theoretical calculations predicted the topologically non-trivial band structure. However, the detail band structure near the $\Gamma$ point depends on the specific calculation methods, and also the band structure inferred from experimental results show discrepancy from the theoretical one. Particularly in RPtBi (R=Y, Lu, Dy, Gd), the surface metallic states, which is absent in most of the theoretical results, were evident by recent ARPES measurements, but there has not been any detailed study on the metallic surface states. Moreover, the observation of topological nodal superconductivity in YPtBi makes the knowledge of Fermiology crucial to understand the pairing mechanism in the half-Heusler superconductors. Here, we present experimental results on angular dependence of magnetoresistance at various temperatures in single crystals of YPtBi. Based on observation of the angular dependence of Shubnikov-de Haas quantum oscillations, we discuss possible topologies of the bulk as well as the surface Fermi surfaces. [Preview Abstract] |
Wednesday, March 16, 2016 4:06PM - 4:18PM |
P28.00007: Large transverse current in topological Dirac semimetal Cd3As2 Wei-Li Lee, Shih-Ting Guo, R. Sankar, Yung-Yu Chien, Tay-Rong Chang, Horng-Tay Jeng, Guang-Yu Guo, F. C. Chou Cadmium arsenide ($\rm Cd_3As_2$) is known for its inverted band structure and ultra-high electron mobility. It has been theoretically predicted and also confirmed by ARPES experiments to exhibit a 3D Dirac semimetal phase containing degenerate Weyl nodes. From magneto-transport measurements in high quality single crystals of $\rm Cd_3As_2$, a small effective mass $m^* \approx$ 0.05 $m_e$ is determined from the Shubnikov-de Haas (SdH) oscillations. In certain field orientations, we find a splitting of the SdH oscillation frequency in the FFT spectrum suggesting a possible lifting of the double degeneracy in accord with the helical spin texture at outer and inner Fermi surfaces with opposite chirality predicted by our \textit{ab initio} calculations. Strikingly, a large antisymmetric magnetoresistance with respect to the applied magnetic fields is uncovered over a wide temperature range in needle crystal of $\rm Cd_3As_2$ with its long axis along [112] crystal direction. It reveals a significant contribution of intrinsic anomalous velocity term in the transport equation intimately related to the unique 3D Rashba-like spin splitted bands in defected $\rm Cd_3As_2$. [Preview Abstract] |
Wednesday, March 16, 2016 4:18PM - 4:30PM |
P28.00008: Magnetic Torque Anomaly in the Quantum Limit of Weyl and Dirac Semimetals Nityan L. Nair, Philip J.W. Moll, Andrew C. Potter, Brad Ramshaw, Kimberly Modic, Scott Riggs, Bin Zeng, Nirmal Ghimire, Eric Bauer, Robert Kealhofer, Zhenglu Li, Steven Louie, Filip Ronning, James G. Analytis Three dimensional Dirac and Weyl semimetals, characterized by bulk quasiparticles that behave as massless, linearly dispersing Dirac or Weyl fermions, have excited physicists with their unique topological properties and potential for applications. The experimental signatures of Weyl or Dirac fermions, however, are often subtle and indirect, especially in systems where they coexist with trivial electrons. Here, we report a novel method by which these topological systems can be unambiguously experimentally identified. Magnetic torque measurements were performed on the Weyl semimetal NbAs in high magnetic field, showing a large anomaly upon entering the quantum limit. The torque exhibits a striking sign reversal, corresponding to a change in the magnetic anisotropy that is a direct result of the topological properties of the charge carriers. This result can be generalized to other Dirac and Weyl semimetal systems and establishes quantum limit torque measurements as a simple and direct experimental method of distinguishing topologically non-trivial Weyl and Dirac systems from trivial semiconductors. [Preview Abstract] |
Wednesday, March 16, 2016 4:30PM - 4:42PM |
P28.00009: Large magnetoresistance and electronic anisotropy in NbAs$_2$ Bing Shen, Shan Jiang, Ni Ni Recently, extremely large magnetoresistance (XMR) was discovered in semimetal such as WTe$_2$ LaSb and so on, triggering extensive reseach on these materials and the origin of XMR. In this talk, we will report the transport properties of non-magnetic layered pnictide material NbAs$_2$. Large transverse magnetoresistance is observed. At 10 K, the magnetoresistance is around 13000 $\%$ in the field of 9 T and shows no saturation behavior. The temperature dependent resistivity at various fields exhibits metal-to-semiconductor transition behavior around 100 K, which is coincident with the sudden increase of the Hall signal in the same temperature region. The angle dependent magnetoresistance at various temperatures follows the 3D scaling behavior with the mass anisotropy around 1.3-1.4, indicative of its 3D electron structure. Quantum oscillation data reveal the existence of at least three Fermi pockets in this material. [Preview Abstract] |
Wednesday, March 16, 2016 4:42PM - 4:54PM |
P28.00010: Magnetic torque study of Weyl semimetal compounds TaP and NbP up to 45 Tesla Gang Li, Tomoya Asaba, Colin Tinsman, Fan Yu, Benjamin Lawson, Yulin Chen, Lu Li Weyl semimetal is a recently proposed new state in condensed matter physics, in which the bulk bands could have three dimensional linear dispersion but the degeneracy at the cross point is lifted into a pair of Weyl points with opposite chirality. Among the predicted candidates, Tantalum monophorspide (TaP) and Niobium monophorspide (NbP) have the simplest composition and do not require extrinsic tuning. Photoemission data is accumulating and the unique Fermi-arc surface state is observed. Magnetotransport experiments has shown highly anisotropic magnetoresistance and quantum oscillations has been observed. Because both linear dispersive bands and conventional bands exist in these materials, a detailed study of the electronic strucuture of the bulk is highly desirable. We use torque magnetometry to study quantum oscillations of TaP and NbP down to 300 mK, and up to 45 Tesla, with focus on the angular dependence of oscillation frequencies. Our comparison shows clear difference in geometry of different bulk bands in these materials. Besides, a discussion will be made on high field torque data since 45 Tesla is high enough to push several of the bands into quantum limit. [Preview Abstract] |
Wednesday, March 16, 2016 4:54PM - 5:06PM |
P28.00011: $\pi$ Berry phase and Zeeman splitting of Weyl semimetal TaP Jin Hu, Jinyu Liu, David Graf, Seyed Radmanesh, Daniel Adams, Alyssa Chuang, Yu Wang, Irinel Chiorescu, Jiang Wei, Leonard Spinu, Zhiqiang Mao The recent breakthrough in the discovery of Weyl fermions in monopnictide semimetals provides opportunities to explore the exotic properties of relativistic fermions in condensed matter. The chiral anomaly-induced negative magnetoresistance and $\pi$ Berry phase are two fundamental transport properties associated with the topological characteristics of Weyl semimetals. Since monopnictide semimetals are multiple-band systems, resolving clear $\pi$ Berry phase for each Fermi pocket remains a challenge. We report the determination of Berry phases of multiple Fermi pockets of Weyl semimetal TaP through high field quantum transport measurements. We show our TaP single crystal has the signatures of a Weyl state, including light effective quasiparticle masses, ultrahigh carrier mobility, as well as negative longitudinal magnetoresistance. Furthermore, we have generalized the Lifshitz-Kosevich (LK) formula for multiple-band Shubnikov-de Haas (SdH) oscillations and extracted the Berry phases of $\pi$ for multiple Fermi pockets in TaP through the direct fits of the modified LK formula to the SdH oscillations. In high fields, we also probed signatures of Zeeman splitting, from which the Land\'{e} \textit{g}-factor is extracted. [Preview Abstract] |
Wednesday, March 16, 2016 5:06PM - 5:18PM |
P28.00012: Quantum Oscillations from generic surface Fermi arcs in Weyl semi-metals Yi Zhang, Daniel Bulmash, Pavan Hosur, Andrew Potter, Ashvin Vishwanath We re-examine the semiclassical derivation of quantum oscillations, and emphasize the correct definition of the chemical potential from the energy-time quantization perspective. In particular, for a Weyl semi-metal with surface Fermi arcs, the most natural energy reference point does not necessarily coincide with the energy of the bulk Weyl nodes. This results in several important amendments to previous conclusions for generic Weyl semi-metals. We also propose a simple lattice realization of Weyl semi-metals following the layered prescription and verify our theoretical conclusions with exact numerical studies. [Preview Abstract] |
Wednesday, March 16, 2016 5:18PM - 5:30PM |
P28.00013: Thermo-electric transports in double-Weyl semimetals Qi Chen, Gregory A. Fiete Topological Weyl semimetals with linearly dispersing nodal points have received a surge of interest due to their experimental realization in real materials. Another nontrivial type of band crossing whose dispersion is not simply linear is the double Weyl point, around which the spectrum disperses linearly along one momentum direction but quadratically along the two remaining directions. In this work, we apply the semi-classical Boltzmann transport theory to study the thermo-electric conductivity of a double-Weyl fermion model. We find that the transport quantities exhibit an interesting dependence on the chemical potential and spatial anisotropic model parameters, differing from a simple quadratically or linearly dispersing electron gas. By applying a static magnetic field, we find that the double-Weyl point is only stable for a magnetic field along the linearly dispersing direction. The longitudinal and transverse electrical and thermal magneto-conductivity show a similar dependence on the in-plane cyclotron frequency to the linearly dispersing Weyl nodes. In the extreme quantum limit of chemical potential being much smaller than the cyclotron energy, we find that the lowest Landau levels are both chiral and doubly degenerate. The chiral anomaly contribution to the longitudinal magneto-conductivity is double that of a linearly dispersing Weyl node. [Preview Abstract] |
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