Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session V29: Weyl Semimetals: Theory and Experiments |
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Sponsoring Units: DCMP Chair: Rudro Biswas, Purdue University Room: 328 |
Thursday, March 17, 2016 2:30PM - 2:42PM |
V29.00001: Topological Nodal-Line Fermions in the Non-Centrosymmetric Spin-Orbit Metal PbTaSe2. Guang Bian We report on the existence of topological nodal-line states in the non-centrosymmetric compound single-crystalline PbTaSe2 with strong spin-orbit coupling. Remarkably, the spin-orbit nodal lines in PbTaSe2 are not only protected by the reflection symmetry but also characterized by an integer topological invariant. Our detailed angle-resolved photoemission measurements, first-principles simulations and theoretical analysis illustrate the physical mechanism underlying the formation of the topological nodal-line states and associated surface states. Our work paves the way towards exploring the exotic properties of the topological nodal-line fermions in condensed matter systems and, potentially, the rich physics arising from the interplay between the topological nodal-line states and the emergent superconductivity in this compound. [Preview Abstract] |
Thursday, March 17, 2016 2:42PM - 2:54PM |
V29.00002: \textbf{Magnetic structure and its role in the possible Weyl state in topological semimetal Sr}$_{\mathbf{1-y}}$\textbf{Mn}$_{\mathbf{1-z}}$\textbf{Sb}$_{\mathbf{2}}_{\, \, }$(0\textless y,z\textless 0.1) Qiang Zhang, Huibo Cao, Jinyu Liu, Alan Tennant, John Ditusa, Zhiqiang Mao Very recently, Liu et al (\textit{arxiv.org/pdf/1507.07978, (2015)}) discovered the first magnetic topological semimetal Sr$_{1-y}$Mn$_{1-z}$Sb$_{2\, \, }$(0\textless y,z\textless 0.1), in which a possible Weyl state arising from time reversal symmetry breaking is expected. However, the origin of ferromagnetic (FM) behavior of this material has not been clarified. By employing the neutron diffraction at the four-circle diffractometer HB3A, HFIR, we found a long-range FM order with Mn moments along $b$-axis below $T_{C}=$565 K, followed by another magnetic transition to a canted $C$-type antiferromagnetic (AFM) order at $T_{FM-AFM}=$ 304 K. In the canted $C$-type AFM state, the Mn moments are aligned with the $a$-axis along with a canting toward the $b$-axis, leading to a net FM moment lying along the$ b-$axis. The Mn moments along the $a-$ and $b$-axes at 5 K are found to be 3.789(3) and 0.741(4) \textmu $_{B}$, respectively. The discovered FM order in Sr$_{1-y}$Mn$_{1-z}$Sb$_{2}$, either the FM ordering at 304\textless $T$\textless 565 K or the FM component of the canted AFM order for $T$\textless 304 K, is sufficient to break time-reversal symmetry likely creating a Weyl semimetal. [Preview Abstract] |
Thursday, March 17, 2016 2:54PM - 3:06PM |
V29.00003: An optical investigation of the Magnetic Weyl semi-metal candidate YbMnBi$_2$ Dipanjan Chaudhuri, Bing Cheng, Quinn D. Gibson, Robert J. Cava, N. Peter Armitage The discovery of Dirac and Weyl fermions in condensed matter systems has sparked tremendous interest in both condensed matter and high energy physics communities alike. While the existence of Dirac electrons in graphene and topological insulator materials is now well established, only a handful of experiments provide direct evidence of Weyl fermions in non-centrosymmetric systems. Moreover, the theoretically predicted Weyl fermions in magnetic materials with strong spin-orbit coupling have continued to be elusive. A potent candidate for a Weyl semimetal with broken time reversal symmetry (TRS) and antiferromagnetic ordering is the newly discovered YbMnBi$_2$ compound. Although the complete magnetic structure of this material is still unknown, preliminary ARPES measurements are consistent with TRS breaking and a Weyl fermionic band dispersion. In this work, we investigate YbMnBi$_2$ single crystal with FTIR spectroscopy and study its low energy electrodynamic response. The energy scale of IR light is ideally suited to probe for the linear band dispersion of these materials close to the Weyl points. Temperature dependent optical conductivity calculations reveal crucial information on the fundamental scattering processes in these materials. [Preview Abstract] |
Thursday, March 17, 2016 3:06PM - 3:18PM |
V29.00004: Scanning tunneling microscopic investigation of the topological surface of a Weyl semimetal Hao Zheng Weyl semimetals are believed to open the next era of condensed matter physics after graphene and topological insulators because they provide the first ever realization of Weyl fermions in all physics and extend the classification of topological phases beyond insulators. For many years, experimental studies have been held back due to the absence of material realization of the Weyl semimetal state. Very recently, the first Weyl semimetal has been experimentally discovered in TaAs class of materials. So far only preliminary ARPES and transport experiments have been reported. In this talk, we will present some interesting results of the scanning tunneling microscopy/spectroscopy (STM/STS) study on a Weyl semimetal. [Preview Abstract] |
Thursday, March 17, 2016 3:18PM - 3:30PM |
V29.00005: SdH oscillations and pressure effect of the Weyl semimetal NbAs Yongkang Luo, N. J. Ghimire, M. Wartenbe, Hongchul Choi, M. Neupane, R. D. McDonald, E. D. Bauer, Jianxin Zhu, J. D. Thompson, F. Ronning Via angular Shubnikov-de Hass (SdH) quantum oscillations measurements, we determine the Fermi surface topology of NbAs. The SdH oscillations consist of two frequencies, corresponding to two Fermi surface extrema: 20.8 T ($\alpha $-pocket) and 15.6 T ($\beta $-pocket). The analysis shows that the $\beta $-pocket has a Berry phase of $\pi $ and a small effective mass 0.033 m$_{\mathrm{0}}$, indicative of a nontrivial topology; whereas the $\alpha $-pocket has a trivial Berry phase of 0 and a heavier effective mass 0.066 m$_{\mathrm{0}}$. Subtle changes can be seen in the $\rho _{\mathrm{xx}}$(T) profiles with pressure up to 2.31 GPa. The Fermi surfaces undergo an anisotropic evolution under pressure, while the topological features of the two pockets remain unchanged. Specific heat measurements reveal a small Sommerfeld coefficient $\gamma _{\mathrm{0}}=$0.09(1) mJ/(mol\textbullet K$^{\mathrm{2}})$ and a large Debye temperature, $\Theta_{\mathrm{D}}=$450(9) K, confirming a ``hard'' crystalline lattice that is stable under pressure. We also studied the Kadowaki-Woods ratio of this low-carrier-density massless system, R$_{\mathrm{KW}}=$3.2\texttimes 10$^{\mathrm{4}}$\textmu $\Omega $ cm mol$^{\mathrm{2}}$K$^{\mathrm{2}}$J$^{\mathrm{-2}}$. After accounting for the small carrier density in NbAs, this R$_{\mathrm{KW}}$ indicates a suppressed transport scattering rate relative to other metals. \textbf{References:} [1] N. J. Ghimire \textit{et al}., J. Phys.: Condens. Matter \textbf{27}, 152201 (2015) [2] Y. Luo \textit{et al.}, arXiv: 1506.01751 (2015) [3] Y. Luo \textit{et al.}, arXiv: 1510.08538 (2015) [Preview Abstract] |
Thursday, March 17, 2016 3:30PM - 3:42PM |
V29.00006: Topological semimetal Sr$_{\mathrm{1-y}}$Mn$_{\mathrm{1-Z}}$Sb$_{\mathrm{2}}$ Jinyu Liu, Jin Hu, David Graf, S.M.A. Radmanesh, D.J. Adams, Y.L. Zhu, G.F. Chen, X. Liu, J. Wei, I. Chiorescu, L. Spinu, Z.Q. Mao Recent discoveries of topological Weyl semimetals in noncentrosymmetric monopnictides TX (T$=$Ta/Nb, X$=$As/P) [1-4] and photonic crystals$^{\mathrm{\thinspace }}$[5] have generated immense interests since they represent new topological states of quantum matter. Time reversal symmetry (TRS) breaking Weyl semimetal was also recently reported in YbMnBi2 [6]. In this talk, we report a new type of topological semimetal phase arising from two-dimensional Sb layers in Sr$_{\mathrm{1-}}_{y}$Mn$_{\mathrm{1-}}_{z}$Sb$_{\mathrm{2}}$ ($y$, $z $\textless 0.1), which coexists with ferromagnetism. Through quantum transport measurements on this material, we reveal remarkable signatures of relativistic fermions, including light effective quasiparticle mass, high carrier mobility, a $\pi $ Berry phase and valley polarized interlayer conduction. Given Sr$_{\mathrm{1-}}_{y}$Mn$_{\mathrm{1-}}_{z}$Sb$_{\mathrm{2}}$ shows ferromagnetism, it offers a wonderful opportunity to explore the TRS breaking Weyl state. [1]. H. Weng \textit{et al}., \textit{Phys. Rev. X} \textbf{5}, 011029, (2015). [2]. S.M. Huang \textit{et al.}, \textit{Nature Commun.} \textbf{6}, (2015). [3]. S.Y. Xu\textit{ et al., Science } \textbf{349}, 613-617 (2015). [4]. B.Q. Lv\textit{ et al.,} \textit{Phys. Rev. X} \textbf{5}, 031013, (2015). [5]. L. Lu \textit{et al}., \textit{Science} \textbf{349}, 622-624 (2015). [6] S. Borisenko \textit{et al.,} \textit{arXiv:1507.04847}, (2015). [7] J.Y. Liu et al., \textit{arXiv:1507.07978}, (2015). [Preview Abstract] |
Thursday, March 17, 2016 3:42PM - 3:54PM |
V29.00007: Chiral magnetic effect in ZrTe5 Qiang Li, Cheng Zhang, Genda Gu, T. Valla, Dmitri Kharzeev, I. PLETIKOSIC The chiral magnetic effect is the generation of electric current induced by chirality imbalance in the presence of magnetic field. Here we report on the measurement of magneto-transport in zirconium pentatelluride, ZrTe5 that provides a strong evidence for the chiral magnetic effect. Our angle-resolved photoemission spectroscopy experiments show that this material's electronic structure is consistent with a 3D Dirac semimetal. We observe a large negative magnetoresistance when magnetic field is parallel with the current. The measured quadratic field dependence of the magnetoconductance is a clear indication of the chiral magnetic effect. The observed phenomenon stems from the effective transmutation of Dirac semimetal into a Weyl semimetal induced by the parallel electric and magnetic fields that represent a topologically nontrivial gauge field background. We expect that chiral magnetic effect may emerge in a wide class of materials that are near the transition between the trivial and topological insulators. [Preview Abstract] |
Thursday, March 17, 2016 3:54PM - 4:06PM |
V29.00008: Optical evidence for a Weyl semimetal state in pyrochlore Eu2Ir2O7 Andrei Sushkov, Johannes Hofmann, Gregory Jenkins, Dennis Drew, Jun Ishikawa, Satoru Nakatsuji Possible realization of a Weyl semimetallic state with the broken time-reversal symmetry in pyrochlore iridates is still under debate. In the absense of ARPES and neutron data, optical evidence become very important. We found that the THz optical conductivity and temperature dependence of the free carrier response in pyrochlore Eu$_2$Ir$_2$O$_7$ match the predictions for a Weyl semimetal and suggest novel Dirac liquid behavior. The interband optical conductivity vanishes continuously at low frequencies signifying a semimetal. The metal-semimetal transition at $T_N=110$~K is manifested in the Drude spectral weight, which is independent of temperature in the metallic phase, and which decreases smoothly in the ordered phase. The temperature dependence of the free carrier weight below $T_N$ is in good agreement with theoretical predictions for a Weyl semimetal. The fit of experimental Drude weight yields a Fermi velocity 4x10$^7$ cm/s, a logarithmic renormalization scale $\Lambda_L \approx 600$~K, and require a Fermi temperature of ~ 100 K associated with residual unintentional doping to account for the low temperature optical response and dc resistivity. [Preview Abstract] |
Thursday, March 17, 2016 4:06PM - 4:18PM |
V29.00009: Weak Topological Insulators and Composite Weyl Semimetals: $\bm\beta$-Bi$_{\bm 4}$X$_{\bm 4}$ (X=Br,~I) Fan Zhang, Cheng-Cheng Liu, Jin-Jian Zhou, Yugui Yao While strong topological insulators (STI) have been experimentally realized soon after their theoretical predictions, a weak topological insulator (WTI) has yet to be unambiguously confirmed. A major obstacle is the lack of distinct natural cleavage surfaces to test the surface selective hallmark of WTI. With a new scheme, we discover that Bi$_{4}$X$_{4}$ (X=Br,~I), stable or synthesized before, can be WTI with two natural cleavage surfaces, where two anisotropic Dirac cones stabilize and annihilate, respectively. We further find four surface state Lifshitz transitions under charge doping and two bulk topological phase transitions under uniaxial strain. Near the WTI-STI transition, there emerges a novel Weyl semimetal phase, in which the Fermi arcs generically appear at both cleavage surfaces whereas the Fermi circle only appears at one selected surface. [Preview Abstract] |
Thursday, March 17, 2016 4:18PM - 4:30PM |
V29.00010: STM studies of Weyl semimetals Hiroyuki Inoue, Andras Gyenis, Seong Woo Oh, Jian Li, Zhi Jun Wang, Andrei Bernevig, Ni Ni, Ali Yazdani Weyl semimetal exhibits a new gapless topological phase, which is characterized by an even number of band touching points of two non-degenerate bands in the bulk, called Weyl nodes. The surfaces of these compounds are expected to harbor topologically protected surface states with disconnected Fermi surfaces, called Fermi arcs, which connect surface projections of the Weyl nodes with opposing Chern numbers. Among the theoretically predicted Weyl semimetals, there have been several experimental reports on the presence of Fermi arcs in inversion-symmetry-broken monoarsenides, such as TaAs. In this talk, we will present atomic-scale imaging and spectroscopic mapping of the electronic properties of TaAs and other Weyl semimetal candidates. Such measurements have the potential to directly visualize the Fermi arc surface states of these compounds and to probe their properties. This work is supported by ARO and NSF. [Preview Abstract] |
Thursday, March 17, 2016 4:30PM - 4:42PM |
V29.00011: Noncentrosymmetric Line-Node Dirac semimetal CaAgX (X=P, As) Ai Yamakage, Youichi Yamakawa, Yukio Tanaka, Yoshihiko Okamoto Noncentrosymmetric ternary pnictide CaAg$X$ ($X=$P, As) is shown to be a topological line-node semimetal protected solely by mirror-reflection symmetry. The band gap vanishes on a circle in momentum space and surface states emerge within the circle. The $Z_2$ topological invariant $\nu$ related to the surface states is defined from the Berry phase and mirror-reflection symmetry. Extending this study to spin-orbit coupled systems reveals that, compared with CaAgP, a substantial band gap is induced in CaAgAs by large spin-orbit interaction. The resulting states are a topological insulator, in which the $Z_2$ topological invariant is given by 1;000. We have found that the $Z_2$ topological invariants $\nu_0$, $\nu_1$, $\nu_2$, and $\nu_3$ for time-reversal-invariant insulators without spatial-inversion symmetry and with mirror-reflection symmetry are calculated from the $Z_2$ invariant $\nu$ for a line node in the absence of spin-orbit interaction. Namely, line-node Dirac semimetals protected by mirror-reflection symmetry turn into strong topological insulators owing to spin-orbit interaction. [AY, Y. Yamakawa, Y. Tanaka, and Y. Okamoto, arXiv:1510.00202] [Preview Abstract] |
Thursday, March 17, 2016 4:42PM - 4:54PM |
V29.00012: Signature of Instability of Chiral Fermion in Ultraquantum Weyl Semimetal TaAs Chenglong Zhang, Bing-Bing Tong, Zhujun Yuan, Shuang Jia, Chi Zhang In the compound of Tantalum Monoarsenide (TaAs), the novel Weyl fermions are hosted. In our high mobility single crystal samples, the Shubnikov-de Haas oscillations are distinct below the quantum limit (B \textasciitilde 10 T). Beyond the quantum limit, within the configuration-B (electric field E // B-fields), we observed the novel quantum phase transitions in the $\rho _{zz}$-measurements, which may come from the nesting of the vector over a wide range of B-T plane. Due to the similarity to those observed density wave phases or the excitonic states in three-dimensional graphite semimetal, the novel phase may from many body effect beyond the quantum limit. However, in the other measurements (R$_{xx}$ in configuration-A: E $\bot $ B-fields), the novel quantum phase transition does not exist. In general, our results on the density wave phase support its non-trivial topological order. [Preview Abstract] |
Thursday, March 17, 2016 4:54PM - 5:06PM |
V29.00013: Optical spectroscopy study of Weyl Semimetal NbP Jeremy Yang, Yuxuan Jiang, Zhiling Dun, Haidong Zhou, Dmitry Smirnov, Zhigang Jiang Weyl semimetals have attracted much interest lately because of its unique band structure, where conduction band and valence band touch at discrete points. Here, we report on optical spectroscopy study of Weyl semimetal NbP, seeking evidence for the existence of Weyl fermions. Specifically, using Raman spectroscopy we investigate the anisotropic response of Raman-active phonon modes in NbP and compare with Quantum Espresso simulations. Using magneto-infrared spectroscopy in a high magnetic field up to 17.5T, we observe several Landau level transitions and compare with the theoretical model of three-dimensional massless Dirac/Weyl fermions. By combining our data with low-temperature magneto-transport measurement, the magnetic field dispersion of Landau levels in NbP is obtained. [Preview Abstract] |
Thursday, March 17, 2016 5:06PM - 5:18PM |
V29.00014: Non-stoichiometry and Defects in the Weyl Semimetals TaAs, TaP, NbAs, and NbP Tiglet Besara, Daniel A. Rhodes, Kuan-Wen Chen, Qui Zhang, Bin Zheng, Yan Xin, Luis Balicas, Ryan E. Baumbach, Theo Siegrist We report on a structural study of the Weyl semimetals TaAs, TaP, NbAs, and NbP, utilizing diffraction techniques (single crystal x-ray diffraction and energy dispersive spectroscopy) and imaging techniques (transmission electron microscopy). We observe defects of various degrees, leading to non-stoichiometric single crystals of all four semimetals. While TaP displays a very large pnictide deficiency with composition TaP$_{0.83(3)}$ and stacking faults accompanied by anti-site disorder and site vacancies, TaAs displays transition metal deficiency with composition Ta$_{0.92(2)}$As and a high density of stacking faults. NbP also displays pnictide deficiency, yielding composition NbP$_{0.95(2)}$, and lastly, NbAs display very little deviation from a 1:1 composition, NbAs$_{1.00(3)}$, and is therefore recommended to serve as the model compound for these semimetals. [Preview Abstract] |
Thursday, March 17, 2016 5:18PM - 5:30PM |
V29.00015: Chiral Magnetic Effect in Condensed Matter Tonica Valla, Qiang Li, Dmitri Kharzeev, Cheng Zhang, Yuan Huang, Ivo Pletikosic, Alexei Fedorov, Ruidan Zhong, John Schneeloch, Genda Gu The chiral magnetic effect (CME) is the generation of electric current induced by chirality imbalance in the presence of magnetic field - a macroscopic manifestation of the quantum anomaly in relativistic field theory of chiral fermions. The recent discovery of Dirac and Weyl semimetals opened a fascinating possibility to study this phenomenon in condensed matter experiments. Magneto-transport in ZrTe$_5$ shows a strong evidence for CME. Our ARPES experiments show that this material's electronic structure is consistent with a highly anisotropic 3D Dirac semimetal. We observe a large negative magnetoresistance in parallel magnetic field, with the quadratic field dependence of the magneto-conductance - a clear indication of the CME. [Preview Abstract] |
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