Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S43: Weyl Semimetals: Theory and Experiment |
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Sponsoring Units: DCMP Chair: Steven Disseler, National Institute of Standards and Technology Room: Mile High Ballroom 4B |
Thursday, March 6, 2014 8:00AM - 8:12AM |
S43.00001: Weyl nodes in the bandstructure of bcc iron Ivo Souza, David Vanderbilt Weyl points in 3D bandstructures are receiving increasing attention in connection with topological states of matter. In addition, a controversial suggestion has recently appeared\footnote{Y. Chen \textit{et al.}, Phys. Rev. B \textbf{88}, 125110 (2013).} to the effect that isolated touching points between fully occupied and partially occupied bands, which act as monopole sources of Berry curvature, give a non-quantized, non-Fermi-surface contribution to the intrinsic anomalous Hall conductivity of ferromagnets. With these motivations, we carry out a systematic search for Weyl nodes in the bandstructure of bcc Fe using first-principles calculations. We trace the evolution of the Chern index of the fully occupied 2D bands in the $(k_x,k_y)$ plane as a function of $k_z$, and find several touching events taking place at high-symmetry points and lines in the 2D BZ. The amount of Chern-number transfer is analyzed in terms of the symmetry labels of the crossing bands.\footnote{C. Fang \textit{et al.}, Phys. Rev. Lett. \textbf{108}, 266802 (2012).} In addition to conical intersections with a topological charge $q=1$, we find quadratic touching points with $q=2$ that are protected by $C_4^z$ symmetry, as well as lines of degeneracy carrying a net topological charge. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S43.00002: Weyl semimetal emerging from LaBiTe$_3$-class topological insulators Jianpeng Liu, David Vanderbilt We study the topological-to-normal transition in LaBiTe$_3$ and LuBiTe$_3$ by tuning the strength of the spin-orbit coupling (SOC). For centrosymmetric 3D topological insulators (TIs), the strong $Z_2$ index can be changed only by an accidental band touching at an odd number of time-reversal invariant momenta in the Brillouin zone (BZ), achieved at some critical value of an external parameter $\lambda$. These band-touching points (BTPs) are ``Dirac-like,'' carrying zero chiral charge. For general noncentrosymmetric TIs, however, one expects to see a stable Weyl semimetal phase over some finite interval of $\lambda$. As $\lambda$ is varied, one expects first the appearance of $2(2n+1)$ Dirac-like BTPs in the BZ, which then split into pairs of Weyl points carrying opposite chiral charges. These BTPs then migrate in the BZ and finally annihilate after exchanging partners, leaving behind an inverted strong $Z_2$ index. Based on first-principles calculations, we predict that this phenomenon can be realized as the SOC is tuned in LaBiTe$_3$ and LuBiTe$_3$. We also construct a low-energy effective model to describe the topological phases in these materials. Preliminary results suggest that other interesting phases could be observed when a Zeeman field is applied. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S43.00003: Discovery of a Three-dimensional Topological Dirac Semimetal, Na3Bi Y.L. Chen, Z.K. Liu, B. Zhou, Y. Zhang, Z.J. Wang, H.M. Weng, D. Prabhakran, S.-K. Mo, Z.X. Shen, Z. Fang, X. Dai, Z. Hussain Three-dimensional (3D) topological Dirac semimetals (TDSs) represent a novel state of quantum matter that can be viewed as ``3D graphene''. In contrast to two-dimensional (2D) Dirac fermions in graphene or on the surface of 3D topological insulators, TDSs possess 3D Dirac fermions in the bulk. The TDS is also an important boundary state mediating numerous novel quantum states, such as topological insulators, Weyl semi-metals, Axion insulators and topological superconductors. By investigating the electronic structure of Na3Bi with angle resolved photoemission spectroscopy, we discovered 3D Dirac fermions with linear dispersions along all momentum directions for the first time. Furthermore, we demonstrated that the 3D Dirac fermions in Na3Bi were protected by the bulk crystal symmetry. Our results establish that Na3Bi is the first model system of 3D TDSs, which can also serve as an ideal platform for the systematic study of quantum phase transitions between rich novel topological quantum states. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S43.00004: Tunable optical activity as a probe of the chiral anomaly in Weyl semimetals Pavan Hosur, Xiaoliang Qi Weyl semimetals are a three dimensional gapless topological phase in which bands intersect at arbitrary points in the Brillouin zone. These points carry a topological quantum number known as the ``chirality'' and always appear in pairs of opposite chiralities. The notion of chirality leads to the ``chiral anomaly,'' according to which charge associated with a given chirality is not conserved in an electromagnetic field E$\cdot$B. Since Weyl nodes are separated in momentum space, it is difficult for ordinary real space probes to probe this anomaly. Here, we propose a technique to probe the chiral anomaly optically. In particular, we observe that an E·B field induces a form of optical activity known as gyrotropy, which is directly proportional to the chirality of the underlying Hamiltonian, in Weyl semimetals. This dynamically induced gyrotropy can then be seen in routine Faraday and Kerr effect experiments. We estimate typical sizes of these effects and find them to be within experimental reach. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S43.00005: Weyl Semimetal in Hg$_{1-x-y}$Cd$_x$ Mn$_y$ Te Daniel Bulmash, Chao-Xing Liu, Xiao-Liang Qi We study strained Hg$_{1-x-y}$Cd$_x$Mn$_y$Te in a magnetic field using a $\mathbf{k}\cdot\mathbf{p}$ model and predict that the system is a Weyl semimetal with two nodes in an experimentally reasonable region of the phase diagram. We also predict two signatures of the Weyl semimetal phase which arise from tunability of the Weyl node splitting. First, we find that the Hall conductivity is proportional to the average Mn ion spin and thus is strongly temperature dependent. Second, we find an unusual magnetic field angle dependence of the Hall conductivity; in particular, we predict a peak in $\sigma_{xy}$ as a function of field angle in the $xz$-plane and a finite $\sigma_{yz}$ as the $x$-component of the field goes to 0. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S43.00006: Anomalous domain-wall conductance in pyrochlore-type Nd$_{2}$Ir$_{2}$O$_{7}$ on the verge of metal-insulator transition Kentaro Ueda, Jun Fujioka, Youtarou Takahashi, Takehito Suzuki, Shintaro Ishiwata, Yasujiro Taguchi, Masashi Kawasaki, Yoshinori Tokura Pyrochlore iridates have attracted much attention since the interplay between electron correlation and strong spin-orbit coupling can lead to various topologically-nontrivial phases such as Weyl semimetal. The Weyl semimetal phase shows k-linear dispersing excitations as described by the Weyl equation in the three-dimensional bulk and remarkable edge states (Fermi arcs) at the surface or domain boundary. Recent theoretical studies have shown that such metallic edge modes can survive at the magnetic domain wall even in the fully-gapped bulk state subsequent to the pair-annihilation of Weyl fermions. In this study, we have investigated the charge transport and the low-energy charge dynamics originating from the magnetic domain walls in pyrochlore-type Nd$_{2}$Ir$_{2}$O$_{7}$, whose bulk is a fully-gapped antiferromangnetic insulator in vicinity to Weyl semimetal. We observed that the antiferromagnetic domain wall is metallic, despite the fully-gapped insulating state in the bulk by means of charge transport and optical measurements. We discuss the origin of such highly conductive magnetic domain wall in terms of edge states inherent to the Weyl semimetal. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S43.00007: Synthesis and Characterization of Epitaxial Nd2Ir2O7 Thin Films Fabricated with Off-Axis Magnetron Sputtering James Gallagher, Robert Williams, Sam Wagers, David McComb, Fengyuan Yang There has been a recent interest in the study of 5d transition metal oxides due to the large spin-orbit coupling. In particular, the pyrochlore iridates $A_2Ir_2O_7$ are a group of interesting materials with geometric frustration of magnetic moments and large spin-orbit coupling, allowing the possibility of Mott insulators, topological insulators and Weyl semimetals. $Nd_2Ir_2O_7$ is of particular interest because it is near the metal-insulator transition, making it a potential candidate for search of topological insulators and Weyl semimetals. We grow pure phase, fully epitaxial $Nd_2Ir_2O_7$ thin films using ultrahigh vacuum off-axis magnetron sputtering on yittrium-stabalized zirconia. X-ray diffraction verified that the pure phase epitaxial relationship of the film to the substrate. Scanning transmission electron microscopy (STEM) images reveal the pyrochlore ordering between Nd and Ir and epitaxial nature of the film. Transport measurements show that the films undergo a metal-insulator transition around 70 K, up from around 35 K in the bulk. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S43.00008: Observation of a topological 3D Dirac semimetal phase in high-mobility Cd$_{3}$As$_{2}$ M. Neupane, S.-Y. Xu, R. Sankar, N. Alidoust, G. Bian, Chang Liu, I. Belopolski, T.-R. Chang, H.-T. Jeng, H. Lin, A. Bansil, Fangcheng Chou, M.Z. Hasan Experimental identification of three-dimensional (3D) Dirac semimetals in solid state systems is critical for realizing exotic topological phenomena and quantum transport. Using high-resolution angle-resolved photoemission spectroscopy, we performed systematic electronic structure studies on well-known compound Cd$_{3}$As$_{2}$. For the first time, we observe a highly linear bulk Dirac cone located at the Brillouin zone center projected onto the (001) surface, which is consistent with a 3D Dirac semimetal phase in Cd$_{3}$As$_{2}$. Remarkably, an unusually high Dirac Fermion velocity is seen in samples where the mobility far exceeds 20,000 cm$^{2}$/V.s suggesting that Cd$_{3}$As$_{2}$ can be a promising candidate as a hypercone analog of graphene in many device-applications, which can also incorporate topological quantum phenomena in a large gap setting. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S43.00009: Shubnikov-de Haas Oscillations in 3D Dirac semimetal Cd3As2 Shih-Ting Guo, R. Sankar, Yung-Yu Chien, Horng-Tay Jeng, F.C. Chou, Wei-Li Lee Cadmium arsenide (Cd3As2) was known for its inverted band structure and ultra-high electron mobility. It has been theoretically predicted to exhibit a 3D Dirac semimetal phase containing degenerate Weyl nodes, which was recently confirmed by ARPES experiments. By conducting magneto-transport measurements in high quality single crystals of Cd3As2, we found a single frequency F $=$ 67 Tesla in the Shubnikov-de Haas (SdH) oscillations giving an effective mass m* $=$ 0.057 me with electron mobility as high as 99,500 cm$^{2}$/V-sec, which is consistent with the calculated 3D Dirac semimetal band. In a certain field orientation, we found a splitting of the SdH oscillation frequency in the FFT spectrum, which infers a possible lifting of the degeneracies. Detailed magnetotransport data in several doped and undoped single crystals of Cd3As2 will be presented and discussed. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S43.00010: Transport properties of Dirac semimetal Cd$_3$As$_2$ Tian Liang, Quinn Gibson, Jun Xiong, Minhao Liu, Maximilian Hirschberger, Robert Cava, Nai Phuan Ong The semimetal Cd$_3$As$_2$ has emerged as an attractive candidate for a Dirac semimetal. A recent LDA calculation reveals that, at the Fermi energy, it has two bulk Dirac nodes which straddle the $\Gamma$ point along the k$_z$ axis. The Dirac nodes were recently observed by ARPES. We have made extensive transport measurements of Cd$_3$As$_2$. Because of possible Cd vacancy disorder in the very large unit cell (160 atoms), the SdH oscillations reveal a quantum lifetime that is moderately damped. Despite the disorder, the observed resistivity $\rho$ in some crystals displays a RRR of 1000. At 4 K, the residual resistivity is anomalously low (30 n$\Omega$ cm). We estimate that the mobility exceeds 10$^6$ cm$^2$V$^{-1}$s$^{-1}$. A magnetic field H strongly increases $\rho$ by factors of 100 to 1000 at 10 Tesla. This giant magnetoresistance (MR) is highly anisotropic. The MR is largest when H is perpendicular to the axis (110) and minimal when H is $ \parallel $(110). We will discuss possible origins of this unusual anisotropic giant MR. We also discuss the possibility of detecting an enhanced longitudinal MR associated with charge pumping between Weyl nodes (the chiral anomaly). [1] Wang et al. arXiv:1305.6780 [2] Borisenko et al. arXiv:1309.7978 [3] Neupane et al. arXiv:1309.7892 [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S43.00011: Superconducting Proximity Effect in the 3D Dirac Semimetal Cd3As2 Jun Xiong, Quinn Gibson, Tian Liang, Robert J. Cava, Nai Phuan Ong Cd3As2 is a semimetal that is a candidate for a 3D Dirac semimetal. Angle-resolved photoemission has observed bulk, massless Dirac nodes whose Fermi velocity is in good agreement with the value determined by quantum oscillation measurements. A number of novel transport features have been identified. The system is protected from 2kF backscattering despite a short quantum lifetime. Theory predicts the existence of surface states with Fermi arcs. We search for evidence for the arcs using Nb pads to proximitize the surface electrons. We have fabricated a Nb-Cd3As2-Nb hybrid structure to study the Josephson effect. Given the long electronic mean-free-path in Cd3As2, we have made Josephson junctions with different lengths to study the coherent transport of cooper pairs in 3D Dirac semimetal. To reveal the exotic nature of 3D Dirac electrons, we have measured the dc I-V curves of the junction, in the presence of a weak magnetic field. Supported by NSF-MRSEC (DMR 0819860), Army Research Office (ARO W911NF-11- 1-0379) and DARPA under SPAWAR program (Grant N66001-11-1-4110). [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S43.00012: Electronic Indication of Three-Dimensional Dirac Cone in Cd3As2 from Angle-Resolved Photoemission H.M. Yi, C.Y. Chen, Y.G. Shi, Z.J. Wang, Z.J. Xie, Y. Feng, A.J. Liang, S.L. He, J.F. He, Y.Y. Peng, X. Liu, Y. Liu, L. Zhao, G.D. Liu, X.L. Dong, J. Zhang, Arita M, Shimada K, Namatame H, Taniguchi M, Z.Y Xu, C.T. Chen, X. Dai, Z. Fang, X.J. Zhou The narrow gap semiconductor, Cd3As2 is well known to have inverted band structure. It draws much attention recently because of its none-trivial properties which is predicted to be a three-dimensional Dirac semi-metal. Analogous to two-dimensional layered material graphene, Cd3As2 can be viewed as a 3D version of Dirac Fermion material whose bulk conduction and valence band contact only at discrete (Dirac) points in the Brillouin zone and disperse linearly in all directions around these critical points. Here we report direct observation of three-dimensional Dirac cones in Cd3As2 by using high resolution angle resolved photoemission spectroscopy(ARPES). Our ARPES results reveal the unique band structures for this topological 3D Dirac material that will provide key information in understanding and exploring exotic phenomenon in Cd3As2. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S43.00013: Atomic scale imaging and spectroscopic investigation of Cd$_{3}$As$_{2}$ with the STM Sangjun Jeon, Brian Zhou, Andras Gyenis, Quinn Gibson, Robert Cava, Ali Yazdani Cd$_{3}$As$_{2}$ is known for having high carrier mobility and inverted HgTe-type band structure and is theoretically expected to be a three-dimensional Dirac semimetal. Recently, ARPES measurements on this material show the band structure to have linear dispersions in three dimensions to form a three-dimensional Dirac conelike structure. Much remains to be understood about the nature of the electronic states in this compound. Here we probe with high spatial and energy resolution the electronic structure of Cd$_{3}$As$_{2}$ using STM measurements. Cd$_{3}$As$_{2}$ single crystals grown by Bridgman method were cleaved in UHV environment and investigated in cryogenic STM. We will report on various STM measurements to determine the atomic structure of the cleaved surfaces and use spectroscopic measurements to probe its unique bulk and surface electronic properties. Work supported by ARO-MURI. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S43.00014: STM Spectroscopic Mapping of Cd$_{3}$As$_{2}$ Andras Gyenis, Sangjun Jeon, Brian Zhou, Benjamin Feldman, Quinn Gibson, Robert Cava, Ali Yazdani Recently, theoretical studies and experimental findings suggest that the well-known semiconductor, cadmium arsenide (Cd$_{3}$As$_{2})$ is one of the realizations of the three-dimensional Dirac semimetal state of matter. This new topological phase has various exotic physical properties originating from the existence of the single pair of bulk Dirac points and the unusual Fermi arcs of the surface states. To investigate the unique electronic structure of Cd$_{3}$As$_{2}$, we perform scanning tunneling spectroscopy measurements on samples with different doping levels. Similar to STM experiments on other materials, spectroscopic mapping with the STM can be used to understand the role of the impurities on the surface and to examine scattering transitions, which helps to establish topological properties of the 3D Dirac state. Work supported by ARO-MURI [Preview Abstract] |
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