Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S7: Focus Session: Dirac & Weyl Semimetals |
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Sponsoring Units: DMP DCMP Chair: Madhab Neupane, Princeton University Room: 006B |
Thursday, March 5, 2015 8:00AM - 8:12AM |
S7.00001: Prediction of a strain-tunable 2D Topological Dirac semimetal in monolayers of black phosphorus Xiuwen Zhang, Qihang Liu, Alex Zunger N-dimensional Topological Nonmetals (TNM) such as N $=$ 2D HgTe/CdTe quantum wells or N $=$ 3D Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ have a finite (often tiny) band gap between occupied and unoccupied bands, and show conductive Dirac cones in their N-1 dimensional geometric boundaries. On the other hand, examples of topological semimetals (TSM) are known for 3D solids (Cd$_{\mathrm{3}}$As$_{\mathrm{2}})$ where they have Dirac cones in the 3D system itself. Using density functional calculation of bands and the topological invariant Z$_{\mathrm{2}}$ we predict the existence of 2D topological Dirac semimetal in few monolayers of strain tuned black phosphorus (BP), with Dirac cones induced by band inversion. The band structures of few monolayers and bulk crystal of BP under a few percent biaxial and uniaxial strains were calculated using state-of-art electronic structure methods. The critical strain of the transition to TSM was found to decrease as the layer thickness increases. We will discuss the protection of the Dirac cones by the crystalline symmetry in the 2D TSM and the manipulation of crystalline symmetry, which induces further topological phase transitions. Supported by the NSF-DMREF-13-34170. [Preview Abstract] |
Thursday, March 5, 2015 8:12AM - 8:24AM |
S7.00002: Interaction effects of a topological Dirac semimetal Na3Bi Ruixing Zhang, Jimmy Hutasoit, Chaoxing Liu We study the interaction effects of a topological Dirac semimetal Na3Bi based on the mean field theory. The phase diagram can be classified by two kinds of chiral-symmetry-breaking order parameters: nematic orders that break rotational symmetry and charge-density-wave (CDW) order that break translational symmetry. Under strong magnetic field, gapless Landau levels will be formed and result in instabilities due to the above order parameters. These order parameters are generally complex, and are identified as complex mass terms, which introduce axions into the low energy theory. The possible experimental consequence is also studied. [Preview Abstract] |
Thursday, March 5, 2015 8:24AM - 8:36AM |
S7.00003: Density of states and transport in Weyl semimetals with long-range disorder Dmytro Pesin, Alex Levchenko We consider the density of states in a Weyl semimetal with long-range Coulomb impurities. We show that at energies close to the nodal point the motion of electrons is essentially classical, and compute the density of states at low energies for the cases of self-consistent screening, and screening by non-Weyl bands. As an alternative to the recent renormalization-group calculations, we develop a diagrammatic technique that is capable of capturing quantum corrections to the classical result, and determine the applicability range of the classical treatment, as well as the energy dependence of the density of states away from the nodal points. Finally, the obtained results are contrasted with those obtained from the self-consistent Born approximation, and the implications for the electrical, thermal and thermomagnetic transport properties of the Weyl semimetal are discussed. [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 8:48AM |
S7.00004: Theory of Symmetry Protected 2D Dirac Semimetals and Derivative Topological States Steve Young, Charles Kane We present the theory of symmetry-protected 2D Dirac semimetals. These systems are distinguished from graphene by the presence of spin-orbit coupling, under which the latter becomes a quantum spin hall insulator. 2D Dirac semimetals exhibit behavior distinct from both graphene and their three-dimensional counterparts. We discuss the symmetry requirements for such systems, and using the simplest tight-binding model satisfying them, describe their properties, as well as the states that result from relaxing various constraints, including Weyl semimetal and topological insulator states. Additionally, we provide suggestions for realizing these systems based on Density Functional Theory calculations. [Preview Abstract] |
Thursday, March 5, 2015 8:48AM - 9:00AM |
S7.00005: Thermoelectric properties of Weyl and Dirac semimetals Pontus Laurell, Rex Lundgren, Gregory Fiete We study the electronic contribution to the thermal conductivity and the thermopower of Weyl and Dirac semimetals using a semiclassical Boltzmann approach. We investigate the effect of various relaxation processes including disorder and interactions on the thermoelectric properties, and also consider doping away from the Weyl or Dirac point. We find that the thermal conductivity and thermopower have a dependence on the chemical potential that is characteristic of the linear electronic dispersion, and that electron-electron interactions modify the Lorenz number. For the interacting system, we also use the Kubo formalism to obtain the transport coefficients, finding exact agreement with the Boltzmann approach at high temperatures. We also consider the effect of electric and magnetic fields on the thermal conductivity in various orientations with respect to the temperature gradient. Notably, when the temperature gradient and magnetic field are parallel, we find a large contribution to the longitudinal thermal conductivity that is quadratic in the magnetic field strength, similar to the magnetic field dependence of the longitudinal electrical conductivity due to the presence of the chiral anomaly when no thermal gradient is present. [Preview Abstract] |
Thursday, March 5, 2015 9:00AM - 9:12AM |
S7.00006: Kondo effect and non-Fermi liquid behavior in Dirac and Weyl semimetals E. Rossi, Alessandro Principi, Giovanni Vignale We study the Kondo effect in three-dimensional (3D) Dirac materials and Weyl semimetals [1]. We find the scaling of the Kondo temperature with respect to the doping n and the coupling J between the moment of the magnetic impurity and the carriers of the semimetal. We find that when the temperature is much smaller than the Kondo temperature the resistivity due to the Kondo effect scales as the n to the -4/3.We also study the effect of the interplay of long-range scalar disorder and Kondo effect. In the presence of disorder-induced long-range carrier density inhomogeneities the Kondo effect is not characterized by a Kondo temperature but by a distribution of Kondo temperatures. We obtain the expression of such distribution and show that its features cause the appearance of strong non-Fermi liquid behavior. Finally we compare the properties of the Kondo effect in 3D Dirac materials and 2D Dirac systems like graphene and topological insulators. \\[4pt] [1] Alessandro Principi, Giovanni Vignale, E. Rossi, arXiv:1410.8532. \\[4pt] Work supported by ONR-N00014-13-1-0321. [Preview Abstract] |
Thursday, March 5, 2015 9:12AM - 9:48AM |
S7.00007: Visualizing the Electronic Structure of Topological Dirac Semimetals Invited Speaker: Yulin Chen Three-dimensional (3D) topological Dirac semimetals (TDSs) represent an unusual state of quantum matter that can be viewed as ``3D graphene.'' In contrast to 2D Dirac fermions in graphene or on the surface of 3D topological insulators, TDSs possess 3D Dirac fermions in the bulk. Moreover, a TDS can potentially be driven into other exotic phases (such as Weyl semimetals, axion insulators or topological superconductors), making it a unique parent compound for the study of these states and the phase transitions between them. By investigating the electronic structure of Na3Bi and Cd3As2 with angle-resolved photoemission spectroscopy including kz information, we discovered bulk (3D) Dirac fermions with linear dispersions along all three momentum directions in both materials. Furthermore, we can demonstrate the robustness of these 3D Dirac fermions against in situ surface doping, showing the protection of the bulk crystal symmetry. These findings establish both materials as model systems of 3D TDSs, which can also serve as ideal platforms for exploring exotic physical phenomena and novel applications. [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:00AM |
S7.00008: Tendency to Localization in Interacting Weyl Semimetals Haizhou Lu, Shunqing Shen Weyl semimetals are novel topological states of quantum matter, in which electrons or Weyl fermions are robust against impurity or disorder, and tend to be delocalized. In a weak external magnetic field, a negative magnetoconductivity is found to be proportional to the square root of magnetic field at low temperatures, giving the signature of the delocalization. However, here we demonstrate that the metallic and delocalization behavior of Weyl semimetals can be sabotaged by electron-electron interaction and inter-valley effects. An ``insulating'' tendency is therefore illustrated in disordered and interacting Weyl semimetals. Reference: Hai-Zhou Lu and Shun-Qing Shen, arXiv:1411.2686 [Preview Abstract] |
Thursday, March 5, 2015 10:00AM - 10:12AM |
S7.00009: Topological Imbert-Fedorov shift in Weyl semimetals Qing-Dong Jiang, Hua Jiang, Haiwen Liu, Qing-feng Sun, X.C. Xie When a beam of light (photons) is reflected at an interface, the wave nature of photons can result in spatial shifts at the interface in the plane of incidence (longitudinal shift) and normal to the plane (transverse shift), which are referred as Goos-H\"anchen (GH) shift and Imbert-Fedorov (IF) shift, respectively. As the massless fermionic cousin of photons, Weyl fermions are expected to share certain similar characteristics as photons. Here, we report the GH and IF effects in Weyl semimetals---a promising material harboring low energy Weyl fermions. Our results show that GH effect in WSMs is analogous to that discovered in a 2D relativistic material---graphene; however, the IF effect has no 2D counterpart, since it is genuinely a 3D effect. We emphasize that the IF shift actually originates from the topological effect of the systems, and can further lead to valley-dependent anomalous velocities. Experimentally, topological related IF shift can be utilized to characterize the Weyl semimetals and further to measure the Berry curvature. The valley-dependent anomalous velocities provide new ways for designing valleytronic devices of high efficiency. [Preview Abstract] |
Thursday, March 5, 2015 10:12AM - 10:24AM |
S7.00010: Chiral Anomaly and Diffusive Magnetotransport in Weyl Metals Anton Burkov We present a microscopic theory of diffusive magnetotransport in Weyl metals and clarify its relation to chiral anomaly. We derive coupled diffusion equations for the total and axial charge densities and show that chiral anomaly manifests as a magnetic-field-induced coupling between them. We demonstrate that a universal experimentally-observable consequence of this coupling in magnetotransport in Weyl metals is a quadratic negative magnetoresistance, which will dominate all other contributions to magnetoresistance under certain conditions. [Preview Abstract] |
Thursday, March 5, 2015 10:24AM - 10:36AM |
S7.00011: Plasmon mode as a detection of the chiral anomaly in 3D Weyl semimetals Jianhui Zhou, Hao-ran Chang, Di Xiao Weyl semimetals (SMs) are one kind of three-dimensional gapless SMs with nontrivial topology in the momentum space. The chiral anomaly in Weyl SMs manifests as a charge imbalance between the Weyl nodes of opposite chiralities induced by parallel electric and magnetic fields. We investigate the chiral anomaly effect on the plasmon mode in Weyl SMs within the RPA. We prove that the chiral anomaly gives rise to a new plasmon mode in intrinsic Weyl SMs. The chiral anomaly leads to some exotic properties in the plasmon dispersion in doped Weyl SMs. Consequently, the unconventional plasmon mode acts as a signature of the chiral anomaly in Weyl SMs, by which the spectrum of plasmon provides a proper way to detect the Lifshitz transition. Our work sheds light on the probing of the chiral anomaly in 3D Weyl SMs via the plasmon mode. The tunability of plasmons due to the chiral anomaly also makes Weyl SMs promising candidates for plasmonics. Reference:arXiv:1408.4876 [Preview Abstract] |
Thursday, March 5, 2015 10:36AM - 10:48AM |
S7.00012: Terahertz conductivity study of predicted Weyl semimetal Nd$_2$Ir$_2$O$_7$ and Eu$_2$Ir$_2$O$_7$ thin films Matthew T. Warren, J.C. Gallagher, T.T. Mai, J. Brangham, F. Yang, C.M. Morris, N.P. Armitage, R. Vald\'es Aguilar There is currently a growing interest in identifying materials with novel topological properties outside of the s- or p-orbital based bismuth chalcogenide topological insulators. One such proposal is the pyrochlore iridate materials $R_2$Ir$_2$O$_7$, where $R$ is a lanthanide series atom or yttrium. These have been predicted to be Weyl semimetals, containing exotic Fermi arc surface states and also an anomalous Hall effect. We have studied the temperature dependent terahertz conductivity of thin films of Nd$_2$Ir$_2$O$_7$ and Eu$_2$Ir$_2$O$_7$ grown by a novel off-axis sputtering technique. We find a close correspondence between DC transport properties and the extracted Drude parameters from fits to the THz conductivity. We will discuss these results in light of the predicted Weyl semimetal state of these materials. [Preview Abstract] |
Thursday, March 5, 2015 10:48AM - 11:00AM |
S7.00013: Coulomb disorder in three-dimensional Dirac materials Brian Skinner In three-dimensional materials with a Dirac spectrum, weak short-ranged disorder is essentially irrelevant near the Dirac point. This is manifestly not the case for Coulomb disorder, where the long-ranged nature of the potential produced by charged impurities implies large fluctuations of the disorder potential even when impurities are sparse, and these fluctuations are screened by the formation of electron/hole puddles. Here I outline a theory of such nonlinear screening of Coulomb disorder in three-dimensional Dirac systems, and present results for the typical magnitude of the disorder potential, the corresponding density of states, and the size and density of electron/hole puddles. The resulting conductivity is also discussed. [Preview Abstract] |
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