Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session P44: Dirac and Weyl Semimetals: Theory III |
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Sponsoring Units: DMP Chair: Wei Zhu, Los Alamos National Laboratory Room: 391 |
Wednesday, March 15, 2017 2:30PM - 2:42PM |
P44.00001: Electromagnetic signatures of the chiral anomaly in Weyl semimetals Edwin Barnes, Jean Heremans, Djordje Minic Weyl semimetals are predicted to realize the three-dimensional axial anomaly first discussed in particle physics. The anomaly leads to unusual transport phenomena such as the chiral magnetic effect in which an applied magnetic field induces a current parallel to the field. Here we investigate diagnostics of the axial anomaly based on the fundamental equations of axion electrodynamics. We find that materials with Weyl nodes of opposite chirality and finite energy separation immersed in a uniform magnetic field exhibit an anomaly-induced oscillatory magnetic field with a period set by the chemical potential difference of the nodes. In the case where a chemical potential imbalance is created by applying parallel electric and magnetic fields, we find a suppression of the magnetic field component parallel to the electric field inside the material for rectangular samples, suggesting that the chiral magnetic current opposes this imbalance. For cylindrical geometries, we instead find an enhancement of this magnetic field component along with an anomaly-induced azimuthal component. We propose experiments to detect such magnetic signatures of the axial anomaly. [Preview Abstract] |
Wednesday, March 15, 2017 2:42PM - 2:54PM |
P44.00002: Anomalous Hall currents induced by time-varying magnetic fields in a Weyl metal phase Iksu Jang, Jae-Ho Han, Ki-Seok Kim A pair of Weyl points can be identified with a magnetic monopole and anti-monopole pair in momentum space. The Berry curvature generated by this monopole pair plays an essential role in anomalous transport phenomena of Weyl metals. Recalling that the relative position of the monopole pair is controlled by external magnetic fields, we apply time-varying magnetic fields to Weyl metals and investigate the role of an oscillating monopole pair in the transport. Based on Boltzmann transport theory with a topologically modified Drude model, we find that the oscillating monopole pair gives rise to anomalous Hall currents. We classify these Hall currents in all possible situations. We reveal that these anomalous Hall effects are involved with an extended chiral anomaly, where a time-varying chiral gauge field to represent the relative-distance vector of the monopole pair appears in the anomaly equation. [Preview Abstract] |
Wednesday, March 15, 2017 2:54PM - 3:06PM |
P44.00003: Linear Magnetochiral effect in Weyl Semimetals Alberto Cortijo We describe the presence of a linear magnetochiral effect in time reversal breaking Weyl semimetals. The magnetochiral effect consists in a simultaneous linear dependence of the magnetotransport coefficients with the magnetic field and a momentum vector. This simultaneous dependence is allowed by the Onsager reciprocity relations, being the separation vector between the Weyl nodes the vector that plays such role. This linear magnetochiral effect constitutes a new transport effect associated to the topological structures linked to time reversal breaking Weyl semimetals. [Preview Abstract] |
Wednesday, March 15, 2017 3:06PM - 3:18PM |
P44.00004: Kinetic equation approach to chiral anomaly driven magnetoconductivity of Weyl semimetals Xaver Neumeyer, Vladimir Zyuzin We study electric field and temperature gradient driven magnetoconductivity of a Weyl semimetal system. To analyze the responses, we utilize the kinetic equation with semiclassical equation of motions affected by the Berry curvature and orbital magnetization of the wave-packet. Apart from known positive quadratic magnetoconductivity, we show that due to chiral anomaly, the magnetconductivity can become non-analytic function of the magnetic field, proportional to 3/2 power of the magnetic field at finite temperatures. We also show that time-reversal symmetry breaking tilt of the Dirac cones results in linear magnetoconductivity. This is due to one-dimensional chiral anomaly the tilt is responsible for. [Preview Abstract] |
Wednesday, March 15, 2017 3:18PM - 3:30PM |
P44.00005: Quantum Kinetic Theory of Magnetotransport in Weyl Metals Akihiko Sekine, Dimitrie Culcer, Allan MacDonald The Weyl semimetal has attracted much attention as a topological phase of matter. Its topological nature is experimentally manifested by the existence of chiral anomalies, and in particular by a negative magnetoresistance (or equivalently a positive quadratic magnetoconductance) for parallel electric and magnetic fields. Although several theoretical studies have derived expressions for the chiral-anomaly-induced magnetoconductance, the conditions required for its appearance have not yet been fully understood. In this talk, we present a general theory of low-field magnetotransport based on a quantum kinetic equation. Our theory naturally includes the Berry phase effects that arise from topologically nontrivial band structures and are often discussed in terms of semiclassical wavepacket dynamics, and properly accounts for their subtle interplay with electron scattering effects. Using our theory, we attempt to understand from the quantum kinetic theory viewpoint when the chiral anomaly emerges in condensed matter systems. As an illustration, we derive an explicit expression for the quadratic magnetoconductance implied by a simple microscopic model of Weyl semimetals, demonstrating explicitly that the chiral anomaly emerges only when intervalley scattering is sufficiently weak. [Preview Abstract] |
Wednesday, March 15, 2017 3:30PM - 3:42PM |
P44.00006: $\mathbb Z_2$ and Chiral Anomalies in Topological Dirac Semimetals Anton Burkov, Yong Baek Kim We demonstrate that topological Dirac semimetals, which possess two Dirac nodes, separated in momentum space along a rotation axis and protected by rotational symmetry, exhibit an additional quantum anomaly, distinct from the chiral anomaly. This anomaly, which we call the $ \mathbb Z_2$ anomaly, is a consequence of the fact that the Dirac nodes in topological Dirac semimetals carry a $\mathbb Z_2$ topological charge. The $\mathbb Z_2$ anomaly refers to nonconservation of this charge in the presence of external fields due to quantum effects and has observable consequences due to its interplay with the chiral anomaly. We discuss possible implications of this for the interpretation of magnetotransport experiments on topological Dirac semimetals. We also provide a possible explanation for the magnetic field dependent angular narrowing of the negative longitudinal magnetoresistance, observed in a recent experiment on Na$_3$Bi. [Preview Abstract] |
Wednesday, March 15, 2017 3:42PM - 3:54PM |
P44.00007: Intrinsic Magnetoconductivity of Non-magnetic Metals Yang Gao, Shengyuan Yang, Qian Niu In this talk, I will show an intrinsic magnetoconductivity for general three-dimensional non-magnetic metals within the Berry-curvature-corrected semiclassical and Boltzmann framework. It is intrinsic in the sense that its ratio to the zero-magnetic-field conductivity is fully determined by the intrinsic band properties, independent of the transport relaxation time, showing a clear violation of Kohler's rule. Remarkably, this contribution can generally be positive for the longitudinal configuration, providing a new mechanism for the appearance of positive longitudinal magnetoconductivity besides the chiral anomaly effect. [Preview Abstract] |
Wednesday, March 15, 2017 3:54PM - 4:06PM |
P44.00008: Electromagnetic response of a Weyl semimetal with coexisting density waves Zachary Raines, Victor Galitski We consider a minimal model of a Weyl semimetal simultaneously perturbed by two different types of translation symmetry breaking order. The system exhibits a non-trivial electromagnetic response to such terms which can be obtained via Fujikawa's chiral rotation technique in the same way as the chiral anomaly. Such a response is similar to the usual topological term but with the translation symmetry breaking terms combining to act in the place of the magnetic field. We comment upon how the properties of this state might be observed experimentally. [Preview Abstract] |
Wednesday, March 15, 2017 4:06PM - 4:18PM |
P44.00009: Theory of transport property of density wave phases in three-dimensional metals and semimetals under high magnetic field Xiao-Tian Zhang, Ryuichi Shindou Three-dimensional (3D) metals/semimetals under magnetic field have an instability toward density wave (DW) orderings. An effective boson model for the DW phases takes forms of XY models with/without Potts terms. A conductivity is calculated in the DW phases with disorders within Born approximation. A single-particle imaginary-time Green function is identified with a partition function of 3D XY models in the presence of pairs of magnetic monopoles. Using this relation, electronic spectral function is calculated near the DW phase transition by duality mapping. The calculated result shows the absence of well-defined single-particle excitations in the DW/normal phases near the transition. Based on this observation, we discuss a temperature-dependence of an in-plane conductance due to chiral surface Fermi arc states. [Preview Abstract] |
Wednesday, March 15, 2017 4:18PM - 4:30PM |
P44.00010: Accurate Tight-Binding Hamiltonians : Topological Insulators Marcio Costa, Roberto Bechara, Marco Nardelli, Adalberto Fazzio In this work we report results of transport calculations for Topological Insulators using the recently developed pseudoatomic orbital projection technique(1-2). We construct a tight-biding Hamiltonian extract from an first-principles calculation. The Spin-Orbit effect is considered in two different forms. Direct from a DFT calculation, which involves a non collinear, and computational demanding, calculation. A more efficient approach, with comparable accuracy, is to introduce the SOC in a scalar relativistic tight-biding Hamiltonian using first order perturbation theory. We applied this methodology for 2D and 3D Topological Insulators. [1]- Phys. Rev. B 94, 165166 (2016). [2]- Phys. Rev. B 88, 165127 (2013). [Preview Abstract] |
Wednesday, March 15, 2017 4:30PM - 4:42PM |
P44.00011: Topological states and quantized current in helical organic molecules Ai-Min Guo, Qing-Feng Sun Topological quantum states have been attracting intense interest in condensed matter and materials physics. Here, we report a theoretical study of electron transport along helical organic molecules subject to an external electric field, which is perpendicular to molecular helix axis. Our results reveal that topological states can appear in single-helical molecules as well as double-stranded DNA under the perpendicular electric field. In particular, a topological charge pumping can be realized by rotating the electric field in the transverse plane. During each pumping cycle, an integer number of electrons can transport across the helical molecules at zero bias voltage, with pumped current being quantized. This quantized current is associated with the Chern number of occupied energy bands, and constitutes multiple plateaus by scanning either the Fermi energy or the bias voltage. Besides, the plateaus of quantized current persist in a very wide range of model parameters, since the edge states are topologically protected. These results could pave the way to explore topological states and quantized current in the biological systems and the helical molecules, and help in designing stable molecular devices. [Preview Abstract] |
Wednesday, March 15, 2017 4:42PM - 4:54PM |
P44.00012: A Case Study Of Organic Dirac Materials -- Benjamin Commeau, Matthias Geilhufe, Gayanath Fernando, Alexander Balatsky Dirac Materials are characterized by linear band crossings within the electronic band structure. Most research of Dirac materials has been dedicated towards inorganic materials, e.g., binary chalcogenides as toplogical insulators, the Weyl semimetal TaAs or graphene. The purpose of this study is to investigate the formation of Dirac points in organic materials under pressure and mechanical strain. We study multiple structural phases of the organic charge-transfer salt (BEDT-TTF)2I3. We numerically calculate the relaxed band structure near the Fermi level along different k-space directions. Once the relaxed ion structure is obtained, we pick different cell parameters to shrink and investigate the changes in the band structure. We discuss band structure degeneracies protected by crystalline and other symmetries, if any. Quantum Espresso and VASP codes were used to calculate and validate our results. [Preview Abstract] |
Wednesday, March 15, 2017 4:54PM - 5:06PM |
P44.00013: Quantized Zak Phase and Topological Surface Charge originating from the Nodal Line Motoaki Hirayama, Ryo Okugawa, Shuichi Murakami Recent study for the topological phenomena reveals that the singularity of the band structure in the k-space plays a significant role for both the bulk and surface properties. The semimetals having such a singularity include the Dirac semimetals, Weyl semimetals [1], and the nodal-line semimetals [2]. In this study, we focus on the nodal-line physics. One of the typical origin of the nodal-line is mirror symmetry, and the other is the $\backslash $pi Berry phase. In the latter case, the Zak phase is either $\backslash $pi or 0 depending on the momentum regions divided by the nodal lines, and the $\backslash $pi Zak phase is related to bulk charge polarization, appearing as a surface polarization charge. We discuss the relation between the quantized Zak phase and the surface charge by using electronic calculations for the realistic materials such as fcc calcium and other systems. We also propose the concept of the $\backslash $pi Zak phase is useful for novel behaviors of polarizations in insulators. [1] M. Hirayama, R. Okugawa, S. Ishibashi, S. Murakami, and T. Miyake, Phys. Rev. Lett. 114, 206401 (2015). [2] M. Hirayama, R. Okugawa, T. Miyake, and S. Murakami, arXiv:1602.06501 (Nat. Commun., Accepted). [Preview Abstract] |
Wednesday, March 15, 2017 5:06PM - 5:18PM |
P44.00014: Bulk-boundary correspondence from the inter-cellular Zak phase Jun-Won Rhim, Jan Behrends, Jens H. Bardarson The Zak phase $\gamma$, the generalization of the Berry phase to Bloch wave functions in solids, is often used to characterize inversion-symmetric 1D topological insulators; however, since its value can depend on the choice of real-space origin and unit cell, only the difference between the Zak phase of two regions is believed to be relevant. Here, we show that one can extract an origin-independent part of $\gamma$, the so-called inter-cellular Zak phase $\gamma^{\mathrm{inter}}$, which can be used as a bulk quantity to predict the number of surface modes as follows: a neutral finite 1D tight-binding system has $n_s = \gamma^{\mathrm{inter}}/\pi$ (mod 2) number of in-gap surface modes below the Fermi level if there exists a commensurate bulk unit cell that respects inversion symmetry. We demonstrate this by first verifying that $\pm e\gamma^{\mathrm{inter}}/2\pi$ (mod $e$) is equal to the extra charge accumulation in the surface region for a general 1D insulator, while the remnant part of $\gamma$, the intra-cellular Zak phase $\gamma^{\mathrm{intra}}$, corresponds to the electronic part of the dipole moment of the bulk's unit cell. Second, we show that the extra charge accumulation can be related to the number of surface modes when the unit cell is inversion symmetric. [Preview Abstract] |
Wednesday, March 15, 2017 5:18PM - 5:30PM |
P44.00015: Conductivity in nodal line semimetals with charged impurities Brian Skinner, Sergey Syzranov Coulomb disorder has a way of wreaking havoc on electronic systems with vanishing density of states (DoS). The issue is that when the DoS vanishes at the Fermi level, the system becomes incapable of screening the long-ranged potential created by stray charges. As a consequence, materials with vanishing DoS usually manifest large band-bending effects even at arbitrarily small impurity concentrations, and the properties of any nodal points are spoiled. Here we show that this spoilage is, surprisingly, averted in three-dimensional nodal line semimetals, for which the Fermi surface at zero chemical potential is a line or ring in momentum space. We discuss the conductivity in these materials as a function of chemical potential, impurity concentration, and temperature. We show that, due to a strong dielectric screening, nodal line semimetals admit the possibility of a wide regime of minimal conductivity, in which the conductivity is independent of the impurity concentration or the chemical potential. Such a regime was hoped for in graphene but never realized experimentally. [Preview Abstract] |
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