Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session A28: Topological PhasesFocus
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Sponsoring Units: DMP DCMP Chair: Yong Chen, Purdue University Room: 327 |
Monday, March 14, 2016 8:00AM - 8:36AM |
A28.00001: Predicting, Realizing and Exploiting Exotic Topological Phases of Quantum Matter Invited Speaker: Arun Bansil The revolution started by the discovery of topological insulators a few years ago has turned out to be the proverbial tip of the much larger iceberg of exotic phases harbored by quantum matter. Consideration of electronic states protected by time-reversal, crystalline and particle-hole symmetries has led to the prediction of many novel 3D materials, which can support Weyl, Dirac and Majorana fermions, and to new types of insulators such as topological crystalline insulators and topological Kondo insulators, as well as 2D quantum spin Hall insulators with large band gaps capable of surviving room temperature thermal excitations. [1] In this talk, I will discuss our recent theoretical work aimed at predicting topological materials beyond the standard topological insulators and identify cases where robust experimental evidence has been obtained toward their successful materials realization. [2-10] I will also comment on the potential of topological materials as next generation platforms for manipulating spin and charge transport and other applications. [1] A. Bansil, H. Lin and T. Das, Reviews of Modern Physics (2015). [2] S.-Y. Xu et al., Science 349, 613 (2015). [3] I. Zeljkovic et al., Nature Materials 14, 318 (2015). [4] J. He et al., Nature Materials 14, 577 (2015). [5] S.-M. Huang et al., Nature Communications 6, 7373 (2015). [6] S.-Y. Xu et al., Science Advances (2015). [7] I. Zeljkovic, et al., Nature Communications 6, 6559 (2015). [8] M. Neupane et al., Physical Review Letters 114, 016403 (2015). [9] Su-Yang Xu, et al., Nature Physics 11, 748 (2015). [10] C. P. Crisostomo et al., Nano Letters 15, 6568 (2015). [Preview Abstract] |
Monday, March 14, 2016 8:36AM - 8:48AM |
A28.00002: Electronic structure of Na$_3$Bi near the Dirac point: Optical measurements Gregory S. Jenkins, A. B. Sushkov, R. L. Carey, H. D. Drew, J. Krizan, S. Kushwaha, R. Cava, Tay-Rong Chang, Horng-Tay Jeng, H. Lin, C. Lane, B. Barbiellini, A. Bansil The first optical characterization of Na$_3$Bi is reported. Reflection measurements on c-plane oriented single-crystals, over the spectral range from 3 mev to 2.5 eV and temperature ranging from 8 to 250K, show a low frequency response consistent with the low doping level $n\sim10^{17} \text{ cm}^{-1}$. The number of observed phonons in the optical spectra is $>$5, which eliminates the P6$_{3}$/mmc symmetry since point group analysis indicates only 2 IR active phonons. A striking, strongly temperature dependent plasma edge reverses direction at $T\sim100$K. The behavior is consistent with thermal population effects in a Dirac cone permitting an estimation of the Fermi level. The Lifshitz gap energy is reported. [Preview Abstract] |
Monday, March 14, 2016 8:48AM - 9:00AM |
A28.00003: Electronic structure of Na$_3$Bi near the Dirac point: Theory Chris Lane, B. Barbiellini, A. Bansil, Tay-Rong Chang, Horng-Tay Jeng, H. Lin, J. Krizan, S. Kushwaha, R. Cava, G. S. Jenkins, A. B. Sushkov, R. L. Carey, H. D. Drew Band structure calculations have been performed and compared with recent optical experiments. The ground state of the system is found to be not the highly symmetric P6$_{3}$/mmc structure, but instead the P$\bar{3}$c1 that involves buckling of the Na-Bi hexagonal planes. The band structure shows very little change between various symmetry configurations, yet the low-energy optical transition matrix elements are dramatically enhanced in the P$\bar{3}$c1 symmetry compared with P6$_{3}$/mmc, which results in an electronic response that agrees much more closely with optical data. A peak in the joint density of states driven by the particle-hole asymmetry of the band structure along the $\Gamma-A$ momentum direction results in a large peak in the imaginary part of the dielectric function. Systematic changes are observed in the low energy Dirac cone Fermi velocity and Lifshitz gap energy with lattice spacing and spin-orbit coupling. The large anisotropies of the Dirac cone and small energy gaps are discussed. [Preview Abstract] |
Monday, March 14, 2016 9:00AM - 9:12AM |
A28.00004: Universal signatures of Fermi arcs in quasiparticle interference on the surface of Weyl semimetals Stefanos Kourtis, Jian Li, Zhi Jun Wang, B. Andrei Bernevig Weyl semimetals constitute a newly discovered class of three-dimensional topological materials with linear touchings of valence and conduction bands in the bulk. The most striking property of topological origin in these materials, so far only observed in photoemission experiments, is the presence of open constant-energy contours in the boundary density of states --- the so-called Fermi arcs. In this work, we establish the universal characteristics of Fermi-arc contributions to surface quasiparticle interference. Using a general phenomenological model, we determine the defining interference patterns stemming from the existence of Fermi arcs in a surface band structure. We then trace these patterns in both simple tight-binding models and realistic ab initio calculations. Our results show that definitive signatures of Fermi arcs can be observed in existing and proposed Weyl semimetals using current scanning tunneling spectroscopy setups. [Preview Abstract] |
Monday, March 14, 2016 9:12AM - 9:24AM |
A28.00005: Probing spin-momentum locking of Weyl nodes with neutron scattering Michael Bjerngaard, Bogdan Galilo, Ari Turner We explain how a Weyl semimetal phase can be uniquely identified in the differential cross-section measured by an unpolarized neutron experiment. This differential cross-section has unique features that reflect the scattering between Weyl nodes of either same or opposite Chern numbers / spin-momentum locking. Hence, an unpolarized neutron experiment can uniquely identify Weyl semimetals of both inversion-and time-reversal symmetric classes. This is very desirable, as no experimental probe has yet directly confirmed such phases. Further, we describe how the neutron spectrum can distinguish proposed Weyl semimetals from Dirac semimetals. [Preview Abstract] |
Monday, March 14, 2016 9:24AM - 9:36AM |
A28.00006: Tuning Weyl nodes with a magnetic field Jennifer Cano, Barry Bradlyn, Zhijun Wang, Max Hirschberger, N. Phuan Ong, B. Andrei Bernevig For Weyl fermions to exist, either inversion or time reversal symmetry must be broken. Here, we consider materials with a normal and/or inverted band structure that display a four band (Dirac) crossing in the presence of both these symmetries. We show that when a magnetic field is applied, thus breaking time reversal, the four band crossing splits into several Weyl nodes, depending on the direction in which the magnetic field is applied as well as on the symmetry group that protected the Dirac crossing. For a particular material realization, relevant to current experiments performed in Princeton, we use a symmetry analysis to predict the position of the Weyl nodes when the magnetic field is along a high-symmetry axis. While the symmetry is not necessary to protect the Weyl crossings, it is a useful tool to find them. Our results agree with both an ab initio and a $k \cdot p$ effective Hamiltonian analysis. [Preview Abstract] |
Monday, March 14, 2016 9:36AM - 9:48AM |
A28.00007: Spiraling Fermi arcs in Weyl materials Songci Li, Anton Andreev In Weyl materials the valence and conduction electron bands touch at an even number of isolated points in the Brillouin zone. In the vicinity of these points the electron dispersion is linear and may be described by the massless Dirac equation. This results in nontrivial topology of Berry connection curvature. One of its consequences is the existence of peculiar surface electron states whose Fermi surfaces form arcs connecting projections of the Weyl points onto the surface plane. Band bending near the boundary of the crystal also produces surface states. We show that in Weyl materials band bending near the crystal surface gives rise to spiral structure of energy surfaces of arc states. The corresponding Fermi surface has the shape of a spiral that winds about the projection of the Weyl point onto the surface plane. The direction of the winding is determined by the helicity of the Weyl point and the sign of the band bending potential. For close valleys arc state morphology may be understood in terms of avoided crossing of oppositely winding spirals. [Preview Abstract] |
Monday, March 14, 2016 9:48AM - 10:00AM |
A28.00008: Creating chiral anomalies Barry Bradlyn, Jennifer Cano, Zhijun Wang, Max Hirschberger, N. Phuan Ong, B. Andrei Bernevig Materials with intrinsic Weyl points should present exotic magnetotransport phenomena due to spectral flow between Weyl nodes of opposite chirality - the so-called ``chiral anomaly''. However, to date, the most definitive transport data showing the presence of a chiral anomaly comes from Dirac (not Weyl) materials. These semimetals develop Weyl fermions only in the presence of an externally applied magnetic field, when the four-fold degeneracy is lifted. In this talk we examine Berry phase effects on transport due to the emergence of these field-induced Weyl point and (in some cases) line nodes. We pay particular attention to the differences between intrinsic and field-induced Weyl fermions, from the point of view of kinetic theory. Finally, we apply our analysis to a particular material relevant to current experiments performed at Princeton. [Preview Abstract] |
Monday, March 14, 2016 10:00AM - 10:12AM |
A28.00009: Visualizing Weyl Fermions in MoTe$_{2}$ Using Scanning Tunneling Microscopy Ayelet Notis, Erick Andrade, Sang-Wook Cheong, Abhay Pasupathy MoTe$_{2}$, a transition metal dichalcogenide, has a metastable orthorhombic phase at temperatures below 250 K. This phase is predicted to be a type II Weyl semimetal, providing us an exciting new opportunity to explore Weyl Fermions, a type of particle long sought after but only recently realized as a quasiparticle excitation in a crystal. A topological consequence of the existence of Weyl points in a crystal is the existence of Fermi arc surface states that connect pairs of Weyl points. Here, we present scanning tunneling microscopy and spectroscopy (STM and STS) studies investigating the topography and electronic structure of this material. We resolve the crystal structure of the orthorhombic phase in STM topography, and probe the electronic structure of the Fermi arc states using quasiparticle interference imaging. [Preview Abstract] |
Monday, March 14, 2016 10:12AM - 10:24AM |
A28.00010: Stable Dirac semi-metal in the allotrope of IV elements Peizhe Tang, Wendong Cao, Shou-Cheng Zhang, Wenhui Duan, Angel Rubio Three dimensional (3D) topological Dirac semi-metals (SM) represent a novel state of quantum matter with exotic electronic structures, in which a pair of Dirac points with the linear dispersion along all three momentum directions exist in the bulk and are protected by the rotation symmetry. Regarded as the copies of 3D Weyl SMs, the Dirac SMs possess unique Fermi-arcs with helical spin textures on some specific surface planes. Herein, by using first principles calculations with the hybrid functional, we discover a new metastable allotrope of Ge and Sn with the staggered layered dumbbell structure, named as germancite and stancite, to be 3D Dirac SMs with a pair of Dirac points on the rotation axis of $C_3$. On the surface parallel to the rotation axis, a pair of topologically non-trivial Fermi arcs are observed to be coexisting with the trivial surface states; and via tuning the Fermi level, the hybridization between them induces a Lifshitz transition on the Fermi surface. Furthermore, the quantum thin film of the germancite is found to be the quantum spin Hall insulator without applying external electric field. These discoveries explore the metastable allotrope of Ge and Sn as topological Dirac SMs showing novel physical properties and future applications. [Preview Abstract] |
Monday, March 14, 2016 10:24AM - 10:36AM |
A28.00011: Chiral magnetic effect and natural optical activity in (Weyl) metals Dmytro Pesin, Jing Ma We consider the phenomenon of natural optical activity, and related chiral magnetic effect in metals with low carrier concentration. To reveal the correspondence between the two phenomena, we compute the optical conductivity of a noncentrosymmetric metal to linear order in the wave vector of the light wave, specializing to the low-frequency regime. We show that it is the orbital magnetic moment of quasiparticles that is responsible for the natural optical activity, and thus the chiral magnetic effect. While for purely static magnetic fields the chiral magnetic effect is known to have a topological origin and to be related to the presence of Berry curvature monopoles (Weyl points) in the band structure, we show that the existence of Berry monopoles is not required for the dynamic chiral magnetic effect to appear; the latter is thus not unique to Weyl metals. The magnitude of the dynamic chiral magnetic effect in a material is related to the trace of its gyrotropic tensor. We discuss the conditions under which this trace is non-zero; in noncentrosymmetric Weyl metals it is found to be proportional to the energy-space dipole moment of Berry curvature monopoles. The calculations are done within both the semiclassical kinetic equation, and Kubo linear response formalisms. [Preview Abstract] |
Monday, March 14, 2016 10:36AM - 10:48AM |
A28.00012: Symmetry-based Search for Topological Weyl Nodes and Nodal-lines in Realistic Materials Motoaki Hirayama, Shuichi Murakami, Ryo Okugawa, Shoji Ishibashi, Takashi Miyake Topological semimetals such as Weyl semimetals and nodal-line semimetals have been under intensive investigation recently. In this study, to realize such semimetals, we start from any insulators without inversion symmetry, and assume that the gap closes by changing some parameter. We then show that after the gap-closing there are only two possibilities; one is a Weyl semimetal phase, and the other is a nodal-line semimetal, depending on the symmetry and the position of the gap-closing point. Our analysis tells us which cases lead to Weyl semimetal and to the nodal-line semimetal, and this result can be used to find realistic topological semimetals. As an example, we study tellurium [1] using ab initio calculation based on relativistic density functional theory. The electronic structure is calculated by OpenMX [2] and the structural optimization is executed by QMAS [3]. We find that HfS has the nodal line in the mirror symmetry plane, and the nodal-line vanishes by pressure or atomic substitution. We also propose some materials showing the spinless nodal lines and its topological surface states. [1] M. Hirayama, R. Okugawa, S. Ishibashi, S. Murakami, and T. Miyake, Phys. Rev. Lett. 114, 206401 (2015). [2] http://www.openmx-square.org/ [3] http://www.qmas.jp/ [Preview Abstract] |
Monday, March 14, 2016 10:48AM - 11:00AM |
A28.00013: Boundary Conditions and Self-Adjoint Extensions of a Continuum Weyl Semimetal Hamiltonian Michael Vennettilli, Babak Seradjeh, Arijit Kundu, Mostafa Tanhayi Ahari A Weyl semimetal is a Dirac material where the spin degeneracy of the energy-momentum Dirac cones is broken. The surface states of Weyl semimetals are expected to permit Fermi arcs connecting the surface-projections of the two Weyl nodes. The existence and physical properties of these surface states depends crucially on the boundary conditions at the surface. Generally speaking, boundary conditions placed on an unbounded Hermitian operator have an intimate relationship with the possible self-adjoint extensions of the operator. Indeed, determining the self-adjoint extensions of the operator naturally classifies all physical boundary conditions on the wavefunctions. We have studied the self-adjoint extensions of a model continuum Hamiltonian for Weyl semimetals and their corresponding classes of surface states. In this way, all possible physical surface spectra of the Weyl semimetal corresponding to different physical realizations of the surface are contained within our result. [Preview Abstract] |
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