Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E44: Dirac and Weyl Semimetals: Optics I |
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Sponsoring Units: DMP Chair: Ching-Kit Chan, University of California, Los Angeles Room: 391 |
Tuesday, March 14, 2017 8:00AM - 8:12AM |
E44.00001: Localized charge and current on magnetic domain walls in Weyl semimetals Yasufumi Araki, Akihide Yoshida, Kentaro Nomura Magnetic domain walls have been intensely studied in the context of spintronics, to make use of them as carriers of information. Those in topological materials can have even more complex properties than in normal metals and insulators, due to the spin-momentum locking nature. A Weyl semimetal, characterized by the Dirac cone structure isolated in the momentum space, is one of the candidates. In this presentation, we show the properties of magnetic domain walls formed by localized magnetic moments in three-dimensional Weyl semimetals. We investigate the spectrum and the wave-function structure of electronic bound states at the domain wall both analytically and numerically, by solving the Weyl equation coupled with the domain wall texture. We find the discretized spectrum of the bound states, as a result of the Landau quantization under an effective “chiral” magnetic field arising from the magnetic texture of the domain wall. The lowest Landau level among them, giving rise to a “Fermi arc” around the domain wall, contributes to the electric charge and current localized at the domain wall. We propose that the localized charge enables one to manipulate the domain-wall motion with an external electric field without any dissipation. [Preview Abstract] |
Tuesday, March 14, 2017 8:12AM - 8:24AM |
E44.00002: Probing Weyl nodes with neutron scattering Michael Bjerngaard, Bogdan Galilo, Ari Turner We present a general expression for the cross-section and polarization of a scattered beam of neutrons within a band description of Weyl semimetals. This differential cross-section has features that reflect the scattering between nodes of either the same or opposite Chern numbers / spin-momentum locking. The strength of the coupling to neutrons is determined by an "anomalous" magnetic moment of the Weyl electrons, which can be very strong, since it diverges close to a topological phase transition. This coupling is anisotropic along the direction vector separating the two Weyl nodes, even when the dispersions at the nodes themselves are isotropic, as is revealed in the cross-section. [Preview Abstract] |
Tuesday, March 14, 2017 8:24AM - 8:36AM |
E44.00003: Spectroscopy of topological semimetals in electromagnetic fields using resonant x-rays Stefanos Kourtis Angle-resolved photoemission spectroscopy (ARPES) has so far been the definitive method for the characterization of materials as topological semimetals, via direct visualization of band touchings in the bulk and nontrivial states at the boundary. However, several unconventional and potentially useful properties of topological semimetals appear only in sizable electromagnetic fields, which severely limit the resolving power of ARPES. The controlled splitting of Dirac nodes to nondegenerate Weyl nodes in Dirac semimetals and the chiral anomaly in Weyl and Dirac semimetals are important examples of such unique properties. We show how resonant inelastic x-ray scattering (RIXS) offers a viable path for the spectroscopic detection of the aforementioned effects. By low-energy modeling of specific material candidates based on ab initio band structure calculations, we derive the corresponding RIXS spectra and highlight the salient features stemming from topological nontriviality. The proposed measurements are within the resolving capabilities of current instrumentation. [Preview Abstract] |
Tuesday, March 14, 2017 8:36AM - 8:48AM |
E44.00004: Signatures of the chiral anomaly in phonon dynamics Pierre Rinkel, Pedro L. S. Lopes, Ion Garate Discovered in high-energy physics, the chiral anomaly has recently made way to materials science by virtue of Weyl semimetals. Thus far, the main efforts to probe the chiral anomaly in quantum materials have concentrated on electronic phenomena. Here, we show that the chiral anomaly can have a notable impact in phonon properties, including phonon dispersion, infrared absorption, and Raman scattering. Remarkably, in enantiomorphic Weyl semimetals, the chiral anomaly leads to a magnetically induced effective phonon charge with an unusual and potentially measurable resonance. [Preview Abstract] |
Tuesday, March 14, 2017 8:48AM - 9:00AM |
E44.00005: Detecting Chiral Magnetic Effect by Lattice Dynamics in Weyl Semimetals Zhida Song, Jimin Zhao, Zhong Fang, Xi Dai In the present paper, we propose that the chiral magnetic effect, the direct consequence of the presence of Weyl points in the band structure, can be detected by its coupling to certain phonon modes, which behave like pseudo scalars under point group transformations. Such coupling can generate resonance between intrinsic plasmon scillation and the corresponding phonon modes, leading to dramatic modification of the optical response by the external magnetic field, which provides a new way to study chiral magnetic effect by optical measurements. [Preview Abstract] |
Tuesday, March 14, 2017 9:00AM - 9:12AM |
E44.00006: Tunable Weyl Points in Periodically Driven Nodal Line Semimetals Zhongbo Yan, Zhong Wang Weyl semimetals and nodal line semimetals are characterized by linear band touching at zero-dimensional points and one-dimensional lines, respectively. We predict that a circularly polarized light drives nodal line semimetals into Weyl semimetals. The Floquet Weyl points thus obtained are tunable by the incident light, which enables investigations of them in a highly controllable manner. The transition from nodal line semimetals to Weyl semimetals is accompanied by the emergence of a large and tunable anomalous Hall conductivity. Our predictions are experimentally testable by transport measurement in film samples or by pump-probe angle-resolved photoemission spectroscopy. [Reference: Phys. Rev. Lett. 117, 087402 (2016)] [Preview Abstract] |
Tuesday, March 14, 2017 9:12AM - 9:24AM |
E44.00007: Chiral Anomaly and Second Harmonic Generation in Weyl Semimetals Alexander Zyuzin, A.Yu. Zyuzin We study second harmonic generation in centrosymmetric Weyl semimetal with broken time reversal symmetry. We calculate electric current density at the double frequency of propagating electromagnetic field in the presence of applied constant magnetic field, using the method of kinetic equation for electron distribution function. We remind that second harmonic generation in systems with inversion centre requires incident radiation with finite wave-vector, while propagating electromagnetic wave with transverse polarization can not lead to the chiral anomaly. We show that applying a constant magnetic field in addition to propagating electromagnetic wave gives rise to the observable contribution of the chiral anomaly to second harmonic generation. It is shown that the chiral anomaly contribution to second harmonic generation in the lowest order is linearly proportional to the applied magnetic field. The limit when the chiral anomaly dominates over the Lorentz-type contribution to second harmonic generation is discussed. [Preview Abstract] |
Tuesday, March 14, 2017 9:24AM - 9:36AM |
E44.00008: Emergent Electromagnetic Induction in Weyl Semimetals Hiroaki Ishizuka, Tomoya Hayata, Masahito Ueda, Naoto Nagaosa Theoretical studies on the Weyl semimetals predicts various interesting nonlinear responses to the external electromagnetic field, such as chiral magneto-electric effect and photovoltaic effects. In many of these theories, the quantum anomaly of Weyl Hamiltonian or the spin-momentum locking take key roles. In this work, we propose a new mechanism for the photovoltaic effect in the Weyl semimetals. We show that the adiabatic Berry phase also contribute to the photovoltaic currents in Weyl semimetals. This phenomenon can be understood as an emergent electromagnetic induction in the momentum space. When a Weyl semimetal being irradiated by the light, which we treat as a slowly varying field in time, it shifts the position of the nodes, leading to a cyclic motion of the Weyl node in the momentum space, i.e., the light induces the cyclic magnetic charge current. In analogy to the electric current in the solenoids that induces magnetic field, the cyclic motion of magnetic monopoles results in dc electric field in the momentum space; the momentum space electric field brings about electric current. In a generalized Weyl Hamiltonian with k$^{2}$ terms, the charge current is induced by incident circularly polarized lights. In Weyl semimetals with broken inversion symmetry, this phenomenon potentially results in the photocurrent of magnitude that is experimentally observable. [Preview Abstract] |
Tuesday, March 14, 2017 9:36AM - 9:48AM |
E44.00009: Illuminating the chirality of Weyl fermions Qiong Ma, Su-Yang Xu, Ching-Kit Chan, Cheng-Long Zhang, Guoqing Chang, Hsin Lin, Shuang Jia, Patrick Lee, Nuh Gedik, Pablo Jarillo-Herrero In particle physics, Weyl fermions (WF) are elementary particles that travel at the speed of light and have a definite chirality. In condensed matter, it has been recently realized that WFs can arise as magnetic monopoles in the momentum space of a novel topological metal, the Weyl semimetal (WSM). Their chirality, given by the sign of the monopole charge, is the defining property of a WSM, since it directly serves as the topological number and gives rise to exotic properties such as Fermi arcs and the chiral anomaly. Moreover, the two chiralities, analogous to the two valleys in 2D materials, lead to a new degree of freedom in a 3D crystal, suggesting novel pathways to store and carry information. By shining circularly polarized light on the WSM TaAs, we illuminate the chirality of the WFs and achieve an electrical current that is highly controllable based on the WFs' chirality. Our results open up a wide range of new possibilities for experimentally studying and controlling the WFs and their associated quantum anomalies by optical and electrical means, which suggest the exciting prospect of ``Weyltronics''. [Preview Abstract] |
Tuesday, March 14, 2017 9:48AM - 10:00AM |
E44.00010: Ultrafast control of coherent phonon oscillations in the magnetic Weyl semimetal SrMnSb2 Christopher Weber, Madison Masten, Thomas Ogloza, Jinyu Liu, Zhiqiang Mao SrMnSb$_2$ is believed to be a Weyl semimetal. It is centrosymmetric, and has canted antiferromagnetic order that breaks time-reversal symmetry. After exciting crystals of SrMnSb$_2$ with short pulses of near-infrared light, we observe the coherent oscillation of several phonon modes in the range of a few THz, and we measure the phonons' frequency in both zero and high magnetic field. We use an ultrafast two-pump-one-probe experiment to distinguish first-order (coherent) excitation of phonons from second-order (squeezed) excitation, and to control the amplitude of the oscillations. We use angle-dependent Raman measurements to identify the symmetry of the phonon modes, and to relate them to the crystal structure. In light of our observations, we will discuss the prospects for ultrafast control of the Weyl state in SrMnSb$_2$. [Preview Abstract] |
Tuesday, March 14, 2017 10:00AM - 10:12AM |
E44.00011: Temperature-tunable Fano resonance induced by strong Weyl fermion-phonon coupling in TaAs Yaomin Dai, S. A. Trugman, J.-X. Zhu, A. J. Taylor, D. A. Yarotski, R. P. Prasankumar, B. Xu, L. X. Zhao, K. Wang, R. Yang, W. Zhang, J. Y. Liu, H. Xiao, G. F. Chen, X. G. Qiu Strong coupling between discrete phonon and continuous electron-hole pair excitations can give rise to a pronounced asymmetry in the phonon line shape, known as the Fano resonance. We present infrared spectroscopic studies on the recently discovered Weyl semimetal TaAs at different temperatures. Our experimental results reveal strong coupling between an infrared-active $A_1$ phonon and electronic transitions near the Weyl points (Weyl fermions), as evidenced by the conspicuous asymmetry in the phonon line shape. More interestingly, the phonon line shape can be continuously tuned by temperature, which we demonstrate to arise from the suppression of the electronic transitions near the Weyl points due to the decreasing occupation of electronic states below the Fermi level with increasing temperature, as well as Pauli blocking caused by thermally excited electrons above the Fermi level. [Preview Abstract] |
Tuesday, March 14, 2017 10:12AM - 10:24AM |
E44.00012: Infrared and magneto-optical spectroscopy of Cd$_3$As$_2$ Ana Akrap, M. Hakl, S. Tchoumakov, Iris Crassee, J. Kuba, M.O. Goerbig, C.C. Homes, O. Caha, J. Novak, F. Teppe, S. Koohpayeh, Liang Wu, N.P. Armitage, T. McQueen, A. Nateprov, E. Arushanov, Q.D. Gibson, R.J. Cava, D. van der Marel, C. Faugeras, G. Martinez, M. Potemski, M. Orlita We report infrared measurements on Cd$_3$As$_2$ single crystals with different orientations and different doping levels, as well as low-magnetic field measurements of Kerr effect. The resulting optical conductivity is isotropic and independent of the crystal direction. Linear regime of the real part of the optical conductivity qualitativelly agrees with massless particles, and is consistent with massless Kane electrons. Two contributions may be separated in the optical conductivity in zero and finite magnetic field. We propose that these two contributions originate from the bulk of the crystal, and a thin depletion layer. [Preview Abstract] |
Tuesday, March 14, 2017 10:24AM - 10:36AM |
E44.00013: Magneto-Optical Signature of Massless Kane Electrons in Cd3As2 Milan Orlita, Michael Hakl, Benjamin A. Piot, Clement Faugeras, Gerard Martinez, Marek Potemski, Ana Akrap, Iris Crassee, Dirk van der Marel, Serguei Tchoumakov, Mark O. Goerbig, Jakub Kuba, Ondrej Caha, Jiri Novak, Frederic Teppe, Wilfried Desrat, Seyed Koohpayeh, Liang Wu, N. Peter Armitage, Alexandr Nateprov, Ernest Arushanov, Quinn D. Gibson, Robert J. Cava, Christopher C. Homes We report on infrared magneto-specroscopy of Cd$_3$As$_2$. The observed response clearly indicates the presence of 3D massless charge carriers. The specific cyclotron resonance absorption in the quantum limit implies that we are probing massless Kane electrons rather than symmetry-protected 3D Dirac particles [1]. The latter may appear at a smaller energy scale and are not directly observed in our infrared experiments. This our conclusion is fully consistent with the simple Bodnar model developed in the past to describe the Cd$_3$As$_2$ band structure [2]. References: [1] A. Akrap et al., Phys. Rev. Lett. 117, 136401 (2016) [2] J. Bodnar et al., Proc. of Narrow Gap Semicond. Conf., Warsaw, 1977, p. 311. [Preview Abstract] |
Tuesday, March 14, 2017 10:36AM - 10:48AM |
E44.00014: Optical spectroscopy of the half-Heusler semi-metal Dipanjan Chaudhuri, Ana Akrap, Satya K. Kushwaha, Robert J. Cava, Dirk van der Marel, N. Peter Armitage The half-Heusler compound GdPtBi is a fascinating system that has an interesting interplay between antiferromagnetism and strong spin-orbit coupling. The proposed electronic structure of the compound is that of a zero gap semiconductor with degenerate quadratic bands touching at $\Gamma$ point which in presence of externally applied magnetic field splits, giving rise to a pair of Weyl nodes. Evidence in favour of such interesting crossover has been presented through the observation of a “chiral anomaly” in transport experiments. Additionally, a large anomalous Hall effect has also been observed in this compound through neutron scattering experiments. In this work, we have studied high quality GdPtBi single crystals through FTIR spectroscopy. With quadratic band touching at zero field, the compound also provides an opportunity to explore non-Fermi liquid physics as a result of large joint density of states near the Fermi level. Moreover, in magnetic field our optical study offers the advantage of a non-contact transport measurement which can help distinguish the true chiral anomalous effect from possible current jetting effects. Additionally, the magnetic field data can provide useful insight on the crossover into the Weyl semi-metallic phase. [Preview Abstract] |
Tuesday, March 14, 2017 10:48AM - 11:00AM |
E44.00015: Electrodynamic response of type II Weyl semimetal YbMnBi$_2$ M. Chinotti, A. Pal, C. Petrovic, W.J. Ren, L. Degiorgi Weyl fermions play a major role in quantum field theory but have been quite elusive as fundamental particles. Materials based on quasi two-dimensional bismuth layers were recently designed and provide an arena for the study of the interplay between anisotropic Dirac fermions, magnetism and structural changes, allowing the formation of Weyl fermions in condensed matter. Here, we present an optical investigation of YbMnBi$_2$, a representative type II Weyl semimetal, and contrast its excitation spectrum with the optical response of the more conventional semimetal EuMnBi$_2$. Our comparative study allows us disentangling the optical fingerprints of type II Weyl fermions, but also challenge the present theoretical understanding of their electrodynamic response. [Preview Abstract] |
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