Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session R44: Dirac and Weyl Semimetals: Optics IIFocus
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Sponsoring Units: DMP Chair: Suyang Xu, Massachusetts Institute of Technology Room: 391 |
Thursday, March 16, 2017 8:00AM - 8:12AM |
R44.00001: Winding number and optical conductivity of multi-Weyl semimetals Seongjin Ahn, Eugene Mele, Hongki Min Multi-Weyl semimetals are a new type of Weyl semimetals which have anisotropic non-linear energy dispersion and a topological charge larger than one, thus exhibiting a unique quantum response. Using a unified lattice model we calculate the optical conductivity numerically in the multiWeyl semimetal phase and in its neighboring gapped states, and obtain the characteristic frequency dependence of each phase analytically using a continuum model. The frequency dependence of longitudinal and transverse optical conductivities obeys scaling relations that are derived from the winding number of the parent multi-Weyl semimetal phase and can be used to distinguish these electronic states of matter. [Preview Abstract] |
Thursday, March 16, 2017 8:12AM - 8:24AM |
R44.00002: Time-resolved Optical Study of Carrier Dynamics in the Weyl Semimetal TaAs M. Mehdi Jadidi, Yigit Aytac, Ryan J. Suess, Andrei B. Sushkov, Gregory S. Jenkins, James G. Analytis, H. Dennis Drew, Thomas E. Murphy Since their recent discovery in 2015, Weyl semimetals have attracted attention because they are predicted to exhibit a host of novel physical and topological properties not seen in other materials. While the electronic structure of these new materials has been confirmed using surface probe methods such as angle-resolved photoemission spectroscopy (ARPES), the fundamental carrier dynamics and temporal response of these materials cannot be discerned through DC or surface-probe measurements. Here we present an ultrafast optical study of the carrier dynamics in the broken-inversion-symmetry Weyl semimetal tantalum arsenide (TaAs). We employ reflectance two-color pump-probe measurements at photon energies 0.8 eV and 1.6 eV to measure the relaxation of photoexcited electrons in in TaAs, as a function of the lattice temperature. Our measurements reveal a fast time constant ($\approx $2 ps at 10 K) which we associate with the scattering of hot electrons via optical phonons, followed by a slower relaxation rate ($\approx $200 ps at 10 K) attributed to acoustic phonon emission. The temperature dependence measurements show that both relaxation processes become slower with increasing lattice temperature. We present a thermodynamic model based on thermalized Dirac quasi-particles to explain the observed reflectance pump-probe results. [Preview Abstract] |
Thursday, March 16, 2017 8:24AM - 8:36AM |
R44.00003: Photocurrents in Weyl semimetals Ching-Kit Chan, Netanel H. Lindner, Gil Refael, Patrick A. Lee The generation of photocurrent is symmetry-forbidden in two-dimensional Dirac electronic systems with inversion symmetry. In sharp contrast, we show that three-dimensional Weyl semimetals can generically support significant photocurrents due to the combination of inversion symmetry breaking and finite tilts of the Weyl spectra. We will explore the symmetry properties, chirality relations and various dependences of this photovoltaic effect on the system and the external light source. Our results suggest that noncentrosymmetric Weyl materials can be advantageously applied to room temperature photodetections of mid- and far-infrared radiations. [Preview Abstract] |
Thursday, March 16, 2017 8:36AM - 9:12AM |
R44.00004: Linear and nonlinear responses in topological semimetals Invited Speaker: Joel Moore Topological phases that are insulating in the bulk often have quantized electromagnetic responses, such as the quantum Hall effect or axion electrodynamics. Gapless phases with topological features, such as Dirac and Weyl semimetals in 3D, can also have interesting electromagnetic responses, although one might not expect them to be quantized. We first discuss linear response and the possibility of a chiral magnetic effect in Weyl semimetals; instead we find a "gyrotropic magnetic effect" (GME) determined at low frequency by the magnetic moments on the Fermi surface of Bloch electrons. The GME should be observable in optical rotation on mirror-free Weyl semimetals. At higher order in electromagnetic fields, many possible effects occur, which are studied both semiclassically and via a fully quantum Floquet approach. Nonlinear optical properties, specifically photocurrents and second-harmonic generation, are discussed in detail. The circular photogalvanic effect (CPGE) is a piece of the photocurrent that was previously argued to have a Berry-phase origin; we confirm this in a fully quantum calculation and then show that for certain Weyl semimetals the CPGE injection current is actually quantized with no material-specific parameters. [Preview Abstract] |
Thursday, March 16, 2017 9:12AM - 9:24AM |
R44.00005: Transient many-body instability in driven Dirac materials. Anna Pertsova, Christopher Triola, Alexander Balatsky The defining feature of a Dirac material (DM) is the presence of nodes in the low-energy excitation spectrum leading to a strong energy dependence of the density of states (DOS). The vanishing of the DOS at the nodal point~implies a very low effective coupling constant which leads to stability of the node against electron-electron interactions.~Non-equilibrium or driven DM, in which the DOS and hence the effective coupling can be controlled by external drive, offer a new platform for investigating collective instabilities. In this work, we discuss the possibility of realizing transient collective states in driven DMs. Motivated by recent pump-probe experiments which demonstrate the existence of long-lived photo-excited states in DMs, we consider an example of a transient excitonic instability in an optically-pumped DM. We identify experimental signatures of the transient excitonic condensate and provide estimates of the critical temperatures and lifetimes of these states for few important examples of DMs, such as single-layer graphene and topological-insulator surfaces. [Preview Abstract] |
Thursday, March 16, 2017 9:24AM - 9:36AM |
R44.00006: Quantized circular photogalvanic effect in Weyl semimetals Fernando de Juan, Adolfo G. Grushin, Takahiro Morimoto, Joel E. Moore The circular photogalvanic effect (CPGE) is the part of a photocurrent that switches depending on the sense of circular polarization of the incident light. It has been consistently observed in systems without inversion symmetry and depends on non-universal material details. We find that in a class of Weyl semimetals (e.g. SrSi$_2$) and three-dimensional Rashba materials (e.g. doped Te) without inversion and mirror symmetries, the CPGE trace is effectively ${\it quantized}$ in terms of the combination of fundamental constants ${e^3 \over h^2 c \epsilon_0}$ with no material-dependent parameters. This is so because the CPGE directly measures the topological charge of Weyl points near the Fermi surface, and non-quantized corrections from disorder and additional bands can be small over a significant range of incident frequencies. Moreover, the magnitude of the CPGE induced by a Weyl node is relatively large, which enables the direct detection of the monopole charge with current techniques. [Preview Abstract] |
Thursday, March 16, 2017 9:36AM - 9:48AM |
R44.00007: Semiclassical theory of nonlinear magneto-optical responses with applications to topological Dirac/Weyl semimetals Takahiro Morimoto, Shudan Zhong, Joseph Orenstein, Joel E. Moore We study nonlinear magneto-optical responses of metals by a semiclassical Boltzmann equation approach [1]. We derive general formulas for linear and second order nonlinear optical effects in the presence of magnetic fields that include both Berry curvature and orbital magnetic moment. Applied to Weyl fermions, the semiclassical approach (i) captures the directional anisotropy of linear conductivity under magnetic field as a consequence of an anisotropic $B^2$ contribution, which may explain the low-field regime of recent experiments; (ii) predicts strong second harmonic generation proportional to $B$ that is enhanced as the Fermi energy approaches the Weyl point, leading to large nonlinear Kerr rotation. Moreover, we show that the semiclassical formula for the circular photogalvanic effect arising from the Berry curvature dipole is reproduced by a full quantum calculation in the case of two bands using a Floquet approach. [1] arXiv:1609.05932 [Preview Abstract] |
Thursday, March 16, 2017 9:48AM - 10:00AM |
R44.00008: Photogalvanic effects in Type-II Weyl semimetals, WTe2 and MoTe2 Se Joon Lim, Claudia Felser, Aharon Kapitulnik For homogeneous systems with broken inversion symmetry, a direct current can be generated under uniform illumination due to the transfer of angular momentum and linear momentum of photons. In this study, we experimentally look at the photogalvanic effect in Weyl semimetals with broken inversion symmetry. In particular, the recently discovered Type-II Weyl semimetals, WTe$_{2}$ and MoTe$_{2}$, with C$_{2v}$ symmetry group are studied under oblique illumination of elliptically polarized light. We measure the response in directions both perpendicular and parallel to the W-chain (Mo-chain), and we are analyzing the contributions from various effects including the photon drag effect which arises from the transfer of photon momentum. [Preview Abstract] |
Thursday, March 16, 2017 10:00AM - 10:12AM |
R44.00009: Giant anisotropic nonlinear optical response in transition metal monopnictide Weyl semimetals Liang Wu, Shreyas Patankar, Takahiro Morimoto, Nityan Nair, Eric Thewalt, Arielle Little, James Analytis, Joel Moore, Joseph Orenstein Recently Weyl quasiparticles have been observed in transition metal monopnictides (TMMPs) such as TaAs, a class of noncentrosymmetric materials that heretofore received only limited attention. The question that arises now is whether these materials will exhibit novel, enhanced, or technologically applicable electronic properties. The TMMPs are polar metals, a rare subset of inversion-breaking crystals that would allow spontaneous polarization, were it not screened by conduction electrons. Despite the absence of spontaneous polarization, polar metals can exhibit other signatures of inversion-symmetry breaking, most notably second-order nonlinear optical polarizability, $\chi^{(2)}$, leading to phenomena such as optical rectification and second-harmonic generation (SHG). Here we report measurements of SHG that reveal a giant, anisotropic $\chi^{(2)}$ in the TMMPs TaAs, TaP, and NbAs. With the fundamental and second harmonic fields oriented parallel to the polar axis, the value of $\chi^{(2)}$ is larger by almost one order of magnitude than its value in the archetypal electro-optic materials GaAs and ZnTe, and in fact larger than reported in any crystal to date. Reference: arXiv:1609.04894. [Preview Abstract] |
Thursday, March 16, 2017 10:12AM - 10:24AM |
R44.00010: Mahan excitons in Weyl semimetals Ion Garate, Simon Bertrand, Ren\'e C\^ot\'e We report on a theoretical study of excitons in weakly doped Weyl semimetals. Solving a two-body Coulomb problem in the presence of a monopole Berry vector potential, we obtain the binding energies of electron-hole pairs and establish their dependence on the monopole charge and on the sign of the magnetic quantum number. We discuss the implications of our results for optical absorption experiments. [Preview Abstract] |
Thursday, March 16, 2017 10:24AM - 10:36AM |
R44.00011: Optoelectronic Response of the Inversion Breaking Weyl Semimetal TaAs Kenneth Burch, Gavin Osterhoudt, Bing Shen, Ni Ni, Philip Moll The recently discovered class of topological materials known as Weyl semimetals have attracted a large amount of interest due to their bulk topological states, chiral excitations, and high mobilities. However, the scattering mechanisms in these materials are not yet well understood. Insight into these mechanisms may be found through optoelectronic measurements. Furthermore, such measurements could potentially reveal a quantum anomalous Hall state. The work we present in this talk has therefore focused on understanding the response of the inversion symmetry breaking Weyl semimetal TaAs under mid-infrared optical excitation. [Preview Abstract] |
Thursday, March 16, 2017 10:36AM - 10:48AM |
R44.00012: THz magneto-optical study of Weyl semimetal TaAs Andrei Sushkov, Gregory Jenkins, Dennis Drew, Mohammad Jadidi, Bing Shen, Ni Ni Weyl semimetals are predicted to exhibit novel magneto-optical-like effects due to Berry phase singularities associated with the Weyl nodes. It was predicted theoretically and confirmed by ARPES experiments that TaAs is a low doped Weyl semimetal with the broken spatial inversion symmetry. We will report on the results of our experimental broad band THz reflectivity magneto-optical study of TaAs single crystals in Faraday and Voigt geometries. [Preview Abstract] |
Thursday, March 16, 2017 10:48AM - 11:00AM |
R44.00013: Terahertz nonlinear optical response from transition metal monopnictide Weyl semimetal TaAs Shreyas Patankar, Liang Wu, Arielle Little, Eric Thewalt, Dylan Rees, Nityan Nair, James Analytis, Joseph Orenstein Weyl semimetals are a newly discovered class of materials whose low-energy excitations are massless chiral fermions known as Weyl fermions. It has recently been shown that the transition metal monopnictide (TMMP) family of Weyl semimetals has a giant anisotropic nonlinear optical response, [arXiv:1609.04894] as revealed by optical second harmonic generation. We report here the nonlinear response of the TMMP TaAs through optical rectification of amplified femtosecond laser radiation, a phenomenon related to photogalvanic and nonlinear Hall effects. The emitted rectified radiation is in the Terahertz energy range of around 4 meV and allows us to study electron dynamics close to the Fermi surface. [Preview Abstract] |
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