Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session P55: Linear and nonlinear optics of topological materials |
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Sponsoring Units: DCMP Room: Mile High Ballroom 2B |
Wednesday, March 4, 2020 2:30PM - 2:42PM |
P55.00001: THz Emission Spectroscopy of Surface Photogalvanic Effects in a Chiral Weyl Semimetal Darius Torchinsky, Baozhu Lu, Dylan Rees, Kaustuv Manna, Horst Borrmann, Claudia Felser, Joseph Orenstein Topological materials, and Weyl semimetals in particular, exhibit diverse optical effects. If inversion symmetry is broken, second order responses are allowed, expanding the number of methods that can be used to probe the optical properties of a material. These second order effects are also allowed at material interfaces, providing a unique tool to study their topologically important surface states. Here, we present photogalvanic effect data collected via terahertz emission spectroscopy on a chiral Weyl semimetal as a function of polarization and incident wavelength and discuss how our data reveal signatures of the topologically relevant surface states. |
Wednesday, March 4, 2020 2:42PM - 2:54PM |
P55.00002: Non-linear Optical Response of an Exactly Soluble Model of Topological Insulators Peter Riseborough We examine an exactly soluble model of time-reversal and inversion symmetric Topological Insulators and calculate the linear optical conductivity. For frequencies smaller than the bulk gap, the response is quantized and yields information on the surface Weyl cones and their energies. The inclusion of many-body interactions changes the result and may cause an instability due to the spontaneous emission of spin-one k=0 excitations, if the Fermi-energy is in close proximity to the vertex of the Weyl cone. For topological insulators, the low-frequency conductivity is caused by metallic states and the conductivity is highly non-local. The non-linear response functions are shown to diverge at low-frequencies, due to the 1/√ω coupling that lead to the well-known infra-red divergences found in Brehmstrahlung. We show that the infra-red divergences in the response functions due to the interaction with the surface states at both surfaces of the slab, cancel analogously to Furry's Theorem. Cancellation of the divergences seems to requires that the non-local Maxwell's equations are solved and the systematic inclusion many-body interactions. |
Wednesday, March 4, 2020 2:54PM - 3:06PM |
P55.00003: Circular photogalvanic effect in interacting Weyl semimetals Aleksandr Avdoshkin, Vladyslav Kozii, Joel Moore The circular photogalvanic effect (CPGE) is a photocurrent that depends on the sense of circular polarization. In a disorder-free, non-interacting chiral Weyl semimetal, the magnitude of the effect is approximately quantized with a material-independent quantum e^3/h^2 for reasons of band topology. We study the first-order corrections due to the Coulomb and Hubbard interactions in a continuum model of the Weyl points. We find that the inclusion of interactions generically breaks the quantization yet the corrections are non-divergent. The corrections are similar in spirit to the case of interaction corrections to the (non-topological) linear conductivity of graphene. Thus, we conclude that, unlike the quantum Hall effect or the chiral anomaly in field theories, the quantization of the CPGE is not protected but has perturbative corrections in interaction strength. |
Wednesday, March 4, 2020 3:06PM - 3:18PM |
P55.00004: Electronic band structure and free carrier properties of strained tin-germanium alloys on InSb from FTIR ellipsometry Rigo Carrasco, Stefan Zollner, Arnold Kiefer, Bruce Claflin, Stephanie Chastang, Jinsong Duan, Gordon Grzybowski From Fourier-transform infrared ellipsometry, we provide spectroscopic evidence about the valence band structure of α-tin. The mid-infrared dielectric function of α-tin grown pseudomorphically by molecular beam epitaxy shows a strong peak near 0.41 eV. The peak can be attributed to the allowed intra-valence band transitions from the Γ7- (electron-like) to the Γ8+υ heavy hole valence band and/or interband transitions from the Γ7- band to the Γ8+c light “hole” conduction band. Possible sources for the strength of the peak and its temperature dependence will be discussed. Our results are significant, because intra-valence band transitions have not previously been reported as a peak in ellipsometry spectra. This peak cannot be described as a van Hove singularity with a critical point lineshape. It should be universal, i.e., common to all zero-gap semiconductors. We will also show the dependence of this peak on composition and strain in Ge1-xSnx alloys with up to 6% Ge. At photon energies below 0.4 eV, FTIR ellipsometry spectra are dominated by the Drude response of free carriers and a discussion on the species of the carriers will be provided. |
Wednesday, March 4, 2020 3:18PM - 3:30PM |
P55.00005: SHG spectroscopy of the surface states of a chiral Weyl semimetal Dylan Rees, Baozhu Lu, Kaustuv Manna, Claudia Felser, Joseph Orenstein, Darius Torchinsky Second harmonic generation (SHG) is an optical effect that has recently been extensively used to study the electronic band structure as well as lattice and electronic symmetry of various inversion-symmetry breaking solid state materials, including topological systems. Since the surface of any material is a locus of inversion symmetry breaking, SHG spectroscopy has traditionally been used to selectively probe the surface electronic properties of materials. We present an SHG study of a chiral Weyl semimetal as a function of incident photon energy to study SHG response deriving solely from the surface and discuss what our analysis of the nonlinear tensor elements reveals on the topological nature of the material. |
Wednesday, March 4, 2020 3:30PM - 3:42PM |
P55.00006: Kerr and Faraday rotations in a Weyl semimetal in a strong magnetic field Jean-Michel Parent, Rene Cote, Ion Garate One striking property of the Landau level spectrum of a Weyl semimetal (WSM) is the existence of a chiral Landau level (CLL) in which the electrons propagate unidirectionnaly along the magnetic field. This linearly dispersive level influences the optical properties of WSMs. For example, it was shown that a complete optical valley polarization is achievable in a time-reversal symmetric WSM placed in a strong magnetic field [1]. This effect originates from interband transitions involving the CLL and requires a tilt of the Weyl cones in the hamiltonian. In this talk, we show how the presence of the CLL changes the behavior of the Kerr and Faraday rotations of an electromagnetic wave incident on a WSM in a strong magnetic field with respect to a non-topological metal. To calculate the optical conductivity tensor σαβ(ω), we use the minimal model of a WSM developed in Ref. [1] with four tilted Weyl nodes related by mirror and time-reversal symmetry. We present the dependency of the Kerr and Faraday angles on the tilt angle of the cones, the position of the Fermi level, the thickness of the WSM and the magnetic field intensity in both the resonant and non-resonant frequency regimes. |
Wednesday, March 4, 2020 3:42PM - 3:54PM |
P55.00007: Connecting second-order optical response with the band structure geometry of Weyl semimetals Tobias Holder, Daniel Kaplan, Binghai Yan Nonlinear optical response is well studied in the context of semiconductors and has gained a renaissance in studies of topological materials in the recent decade. In non-magnetic materials the response is believed to root in the Berry curvature of the material band structure. Here, we revisit the general formalism for the second-order optical response with finite lifetimes, focusing on the consequences of the time-reversal-symmetry breaking. We identify three physical mechanisms to generate a dc photocurrent, i.e. the Berry curvature, the quantum metric and the diabatic motion. All three effects are non-zero when time-reversal symmetry is broken. They can be understood intuitively from the anomalous acceleration; the first two terms are respectively the antisymmetric and symmetric parts of the quantum geometric tensor. The last term is due to the dynamical antilocalization that refers to the phase accumulation between time-reversed fermion loops. |
Wednesday, March 4, 2020 3:54PM - 4:06PM |
P55.00008: Probing quantum criticality using nonlinear Hall effect in a metallic Dirac system Vladimir Juricic, Habib Rostami Interaction driven symmetry breaking in a metallic (doped) Dirac system can manifest in the spontaneous gap generation at the nodal point buried below the Fermi level. Across this transition linear conductivity remains finite making its direct observation difficult in linear transport. In this talk, we will show how the nonlinear Hall effect can be used as a direct probe of this transition when inversion symmetry is broken [1]. Specifically, for a two-dimensional Dirac material with a tilted low-energy dispersion, we first predict a transformation of the characteristic inter-band resonance peak into a non-Lorentzian form in the collisionless regime. Furthermore, we find that inversion-symmetry breaking quantum phase transition is controlled by an exotic tilt-dependent line of critical points. As this line is approached from the ordered side, the nonlinear Hall conductivity is suppressed owing to the scattering between the strongly coupled incoherent fermionic and bosonic excitations. Our results should motivate further studies of nonlinear responses in strongly interacting Dirac materials. |
Wednesday, March 4, 2020 4:06PM - 4:18PM |
P55.00009: Observation of bulk-to-surface optical transition in topological insulator Bi2Se3 Jiwon Jeon, kwangnam yu, Jiho kim, Jisoo Moon, Seongshik Oh, Eunjip Choi We performed broadband optical transmission measurements of Bi2Se3 and In-doped (Bi1-xInx)2Se3 thin films, where in the latter the spin-orbit coupling (SOC) strength can be tuned by introducing In. An optical absorption peak located at E=1eV in Bi2Se3 becomes completely suppressed at the critical x=0.06 in correlation with the topological surface state (SS) quenched at the same x due to TI-NTI transition. When Bi2Se3 is electrically gated, the 1eV-peak becomes stronger(weaker) when electron is depleted from (accumulated into) the SS. These observations combined together demonstrate that under the 1eV photo illumination, electron is excited from a bulk band into the SS of Bi2Se3. The bulk-to-surface optical transition or equivalently the optical population of the SS, is the first kind of such phenomena observed in TI's, is not only of fundamental significance but also offers an opportunity for optoelectronic application. |
Wednesday, March 4, 2020 4:18PM - 4:30PM |
P55.00010: Chiral Magnetic Photocurrent in Dirac and Weyl Materials Sahal Kaushik, Dmitri E Kharzeev, Evan Philip Circularly polarized light (CPL) can induce an asymmetry between the number of left- and right-handed chiral quasiparticles in Dirac and Weyl semimetals. We show that if the photoresponse of the material is dominated by chiral quasiparticles, the total chiral charge induced in the material by CPL can be evaluated in a model-independent way through the chiral anomaly. In the presence of an external magnetic field perpendicular to the incident CPL, this allows to predict the linear density of the induced photocurrent resulting from the chiral magnetic effect. The predicted effect should exist in any kind of Dirac or Weyl materials, with both symmetric and asymmetric band structure. An estimate of the resulting chiral magnetic photocurrent in a typical Dirac semimetal irradiated by an infrared laser of intensity ~5×106 W/m2 and a wavelength of λ~10μm in an external magnetic field B~2T yields a current J~50nA in the laser spot of size ~50μm. This current scales linearly with the magnetic field and wavelength, opening up possibilities for applications in photonics, optoelectronics, and THz sensing. |
Wednesday, March 4, 2020 4:30PM - 4:42PM |
P55.00011: The Low Energy Second Harmonic Generation Spectrum of the Chiral Multifold Fermion System RhSi Baozhu Lu, Dylan Rees, Kaustuv Manna, Horst Borrmann, Adolfo G Grushin, Claudia Felser, Joseph Orenstein, Darius Torchinsky Lacking any mirror symmetries, the Weyl nodes of the multifold fermion system RhSi are offset in energy, permitting a spectroscopic study of the optical properties of a single node. The lack of any mirror symmetry also implies that RhSi lacks an inversion center and thus can support second order nonlinear optical processes in its bulk. Here, we present a spectroscopic study of one of these processes, the bulk second harmonic generation response as a function of incidence photon energy and measure the single SHG susceptibility tensor element χxyz over the 275 - 900 meV range. We describe how our results directly probe the nonlinear optical response of the isolated Weyl at the Γ point in the Brillouin zone and discuss our findings in the context of the linear optical response of the material. |
Wednesday, March 4, 2020 4:42PM - 4:54PM |
P55.00012: Nonlinear optical response of graphene family materials near topological phase transitions Rajesh Malla, Wilton J De Melo Kort-Kamp
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Wednesday, March 4, 2020 4:54PM - 5:06PM |
P55.00013: Femtosecond to Nanosecond Lifetimes in Optically-Excited Nodal-Line Semimetals Robert Kirby, Greg Scholes, Leslie Schoop Topological semimetals, such as the Dirac semimetal Cd3As2, have been proposed as superior materials for the next generation of highspeed optoelectronic devices due to their previously-observed ultrafast response times and high carrier concentrations [1]. Nodal-line semimetals, such as ZrSiS, should be even better suited to these applications because they have a much higher carrier concentration in Dirac bands, since those bands extend over lines or loops in k-space instead of just around a single point. In this talk, we will discuss our ultrafast optical measurements of the response of ZrSiX (X = S, Se, Te) to narrowband near-infrared pulses. Unlike other studies into this class of materials, multiple time scales are observed in the response, from the femtosecond- to nanosecond-scale. The spectral and kinetic features are connected to the structure and bandstructure of the materials, particularly surrounding the nodal-line straddling the Fermi level. |
Wednesday, March 4, 2020 5:06PM - 5:18PM |
P55.00014: Colliding Electrons and Holes in Semiconductors to Reconstruct Berry Curvature Joseph Costello, Seamus O'Hara, Darren Valovcin, Qile Wu, Mackillo Kira, Loren Pfeiffer, Mark Stephen Sherwin Electron-hole recollisions occur when a probe laser beam excites electron-hole pairs in a semiconductor that is driven by sufficiently strong THz-frequency electric fields [1]. The THz accelerates the electron-hole pairs and causes recollision, leading to the release of photons at higher energy than the probe in a process called high order sideband generation (HSG). |
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