Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session A68: Optical response of topological semimetalsFocus Recordings Available

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Sponsoring Units: DMP Chair: Ken Burch, Boston College Room: Hyatt Regency Hotel Hyde Park B 
Monday, March 14, 2022 8:00AM  8:36AM 
A68.00001: Linear and nonlinear optical responses of multifold fermions Invited Speaker: Liang Wu The absence of mirror symmetry, or chirality, is behind striking natural phenomena found in systems as diverse as DNA and crystalline solids. A remarkable example occurs when chiral semimetals with topologically protected band degeneracies are illuminated with circularly polarized light. Under the right conditions, the part of the generated photocurrent that switches sign upon reversal of the light's polarization, known as the circular photogalvanic effect (CPGE), is predicted to depend only on fundamental constants. The conditions to observe quantization are nonuniversal, and depend on material parameters and the incident frequency. In my talk, I will first present the evidence of multifold fermions by linear optical conductivity. Then I will discuss nonlinear terahertz emission spectroscopy with tunable photon energy from 0.2 eV  1.1 eV in the chiral topological semimetals CoSi and RhSi. Particularly, we identify a large longitudinal photocurrent peaked at 0.4 eV reaching around 550 μ??/??^{2} at room tempreture in CoSi , which is much larger than the photocurrent in any chiral crystal reported in the literature. Using firstprinciples calculations we establish that the peak originates from topological band crossings, reaching 3.3±0.3 in units of the quantization constant. Our calculations indicate that the quantized CPGE is within reach in CoSi upon doping and increase of the hotcarrier lifetime. Finally, I will present our most recent experiments to explore the quantization and interaction effects in these chiral crystals. 
Monday, March 14, 2022 8:36AM  8:48AM 
A68.00002: Spatially Resolved Circular Photogalvanic Effect (CPGE) in Cd_{3}As_{2} Nanobelts Bob M Wang, Henry C Travaglini, Dong Yu Cd_{3}As_{2} is a material on its second rise to fame: its first in the late 20^{th} century following its classification as an inverted band semiconductor with unusually high mobility, and its second now as a gapless Dirac Semimetal. The combination of these two traits paints high promise in its application in fast broadband detector and spintronics technologies. Understanding spindependent transport in this system is paramount to utilizing its capabilities. 
Monday, March 14, 2022 8:48AM  9:00AM 
A68.00003: Nonlinear THz Spectroscopy of the Dirac Semimetal Cd_{3}As_{2} Rishi Bhandia, David Barbalas, Run Xiao, Tanya Berry, Juan Chamorro, Tyrel M McQueen, Nitin Samarth, N. P Armitage Topological semimetals are materials with linear band touchings in their bulk band structure, resulting in lowenergy excitation that behaves like a Weyl or Dirac fermion, resulting in novel electronic properties. Cd_{3}As_{2}, a Dirac topological semimetal, has become a promising candidate for integration into optoelectronic devices due to its chemical stability. Here we investigate the linear and nonlinear THz response of (112) Cd3As2 thin films grown on (111) GaAs via molecular beam epitaxy (MBE). We identify the large nonlinear response as stemming from pumpprobe processes. We then use THzpump, THzprobe to measure the temperaturedependent and fluencedependent relaxation rates of the quasiparticle excitations in this material. 
Monday, March 14, 2022 9:00AM  9:36AM 
A68.00004: Magnetooptical response of chiral fermions. Invited Speaker: Sahal Kaushik Dirac and Weyl materials possess chiral fermions, which are characterized by nontrivial topology and large Berry curvature. Chiral fermions have nontrivial interactions with magnetic fields and light. We propose three different mechanisms for magnetic photocurrents caused by chiral fermions, with different requirements on the symmetry group of the crystal. 
Monday, March 14, 2022 9:36AM  9:48AM 
A68.00005: Faraday rotation in birefringent nonsymmorphic 2D materials Amarnath Chakraborty, Guang Bian, Giovanni Vignale It has been recently reported that materials like Bi monolayer (MBi) and αbismuthene, with crystal lattices described by a nonsymmorphic symmetry group, possess Diraclike band dispersion around specific high symmetry momenta. At variance with graphene, twodimensional nonsymmorphic Dirac semimetals (NSDS) have strongly anisotropic Dirac cones that remain robust in the presence of strong spinorbit coupling and are very sensitive to the application of magnetic fields. In a recent paper, we studied the optical absorption spectra of NSDS based on a minimal model that includes all the relevant aspects of nonsymmorphic symmetry[1], under the assumption that a single Dirac cone dominates the absorption spectrum at low frequency. The absorption spectra were strongly dependent on polarization and frequency near a magneticfield induced van Hove singularity. In this paper, we present a complete study of the magnetooptical properties of the NSDS model, i.e., we calculate both the real and the imaginary parts of all the components of the conductivity tensor in the presence of a magnetic field. From this, we extract the Faraday rotation angle as a function of frequency below the magnetic fieldinduced gap. Our results reveal that the longitudinal conductivities depend on the anisotropy, which gives rise to birefringence. However, the transverse component (the Hall conductivity) is only frequencydependent and proportional to the Faraday rotation angle. At the zerofrequency limit, we get a finite value of Faraday rotation angle, which is 2α, where α is the fine structure constant. This value increases significantly as the frequency approaches the magnetic fieldinduced absorption edge. These results indicate that material like MBi can be suitable polarization rotators with high transmittance overall. More broadly, nonsymmorphic 2D Dirac materials appear to be good platforms for the realization of magnetooptic devices. 
Monday, March 14, 2022 9:48AM  10:00AM 
A68.00006: Evidence for Interband Circularly Polarized Optical Transitions in the Type II Weyl Semimetal NbIrTe_{4} Leigh M Smith, Seyyedesadaf Pournia, Samuel M Linser, Brenden Ortiz, Stephen D Wilson, Giriraj Jnawali, Howard E Jackson, Congcong Le, Yan Sun NbIrTe_{4} is a material with broken inversion symmetry which has 8 Weyl points lying on the kz = 0 plane, all of whjch are all more than ~120 meV above the Fermi surface, and 4 Type II Weyl points lying in each of the kz = ± 0.2 planes which are ~80 meV below the Fermi energy. We use circularly polarized midIR light to excite a twoterminal nanoflake device which shows that the signal associated with the circular photogalvanic effect increases by two orders of magnitude as the excitation energy decreases from 1 eV down to 0.3 eV. In addition, an extremely strong peak is observed between 0.60.8 eV which we associate with a bandtoband transition. Using circularly polarized transient reflectivity, we show that this same transition is seen in the transient circular dichroism signal which undergoes a sign change with the zero near the same energy. Because the 1.5 eV pump pulse can only modulate electronic states near the Fermi level, this allows us to show that either the initial or final state in the optical transition must lie within 80 meV of the Fermi level. A simple model shows that such a transient circular dichroism signal can result from optical transitions from a TypeII Weyl point to a higher lying paraboliclike state. This suggests that these results may be connected with the 8 Weyl points lying in the kz = ± 0.2 planes which are near the Fermi level and are TypeII. 
Monday, March 14, 2022 10:00AM  10:12AM 
A68.00007: Terahertz response of monolayer and fewlayer WTe_{2} at the nanoscale Ran Jing, Yinming Shao, Zaiyao Fei, Chiu Fan Bowen Lo, Rocco A Vitalone, Francesco L Ruta, John Staunton, William J Zheng, Alexander S McLeod, Zhiyuan Sun, Boryuan Jiang, Xinzhong Chen, Michael M Fogler, Mengkun Liu, David H Cobden, Xiaodong Xu, Dmitri N Basov Tungsten ditelluride (WTe_{2}) is an atomically layered transition metal dichalcogenide whose physical properties change systematically from monolayer to bilayer and fewlayer versions. In this presentation, we use apertureless scatteringtype nearfield optical microscopy operating at Terahertz (THz) frequencies and cryogenic temperatures to study the distinct THz range electromagnetic responses of mono, bi and trilayer WTe_{2 }in the same multiterraced microcrystal. THz nanoimages of monolayer terraces uncovered weakly insulating behavior that is consistent with transport measurements. The nearfield signal on bilayer regions shows moderate metallicity with negligible temperature dependence. Subdiffractional THz imaging data together with theoretical calculations involving thermally activated carriers favor the semimetal scenario with over the semiconductor scenario for bilayer WTe_{2}. Also, we observed clear metallic behavior of the nearfield signal on trilayer regions. Our data are consistent with the existence of surface plasmon polaritons (SPPs) in the THz range confined to trilayer terraces in our specimens. In addition, we imaged the THz SPPs of fewlayer WTe_{2} in the phase chanel. Finally, data for microcrystals up to 12 layers thick reveal how the response of a fewlayer WTe_{2} asymptotically approaches the bulk limit. 
Monday, March 14, 2022 10:12AM  10:24AM 
A68.00008: Nonlinear optical Hall effect in a Weyl semimetal WTe2 YoungGwan Choi, ManhHa Doan, Youngkuk Kim, GyungMin Choi The Hall effect indicates generation of a transverse electric current upon exertion of an longitudinal electric field with an perpendicular magnetic field. While ordinary Hall effect shows linear response with timereversal symmetry breaking via an applied external magnetic field or their intrinsic magnetization, a nonlinear Hall effect can generically exist in timereversal symmetric systems resulting from a Berry curvature dipole. Here we report, detections of a nonlinear optical Hall effect in a Weyl semimetal WTe_{2} without an external magnetic field at room temperature. The optical Hall effect results in a optical power dependent polarization rotation of the reflected light, called as the nonlinear Kerr rotation. The nonlinear Kerr rotation linearly depends on the charge current density and optical power, which manifests the currentinduced thirdorder Kerr rotation. We quantitatively determine the fourthorder susceptibility, which shows strong anisotropy depending on the directions of the charge current and the light polarization. Moreover, we conduct light energy dependent nonlinear Kerr rotation measurement. We observe strong variation depending on the light energy implying interband transition near 1.5 eV. Employing symmetry analysis of Berry curvature multipoles, we demonstrate that the nonlinear Kerr rotations can arise from the interband Berry curvature multipole allowed by the crystalline symmetries of WTe_{2}. We also observe the symmetryforbidden Kerr rotation, which suggest a symmetry lowering during the nonlinear process. 
Monday, March 14, 2022 10:24AM  10:36AM 
A68.00009: Nonlinear optical response of a TypeII Weyl semimetal ZiYuan Li, Zhou Li We investigate nonlinear optical responses in the typeI and II Weyl semimetal using timedependent Dirac equations which found good agreement with Kubo formula. As a comparison with typeI, we reveal that strong DC current in typeII semimetal is caused by the inversion symmetry breaking of the electron pockets. Furthermore the superposition of currents in the electron pocket bands weaken the AC contributions, due to the opposedphase interference in time domain. We also verified the evenorder high harmonic generations (HHGs) in anisotropic TypeI and II semimetals at nonzero chemical potential. The symmetry analysis is applied to explain the unusual HHGs excited by the circularly polarized light. We provide a systematic study on tilted dirac cones. 
Monday, March 14, 2022 10:36AM  10:48AM 
A68.00010: Observation of photon drag effect in Dirac semimetal PtSe_{2} using terahertz emission spectroscopy Ee Min E Chia, Liang Cheng, Lixing Kang, Ying Xiong, Qing Chang, Mengji Chen, Jingbo Qi, Hyunsoo Yang, Justin Song, Zheng Liu As a Dirac semimetal, PtSe_{2} has attracted increasing attention due to its interesting fundamental physics and potential application for photonics. In this work, THz generation from PtSe_{2} thin films under linear and circularpolarized infrared femtosecond laser excitation at room temperature is systematically studied. By changing the polarization and helicity of the pump pulses, the THz emission mechanism is confirmed to be a combination of linear and circular photon drag effects, which is consistent with the mechanisms of linear injection current and circular shift current, both of which arise from photoninduced electron transitions in PtSe_{2}. We also demonstrate the ability to control the polarization of the emitted THz radiation by tuning the pump laser. Our observations provide further insights into the nonlinear optical phenomena in PtSe_{2} and new perspectives on the application of PtSe_{2} in THz technology. 
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