High-pressure control of optical nonlinearity in the polar Weyl semimetal TaAs

Chen Li, Xiang Li, T. Deshpande, Xinwei Li, N. Nair, J. G. Analytis, D. M. Silevitch, T. F. Rosenbaum, and D. Hsieh

Phys. Rev. B 106, 014101 – Published 5 July 2022

Abstract

The transition metal monopnictide family of Weyl semimetals recently has been shown to exhibit anomalously strong second-order optical nonlinearity, which is theoretically attributed to a highly asymmetric polarization distribution induced by their polar structure. We experimentally test this hypothesis by measuring optical second harmonic generation (SHG) from TaAs across a pressure-tuned polar-to-nonpolar structural phase transition. Despite the high-pressure structure remaining noncentrosymmetric, the SHG yield is reduced by more than 60% by 20 GPa as compared to the ambient pressure value. By examining the pressure dependence of distinct groups of SHG susceptibility tensor elements, we find that the yield is primarily controlled by a single element that governs the response along the polar axis. Our results confirm a connection between the polar axis and the giant optical nonlinearity of Weyl semimetals and demonstrate pressure as a means to tune this effect in situ.

Received 8 April 2022
Revised 18 June 2022
Accepted 22 June 2022

DOI:https://doi.org/10.1103/PhysRevB.106.014101

Physics Subject Headings (PhySH)

Topological materials

Optical second-harmonic generation Pressure effects

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Chen Li ^1,2, Xiang Li¹, T. Deshpande ¹, Xinwei Li^1,2, N. Nair^3,4, J. G. Analytis^3,4, D. M. Silevitch ¹, T. F. Rosenbaum¹, and D. Hsieh ^1,2,*

¹Department of Physics, California Institute of Technology, Pasadena, California 91125, USA
²Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, California 91125, USA
³Department of Physics, University of California, Berkeley, California 94720, USA
⁴Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

^*Corresponding author: dhsieh@caltech.edu

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Issue

Vol. 106, Iss. 1 — 1 July 2022

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Images

Figure 1
(a) Schematic of the diamond anvil cell (DAC) based SHG-RA setup. Incident and reflected SHG light are shown in red and blue, respectively. The angle between the polarization of the incident beam and the $[1, 1, - 1]$ axis of TaAs is defined as $φ$ . (b) Ambient pressure SHG-RA data from GaAs (001) obtained in a parallel polarization geometry through type Ia and type IIac diamonds (blue lines), compared to data taken on the bare sample outside the DAC (orange lines). The slight asymmetries in the SHG-RA patterns are due to GaAs surface imperfections. (c) Microscope image showing a TaAs (112) crystal sealed inside the DAC and the locations of ruby spheres for checking pressure homogeneity. (d) Ambient pressure SHG-RA data from bare TaAs (112) obtained in parallel and crossed polarization geometries.
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Figure 2
(a) Unit cell of ambient pressure tetragonal ( $I 4_{1} m d$ ) TaAs projected onto the (112) surface. (b) Projection of high-pressure hexagonal TaAs ( $P \bar{6} m 2$ ) onto the same surface. (c) SHG-RA patterns at select pressures measured upon compression and (d) decompression after reaching a maximum pressure of 19.2 GPa. Pressure was tuned in situ, allowing all patterns to be acquired under identical alignment conditions. Horizontal and vertical dashed lines lie along the $[1, 1, - 1]$ and $[1, - 1, 0]$ axes, respectively. The loss of horizontal mirror symmetry in the 14.9 GPa data can be seen from the presence of an intensity dip at $φ \approx 100^{\circ}$ contrasted with hump at $φ \approx 260^{\circ}$ (red arrows).
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Figure 3
Pressure dependence of the $φ$ -integrated SHG intensity from TaAs (112) measured upon compression for both parallel (blue squares) and crossed (orange triangles) polarization geometries. Each curve is normalized to its ambient pressure value. Error bars represent the intensity variation measured across different sample locations. The gray curve is a guide to the eye.
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Figure 4
(a) Representative SHG-RA data acquired in crossed polarization geometry below and above $P_{c}$ (green circles). Fits to the expression for $I_{⊥} (φ)$ given in the main text are overlaid as black lines. A decomposition of the fits into its $A_{1} \to A_{4}$ components is shown to the right. Solid vs open lobes represent opposite signs of the associated trigonometric function. The fitted amplitude of the various terms is shown below, which are normalized to the maximum value of $A_{2}$ for $P < P_{c}$ . (b) Pressure dependence of the fitted amplitudes of $A_{2}$ (blue) and $A_{3}$ (orange).
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