Direct Observation of Coherent Longitudinal and Shear Acoustic Phonons in TaAs Using Ultrafast X-Ray Diffraction

Min-Cheol Lee, N. Sirica, S. W. Teitelbaum, A. Maznev, T. Pezeril, R. Tutchton, V. Krapivin, G. A. de la Pena, Y. Huang, L. X. Zhao, G. F. Chen, B. Xu, R. Yang, J. Shi, J.-X. Zhu, D. A. Yarotski, X. G. Qiu, K. A. Nelson, M. Trigo, D. A. Reis, and R. P. Prasankumar

Phys. Rev. Lett. 128, 155301 – Published 13 April 2022

Abstract

Using femtosecond time-resolved x-ray diffraction, we investigated optically excited coherent acoustic phonons in the Weyl semimetal TaAs. The low symmetry of the (112) surface probed in our experiment enables the simultaneous excitation of longitudinal and shear acoustic modes, whose dispersion closely matches our simulations. We observed an asymmetry in the spectral line shape of the longitudinal mode that is notably absent from the shear mode, suggesting a time-dependent frequency chirp that is likely driven by photoinduced carrier diffusion. We argue on the basis of symmetry that these acoustic deformations can transiently alter the electronic structure near the Weyl points and support this with model calculations. Our study underscores the benefit of using off-axis crystal orientations when optically exciting acoustic deformations in topological semimetals, allowing one to transiently change their crystal and electronic structures.

Received 13 November 2020
Revised 19 January 2022
Accepted 25 February 2022

DOI:https://doi.org/10.1103/PhysRevLett.128.155301

Physics Subject Headings (PhySH)

Acoustic phonons Ultrafast phenomena

Topological materials

X-ray diffraction

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Min-Cheol Lee ^1,*, N. Sirica ¹, S. W. Teitelbaum^2,3, A. Maznev^4,5, T. Pezeril^4,6, R. Tutchton ¹, V. Krapivin^2,3,7, G. A. de la Pena^2,3, Y. Huang^2,3,7, L. X. Zhao⁸, G. F. Chen⁸, B. Xu⁸, R. Yang ⁸, J. Shi⁴, J.-X. Zhu¹, D. A. Yarotski ¹, X. G. Qiu ⁸, K. A. Nelson^4,5, M. Trigo^2,3, D. A. Reis^2,3,7,9, and R. P. Prasankumar^1,†

¹Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
²Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
³Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
⁴Department of Chemistry, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA
⁵Institute for Soldier Nanotechnology, Massachusetts Institute of Technology, 500 Technology Square, NE47-598, Cambridge, Massachusetts, 02139, USA
⁶Institut de Physique de Rennes, Université de Rennes 1, UMR CNRS 6251, 35000 Rennes, France
⁷Department of Applied Physics, Stanford University, Stanford, California 94305, USA
⁸Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China
⁹Department of Photon Science, Stanford University, Stanford, California 94305, USA

^*Corresponding author. mclee@lanl.gov
^†Corresponding author. rpprasan@lanl.gov

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Vol. 128, Iss. 15 — 15 April 2022

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Images

Figure 1
Schematic diagram of (a) the time-resolved x-ray diffraction experiment, including the crystal truncation rod scattering (TRS) signal, performed on the (112) face of TaAs, (b) Bragg reflection, and (c) x-ray TRS. The direction of $t_{3}$ is along the surface normal $n$ , while the in-plane coordinates $t_{1}$ and $t_{2}$ are along $[11 \bar{1}]$ and $[1 \bar{1} 0]$ , respectively. The incident ( $k_{i}$ ) and reflected ( $k_{f}$ ) x rays as well as the reciprocal vector for Bragg reflection ( $G_{h k l}$ ) are shown with black arrows.
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Figure 2
(a)–(c) Two-dimensional XRD images, showing diffuse scattering and TRS signals without pump excitation. Here, variation of the rotation angle $Δ ϕ$ enables the TRS peak (blue arrow) to be resolved. (d)–(f) Time-resolved TRS signal traces, integrated over the $3 \times 3 pixel$ area of the detector ( $0.3 6 ° \times 0.3 6 °$ ) indicated by the blue arrow, reveal clear coherent acoustic phonon oscillations. These oscillations were isolated and extracted from (d)–(f) for $Δ ϕ =$ (g) $- 0.1 5 °$ , (h) $- 0.3 °$ , and (i) $- 0.5 °$ , along with their spectral amplitudes (j)–(l). Experimental data (open symbols) were fit (red lines) as described in the text, where the separate fit components for quasilongitudinal (QL-orange lines) and quasishear (QS-blue lines) modes are shown below.
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Figure 3
(a) Schematic diagram illustrating the polarization of the QL (LA, orange), QS (TA2, blue), and transverse acoustic (TA1, gray) modes with respect to the wave vector along the surface normal ( $n$ ) of the (112) plane, displayed in red. The $a$ , $b$ , and $c$ axes define the vector norms to the (100), (010), and (001) planes, respectively. (b),(c) Calculated phonon dispersion along the (112) direction in TaAs, with the LA (orange), TA2 (blue), and TA1 (gray) modes clearly indicated. (c) Comparison of calculated (lines) and experimentally determined (open symbols) acoustic phonon dispersion measured near the Brillouin zone center (000). Here, the QL (QS) mode is indicated in orange (blue). The solid (dashed) lines are obtained from first-principles (elastic constants) calculations. (d),(e) Fits (solid lines) to the spectral line shape of the (d) QS and (e) QL modes (open circles) for $Δ ϕ = - 0.5 °$ using Fano resonance (blue), time-invariant (unchirped frequency, orange), and time-variant (positive frequency chirp, red) frequency models.
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Figure 4
(a) Simulated electronic structure of TaAs, obtained from the crystal structures of the $2 \times 2 \times 2$ supercell in equilibrium. The red box in (a) indicates the region near one of the Weyl points, magnified in (b) and (c). (b),(c) The electronic structure near the Weyl point in (black dashed line) and out of equilibrium after being shifted by 0.2 pm within the supercell for the QL (orange) and QS (blue) acoustic distortions and for $a b$ -plane shear (red) and $c$ -axis longitudinal distortions (sky blue).
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