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

TL;DR

This study uses ultrafast X-ray diffraction to observe and analyze coherent longitudinal and shear acoustic phonons in TaAs, revealing their dynamics and potential impact on electronic structure in topological semimetals.

Contribution

It demonstrates the simultaneous excitation and detailed analysis of longitudinal and shear phonons in TaAs using femtosecond X-ray diffraction, highlighting the influence of surface symmetry.

Findings

Observation of asymmetric spectral lineshape indicating frequency chirp.

Identification of surface symmetry enabling simultaneous phonon excitation.

Potential transient alteration of electronic structure near Weyl points.

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 lineshape 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.