Direct observation of acoustic phonon oscillations in the Weyl semimetal TaAs using ultrafast x-ray diffraction

In the Weyl semimetal TaAs, we investigated optically excited acoustic phonon oscillations using time-resolved X-ray diffraction. The unique surface orientation (112) of the TaAs crystal enabled the simultaneous observation of longitudinal and shear acoustic modes, whose dispersion closely matches our simulations. The long penetration depth and high temporal resolution of femtosecond X-ray pulses allow us to observe an asymmetric lineshape of the longitudinal mode, which is absent from the shear mode. We attribute this spectral asymmetry to a time-dependent frequency chirp resulting from the thermal diffusion of photoinduced carriers. Our study also highlights the benefit of using off-axis crystal orientations in topological materials to optically excite shear deformations, allowing one to transiently control crystal structures of these materials and potentially their topological properties.