Time-resolved x-ray diffraction study of acoustic phonons in TaAs

We utilized femtosecond x-ray pulses to investigate acoustic phonon oscillations in TaAs. We observed that time-resolved x-ray diffraction reveals coherent phonon oscillations of binary acoustic modes in truncation rod scattering. The phonon oscillations correspond to the transverse and longitudinal acoustic modes propagating along the surface normal vector. We suggest that the off-axis orientation of the surface normal makes it possible to observe the transverse phonon oscillation, in contrast to conventional laser-induced acoustic phonon oscillations due to a single longitudinal mode. We also found an asymmetric Fano-like feature in the longitudinal phonon oscillations, suggestive of strong electron-phonon coupling in TaAs.