Ultrafast non-thermal laser excitation of gigahertz longitudinal and   shear acoustic waves in spin-crossover molecular crystals   [$Fe(PM-AzA)_{2}$$(NCS)_{2}$]
  We report GHz longitudinal as well as shear acoustic phonons photoexcitation and photodetection using femtosecond laser pulses in a spin-crossover molecular crystal. From our experimental observation of time domain Brillouin scattering triggered by the photoexcitation of acoustic waves across the low-spin (LS) to high-spin (HS) thermal crossover, we reveal a link between molecular spin state and photoexcitation of coherent GHz acoustic phonons. In particular, we experimentally evidence an unconventional non-thermal pathway for the laser excitation of GHz phonons. We also provide experimental insight on the optical and mechanical parameters evolution across the LS/HS spin crossover temperature T$_{1/2}$. 
T. Parpiiev and M. Servol and M. Lorenc and I. Chaban and E. Collet   and H. Cailleau and P. Ruello and G. Chastanet and T. ...
cond-mat.mtrl-sci