High-pressure engineering presents a promising avenue to enhance the thermoelectric property of SnSe, but the behaviors of electronic/lattice systems under pressure, as well as their connections to the thermoelectric performance, remain poorly understood. Here, we employ in-situ high-pressure ultrafast optical pump-probe spectroscopy to investigate the nonequilibrium dynamics of both Fermi and bosonic particles in SnSe. Our results reveal an electronic structure phase transition (EPT) occurring in the low-pressure region, alongside a structural phase transition (SPT) at higher pressures. Notably, at 4 GPa, we observe anomalous behaviors of inter-valley and intra-valley scattering processes, which band structure calculations attribute to a pressure-induced abrupt change in electronic structure, identified as a Lifshitz transition. At 7 GPa, the evolution of electron-phonon scattering lifetime and phonon frequency indicates a structural phase transition from Pnma to Bbmm. These insights into nonequilibrium behaviors of SnSe under high pressure provide a foundational understanding for further optimizing its thermoelectric efficiency.

 

Ultrafast Optical Spectroscopy,high pressure,SnSe