Pressure‐Controlled Electronic and Structural Phase Transitions in Thermoelectric Snse Revealed by Ultrafast Optical Spectroscopy

Abstract 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. This study employs in situ high‐pressure ultrafast optical pump‐probe spectroscopy to investigate the nonequilibrium dynamics of both Fermi and bosonic particles in SnSe. These 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, anomalous behaviors of inter‐valley and intra‐valley scattering processes are observed, 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.