Nonequilibrium dynamics of charge and lattice in pressurized SnSe

Improving the performance of thermoelectric (TE) material necessitates a comprehension of its lattice dynamics and electron-phonon $(e\text{\ensuremath{-}}\mathrm{ph})$ interactions. Herein, we examine the nonequilibrium photocarrier dynamics in SnSe, a representative TE compound, under pressure using optical pump-probe spectroscopy. Abrupt changes in $e$-ph couplings are revealed at approximately 0.8 GPa and 5.3 GPa, arising from the electronic Lifshitz transition and the Pnma-Cmcm phase change, respectively. Beyond 5.3 GPa, we observe the emergence of coherent phonon from the ${\mathrm{A}}_{g}^{1}$ mode in the terahertz range, with an enhancement at higher pressure. Furthermore, a significant reduction in lifetime and redshift in frequency highlight the strong anharmonicity of the ${\mathrm{A}}_{g}^{1}$ mode in the high-pressure $Cmcm$ phase. These findings are further supported by Raman spectroscopy and density functional theory calculations. This paper provides nontrivial insights into e-ph and lattice dynamics in the high-pressure phase of SnSe.