Photoexcited Ultrafast Order–Disorder Dynamics in Monolayer SnSe: Dominant Role of Anharmonic Phonon Modes

Photoinduced structural transitions have attracted widespread research interest; however, the order/disorder characteristics within ultrafast nonequilibrium dynamics require further in-depth investigation. Utilizing real-time time-dependent density functional theory simulations, we elucidate how the Raman-active phonon modes (A1(0), A1(1), A1(2), and B1) in monolayer SnSe influence ultrafast structural dynamics under controlled laser intensities. The electron–phonon coupling is governed by the shape of the potential energy surfaces of phonon modes. At low intensity, disordered atomic dynamics are triggered by the third-order anharmonic modes (A1(0), A1(1), and A1(2)), while the contribution of the fourth-order anharmonic B1 mode is negligible. As intensity increases, the A1(1) mode becomes dominant, driving ordered atomic dynamics. The A1(1)-mode-dominated vibrations, coupled with amplified vibrational amplitudes, drive the transitions between Pmn21 and P4/nmm structures and the reversal of Pmn21 phase on picosecond time scales. Our simulations establish an atomic-scale framework for understanding ultrafast nonequilibrium structural dynamics and phase transitions.