Exploring Ultrafast Thermal and Nonthermal Disordering in Semiconductors: A Nonadiabatic Quantum Molecular Dynamics Study

We investigate the phase transition in semiconductors induced by ultrashort and intense laser pulses. To elucidate the origin of ultrafast lattice disordering in semiconductors, we quantify the thermal and nonthermal contributions to disorder in laser-irradiated germanium using nonadiabatic quantum molecular dynamics. This approach, based on time-dependent density functional theory within the linear response framework, captures electronic excitations. The nonadiabatic electron-ion dynamics are described through the fewest switches surface hopping method. This allows us to decompose the time-evolving lattice disorder into its fundamental components, identifying the primary drivers of disorder at each stage. Our results reveal that while bond softening dominates initially—regardless of excitation density—the ultimate ultrafast laser-driven phase transition involves both thermal and nonthermal contributions, with their relative impacts governed by the excitation density.