First-principles study of instantaneous driving force on a lattice system by electronic excitation

This study explores the mechanisms underlying nonthermal instabilities in non-metallic materials exposed to femtosecond laser irradiation. By employing ab initio molecular dynamics (AIMD) simulations and phonon calculations within the harmonic approximation, we investigated the behaviors of SiO2, SiC, Al2O3, and GaN under highly excited electronic states. The AIMD simulations revealed material-specific electronic temperature thresholds beyond which rapid atomic acceleration occurs. The analysis of atomic rearrangement energies associated with nonthermal acceleration revealed a common mechanism across all materials: electron delocalization accompanied by an increase in inter-nuclear Coulomb repulsion. Phonon calculations using harmonic approximation confirmed that elevated electronic temperatures destabilize the crystal lattice. As the electron temperature increases, phonon modes progressively shift to imaginary frequencies, indicating lattice instability. Notably, bending-dominant and optical modes are particularly prone to softening into imaginary modes, thus playing a critical role in structural destabilization. Furthermore, the increase in kinetic energy during nonthermal melting becomes more pronounced as the number of imaginary phonon modes increases, highlighting their critical role in facilitating structural collapse.