Quantifying Thermal and Nonthermal Contributions to Disorder in Ultrashort Laser Irradiated Germanium: Nonadiabatic Quantum Molecular Dynamics Study

We elucidate the origin of the ultrashort laser-driven lattice disorder in germanium through nonadiabatic quantum molecular dynamics simulations. The total disorder is dissected into disorder components arising from electron-phonon coupling, covalent bond softening, and ionic thermal activation caused by potential energy surface modification, using which thermal and nonthermal effects are quantified. We find that, although the bond softening effect initially dominates irrespective of the excitation density, the eventual ultrashort laser-driven phase transition involves both the thermal and nonthermal elements in it, with the level of their effects regulated by the electronic excitation density.