Ultrafast structural phenomena: theory of phonon frequency changes and simulations with code for highly excited valence electron systems

Abstract

When a femtosecond-laser pulse excites a solid it may, among other ultrafast processes, induce coherent phonons, phonon frequency changes, thermal phonon squeezing, and nonthermal melting. Using our in-house code for highly excited valence electron systems, where laser-induced interatomic forces are computed “on the fly” from ab initio theory, we performed molecular dynamics simulations of supercells with up to 800 atoms. For Si we found that thermal phonon squeezing precurses nonthermal melting as a function of fluence. Furthermore, our molecular dynamics trajectories showed that nonthermal melting includes a stage during which the atoms move fractionally diffusive. We also simulated femtosecond-laser-excited Ge. In addition, we explain the electronic origin of laser-induced phonon frequency redshifts and blueshifts in Mg.