Molecular dynamics simulations of a femtosecond-laser-induced solid-to-solid transition in antimony

Abstract

We performed ab initio molecular dynamics (MD) simulations to describe the ultrafast dynamics of laser-excited antimony on a supercell consisting of 864 atoms. For low laser fluences (represented in our theory by moderate electronic temperatures), we obtain the well-known oscillations of the crystal planes in the [111] direction, corresponding to the large amplitude coherent A$_{1\rm g}$ phonon. For large fluences (high electronic temperature) below the melting threshold, simulations suggest a possible transition from the initial, Peierls-distorted A7 structure into a structure without Peierls distortion. However, fluctuations due to finite size effects prevent a clean demonstration of such a nonthermal phase transition. Therefore, and based on the ab initio results, we derived an analytical potential depending on the electronic temperature and used it to perform large-scale MD simulations in supercells containing up to 10$^6$ atoms. The potential can clearly reproduce the nonthermal phenomena and the excitation of the A$_{1\rm g}$ coherent phonon observed in the ab initio results. Most importantly, due to the minimization of finite size effects, our large-scale simulations predict a clean nonthermal transition from the Peierls-distorted A7 structure into a structure without Peierls distortion.