Universal behavior of the band gap as a function of the atomic mean-square displacement in laser-excited silicon

Abstract We performed systematic ab-initio molecular dynamics (MD) simulations of fs-laser-excited silicon (Si) using the Te-dependent density functional theory (DFT). We considered the case in which the potential energy surface (PES) is strongly modified by the laser excitation, so that nonthermal melting occurs. We analyzed the correlation between the time dependence of electronic properties like the band gap and the laser-induced atomic motion. Surprisingly, we found that the indirect electronic band gap decreases as a universal function of the atomic mean-square displacement (MSD) almost independently of the electronic temperature (laser fluence) and that the dependence is linear for a wide range of MSDs. We also found that a universal dependence is also present when analyzing the band gap as a function of the relative Bragg peak intensities, which can be directly measured in experiments.