Ab initio evidence for nonthermal characteristics in ultrafast laser melting

Laser melting of semiconductors has been observed for almost forty years; surprisingly, it is not well understood where most theoretical simulations show a laser-induced thermal process. Ab initio nonadiabatic simulations based on real-time time-dependent density functional theory reveals for the first time intrinsic nonthermal melting of silicon, at a temperature far below thermal melting temperature of 1680~K. Both excitation threshold and time evolution of diffraction intensity agree well with experiment. Nonthermal melting is attributed to excitation-induced drastic changes in bonding electron density, and subsequent decrease in melting barrier, rather than lattice heating as previously assumed in the two-temperature models.