As it is known from visible light experiments, silicon under femtosecond
pulse irradiation can undergo the so-called 'nonthermal melting' if the density
of electrons excited from the valence to the conduction band overcomes a
certain critical value. Such ultrafast transition is induced by strong changes
in the atomic potential energy surface, which trigger atomic relocation.
However, heating of a material due to the electron-phonon coupling can also
lead to a phase transition, called 'thermal melting'. This thermal melting can
occur even if the excited-electron density is much too low to induce
non-thermal effects. To study phase transitions, and in particular, the
interplay of the thermal and nonthermal effects in silicon under a femtosecond
x-ray irradiation, we propose their unified treatment by going beyond the
Born-Oppenheimer approximation within our hybrid model based on tight binding
molecular dynamics. With our extended model we identify damage thresholds for
various phase transitions in irradiated silicon. We show that electron-phonon
coupling triggers the phase transition of solid silicon into a low-density
liquid phase if the energy deposited into the sample is above ∼0.65 eV per
atom. For the deposited doses of over ∼0.9 eV per atom, solid silicon
undergoes a phase transition into high-density liquid phase triggered by an
interplay between electron-phonon heating and nonthermal effects. These
thresholds are much lower than those predicted with the Born-Oppenheimer
approximation (∼2.1 eV/atom), and indicate a significant contribution of
electron-phonon coupling to the relaxation of the laser-excited silicon. We
expect that these results will stimulate dedicated experimental studies,
unveiling in detail various paths of structural relaxation within
laser-irradiated silicon
