ausblenden:
Schlagwörter:
PACS numbers: 41.60.Cr, 64.70.K−, 42.65.Re, 61.80.Ba
Zusammenfassung:
As is known from visible-light experiments, silicon under femtosecond pulse irradiation can undergo 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 nonthermal 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.
