  Laser-induced nonthermal melting in semiconductors has been studied over the
last four decades, but the underlying mechanism is still under debate. Here, by
utilizing an advanced real-time time-dependent density functional theory
simulation, we reveal that the photoexcitation-induced ultrafast nonthermal
melting in silicon occurs via homogeneous nucleation with random seeds
originating from a self-amplified local dynamic instability at the photoexcited
states rather than by simultaneously breaking of all bonds, as suggested by the
inertial model, phonon instability, or Coulombic repulsion mechanisms. Due to
this local dynamic instability, any initial small random thermal displacements
of atoms can be amplified by a charge transfer of photoexcited carriers, which
in turn creates a local self-trapping center for the excited carriers and
yields the random nucleation seeds. This finding provides fresh insights into
photoinduced ultrafast nonthermal melting.
