  Photoinduced phase transition (PIPT) is always treated as a coherent process,
but ultrafast disordering in PIPT is observed in recent experiments. Utilizing
the real-time time-dependent density functional theory (rt-TDDFT) method, here,
we track the motion of individual vanadium (V) ions during PIPT in VO2 and
uncover that their coherent or disordered dynamics can be manipulated by tuning
the laser fluence. We find that the photoexcited holes generate a force on each
V-V dimer to drive their collective coherent motion, in competing with the
thermal-induced vibrations. If the laser fluence is so weak that the
photoexcited hole density is too low to drive the phase transition alone, the
PIPT is a disordered process due to the interference of thermal phonons. We
also reveal that the photoexcited holes populated by the V-V dimerized bonding
states will become saturated if the laser fluence is too strong, limiting the
timescale of photoinduced phase transition.
