  Ultrafast photoexcitation can induce a nonequilibrium dynamic with
electron-lattice interaction, offering an effective way to study photoinduced
phase transitions (PIPTs) in solids. The issue that atomic displacements after
photoexcitation belong to coherent change or disordered process, has become a
controversy in the PIPT community. Using real-time time-dependent density
functional theory (rt-TDDFT) simulations, we obtain both the coherent and the
disordered PIPTs (dimer dissociation) in IrTe2 with the different electronic
occupations. More importantly, we found that in the disordered phase
transition, there exists a local correlation between different dimers regarding
their dissociation status. One can define vertical groups across the layers.
The dimers in the same group will dissociate in a correlated fashion: they
either all dissociate, or all not dissociate. On the other hand, the dimers in
neighboring groups will have an anti-correlation: if the dimers in one group
dissociate, the dimers in the neighboring group tend not to be dissociated, and
vice versus.
