  The hot carrier cooling occurs in most photoexcitation-induced phase
transitions (PIPTs), but its role has often been neglected in many theoretical
simulations as well as in proposed mechanisms. Here, by including the
previously ignored hot carrier cooling in real-time time-dependent density
functional theory (rt-TDDFT) simulations, we investigated the role of hot
carrier cooling in PIPTs. Taking IrTe2 as an example, we reveal that the
cooling of hot electrons from the higher energy levels of spatially extended
states to the lower energy levels of the localized Ir-Ir dimer antibonding
states strengthens remarkably the atomic driving forces and enhances atomic
kinetic energy. These two factors combine to dissolute the Ir-Ir dimers on a
timescale near the limit of atomic motions, thus initiating a deterministic
kinetic phase transition. We further demonstrate that the subsequent cooling
induces nonradiative recombination of photoexcited electrons and holes, leading
to the ultrafast recovery of the Ir-Ir dimers observed experimentally. These
findings provide a complete picture of the atomic dynamics in optically excited
structural phase transitions.
