Ultrafast lattice disordering in VO2 accelerated by electronic collisional forces*

Most ultrafast structural phase transitions are considered as chemical reactions that evolve on a potential energy surface along a reaction coordinate that transform the crystal between two structures. This cooperative motion scenario is not appropriate in the case of the photoexcited vanadium dioxide (VO2), where uncorrelated disorder of the local V-V dimers characterize the transition. We demonstrate via temperature-dependent femtosecond x-ray diffuse scattering that the disorder pathway is independent of the initial velocity distribution of the vanadium atoms, determined by the initial lattice temperature. The rapid loss of memory of the initial velocity suggests the presence of non-conservative forces in the photoexcited phase not accounted for in the potential energy, and that inertial dynamics are negligible in the VO2 ultrafast transition. We give arguments that suggest these non-conservative forces have an electronic origin.