Ultra-fast phase transitions induced by femtosecond lasers in solids: Superheating before the entropy catastrophe

I demonstrate that the entropy change solely due to ultra-fast electronic excitation in a swiftly excited solid is minor in comparison to the entropy catastrophe value. The formation of point defects, one of two major sources of disordering, appears to be the longest of all relaxation processes in fs-laser excited solid. Thus ultra-fast excited solid can be disordered only if a lattice is superheated over the critical temperature determined by the entropy catastrophe. Disordering solely by the electronic excitation is proved to be thermodynamically impossible at excitation levels close to disintegration. Entropy catastrophe criterion in analysis of diverse experiments reveals these salient features of ultra-fast transformation and resolves the controversies in their interpretation.