How superheated crystals melt.

In experiments where intense radiation penetrates into the bulk of a solid and causes ultrafast (femtosecond) heating, the superheated crystalline solid melts from within at a temperature above the equilibrium melting temperature. But what happens on the atomic scale as a solid loses crystalline order remains an open question. Molecular dynamics modelling allows the position of every atom to be traced at each instant, as a crystal transforms from solid to liquid. Here we use such detailed atomistic simulations, relevant for aluminium, to show that the thermal fluctuation initiating melting is an aggregate typically with 6-7 interstitials and 3-4 vacancies. This mechanism differs from those that have traditionally been proposed, which generally involve many more atoms at the initial melting stage.