Melting of Mesoscale Lennard-Jones Crystals with Free Surfaces

It is widely considered that crystalline solids melt heterogeneously, i.e., near the equilibrium melting point (T-m) a liquid phase appears at the surface and gradually penetrates into the interior. However, via molecular dynamics (MD) simulations we find a quite new scenario of melting of mesoscale monatomic Lennard-Jones crystals with free surfaces. In the early stage of pre-melting, a quasi-liquid surface layer occurs which contains both liquidlike and solidlike atoms. Further heating leads to the homogeneous occurrence of liquidlike atoms in the interior. Liquidlike configuration grows fast with temperature and a pure liquid surface layer appears just in the next stage of pre-melting together with a homogeneous occurrence/growth of liquidlike atoms throughout the interior. Melting proceeds further by two different mechanisms: the heterogeneous one in the surface shell and the homogeneous one in the interior leading to the fast collapse of crystal lattice. Our findings are supported by the experimental data of melting of crystaline metal films.