A molecular dynamics study of surface melting

Abstract

The 1,0,0 face of a Lennard-Jones crystal has been examined for surface premelting effects along the crystal-vapour coexistence line by the method of molecular dynamics. The outermost crystal layer becomes quasi-fluid within the narrow temperature range 0.48-0.50 epsilon /k, as evidenced by a rapid increase in diffusivity in the surface plane, and by discontinuities in equilibrium properties characteristic of anomalous first-order transition behaviour. Similar melting is observed for the second and third layers at 0.60-0.62 epsilon /k and 0.67-0.69 epsilon /k respectively, the latter being close to the triple-point temperature. Several properties of the system are reported and discussed in the context of both melting models and available techniques, indicating how surface melting might be detected by LEED experiments. It is conjectured that surface melting should occur for all single-component crystal-vapour interfaces by a similar mechanism to that reported here. A simple empirical melting rule is suggested: Each layer melts when the ratio of kinetic to potential energy per atom in that layer becomes equal to the same ratio as for the bulk crystal at its melting point.