Density functional theory of crystal-fluid interfaces and surface melting

The equilibrium structure of various crystal-fluid interfaces in hard-sphere and Lennard-Jones systems is investigated by a density functional approach based on a weighted density approximation which yields reliable bulk phase diagrams. The practically free minimization of the free energy is achieved. As a result the interfaces between the hard-sphere fluid and the fcc hard-sphere crystal are found to have a width of typically seven hard-sphere diameters. A comparison with previous constrained variational calculations demonstrates that free minimization is indispensable to obtain reliable values for the surface tension. In accordance with recent computer simulations we also find complete wetting of a hard structureless planar wall by the hard-sphere crystal at crystal-fluid coexistence. Finally, the fcc-crystal--gas interface of a Lennard-Jones system exhibits complete surface melting near the triple point for different surface orientations. The width of the interfacial quasiliquid layer depends significantly on temperature and on surface orientation.