Dynamics and Potential Surfaces of Small Clusters

Surface Melting and Surface Diffusion on Clusters

Surface melting on clusters is investigated by a combination of analytic modeling and computer simulation. Homogeneous, argon-like clusters bound by Lennard-Jones forces and Cu-like clusters bound by ‘embedded atom’ potentials are the systems considered. Molecular dynamics (MD) calculations have been carried out for clusters with 40–147 atoms. Well below the bulk melting temperature, the surfaces become very soft, exhibiting well-defined diffusion constants even while the cores remain nearly rigid and solid-like. The simulations, particularly animations, of atomic motion reveal that the surface melting is associated not with amorphous, random surface structures in constant, irregular motion, but rather in large-amplitude, organized, collective motion of most of the surface atoms accompanied by a few “floaters” and holes. At any time, a few of the surface atoms move out of the surface layer, leaving vacancies; these promoted particles wander diffusively, the holes also but less so, and occasionally exchange with atoms in the surface layer. This result is the basis for an analytic, statistical model. The caloric curves, particularly the latent heats, show that surface melting of clusters is a “phase change” different from the bulk melting of clusters.