Mechanisms for adatoms diffusing on metal fcc(111) surfaces

On a series of fcc(111) surfaces modeled by the embedded atom method, adatom diffusion mechanisms are studied systematically by molecular dynamics. On most surfaces, as expected, the diffusion mechanism is only hopping. For some other surfaces, however, we find that the exchange mechanism is possible besides the well-known hopping mechanism. More interesting is that the hcp and fcc sites are now different for adatom diffusion: the exchange diffusion process only occurs between the hcp sites.