Exchange diffusion on unreconstructed face-centered cubic (111) transition metal surfaces was studied through a combination of molecular dynamics with a friction force (quenched molecular dynamics) and the nudged elastic band method. Four homogeneous systems [Au/Au(111), Pt/Pt(111), Ni/Ni(111), and Cu/Cu(111)] provide evidence for the occurrence of exchange via the formation of a transient dimer (concerted exchange) when the following conditions were met. First, adatom and substrate were of the same nature, second, the exchange involves the original adatom and a surface atom in a nearest-neighbor position on the surface (short range exchange). Consideration of two heterogeneous systems, Co/Pt(111) and Co/Au(111), revealed that an alternative mechanism (two-steps exchange) may take place provided the tensile stress of the substrate was sufficiently high. This was the case in Co/Au(111) and in Co/Pt(111), when this latter was submitted to in-plane expansion. By focusing on mechanisms involving surface atoms next-nearest neighbors of the original adatom, two different kinds of medium range diffusion via exchanges were identified. The first amounted to a concerted movement of the adatom and two surface atoms, one of them simply sliding on the surface in a position intermediate between the remaining two, moving upward and downward, respectively. The second occurred through a pair of two correlated short-range exchanges. It was found that exchange diffusion barriers were lowered by the medium-range mechanism on Au(111) substrates.

Mechanisms of Exchange Diffusion on FCC(111) Transition Metal Surfaces. H.Bulou, C.Massobrio: Physical Review B, 2005, 72[20], 205427 (6pp)