3He spin-echo measurements were used to monitor the ps motion of Na atoms on a Cu(001) substrate. Two-dimensional correlated motions arising from repulsive adsorbate interactions were observed at coverages as low as 0.04ML. At coverages greater than 0.05ML, there was a pronounced onset of motion perpendicular to the surface. The latter motion was thermally activated and seemed to be related to the basic translational hopping diffusion process. The correlated motion was modeled successfully by using a molecular dynamics simulation and a dipole-like lateral interaction. A simple model which related the apparent height of the atom to its local coverage was shown to reproduce the experimental observations.

Onset of 3D Collective Surface Diffusion in the Presence of Lateral Interactions - Na/Cu(001). G.Alexandrowicz, A.P.Jardine, H.Hedgeland, W.Allison, J.Ellis: Physical Review Letters, 2006, 97[15], 156103 (4pp)