Measurements, performed using scanning tunnelling microscopy of the motion of impurity atoms on the Cu (001) surface, were reviewed. Like several other elements, the impurity that was introduced (In) tended to embed itself in the first atomic layer of the surface. From the motion of the embedded In atoms, direct information was obtained on the motion of Cu atoms in the surface. That is, the In atoms were used as tracer particles in order to investigate intrinsic motion in the first Cu layer. The peculiar statistics of the 2-dimensional In diffusion showed that the motion was assisted by a rapidly diffusing

entity, which was identified as being a surface vacancy (a single missing Cu atom in the outermost Cu layer). A comparison with model calculations of the statistics of the vacancy-assisted motion of terrace atoms showed that there must be an attractive interaction, between an embedded In atom and a vacancy, which made the In atom slightly more mobile than a Cu surface atom. Such an attraction was found in embedded-atom method calculations. Nevertheless, the temperature dependence of the In motion provided an accurate estimation of the sum of the formation energy of a vacancy and the activation energy for the motion of vacancies through a clean Cu (001) surface.

Diffusion in a Surface - the Atomic Slide Puzzle. J.W.M.Frenken, R.Van Gastel, S.B.Van Albada, E.Somfai, W.Van Saarloos: Applied Physics A, 2002, 75[1] 11-5