1 Diffusion on and in Surfaces : the Atomic Slide Puzzle

This paper presents first results from scanning tunneling microscopy measurements, which show that a close-packed terrace of a metal surface can be far from static, even at temperatures as low as room temperature. We make the motion visible of the atoms in a Cu(001) terrace, by embedding a low density of In atoms in the first Cu layer. The peculiar characteristics of the motion of the In show that the diffusion of surface vacancies is responsible for a continual reshuffling of all the (In and Cu) atoms in the first layer. A comparison with model calculations of the statistics of the vacancy-assisted motion of terrace atoms shows that there must be an attractive interaction between an embedded In atom and a vacancy, which makes the In atom somewhat more mobile than a Cu surface atom. Such an attraction is indeed found in Embedded Atom calculations.