Nothing moves a surface: Vacancy mediated surface diffusion

We report scanning tunneling microscopy observations, which imply that all atoms in a Cu(001) surface move frequently, even at room temperature. Using a low density of embedded indium "tracer" atoms, we visualize the diffusiv e motion of surface atoms. Surprisingly, the indium atoms seem to make conc erted, long jumps. Responsible fur this, motion is an ultralow density of s urface vacancies, diffusing rapidly within the surface. This interpretation is supported by a detailed analysis of the displacement distribution of th e indium atoms, which reveals a shape characteristic fur the vacancy mediat ed diffusion mechanism that we propose.