Experiments and simulations on the effect of a surfactant on atomic di
ffusion are presented. Cu adatoms deposited on a Cu(111) surface cover
ed with a monolayer of Pb quickly get buried and diffuse under the Pb
layer by exchanging sites with other Cu atoms from the substrate. The
surfactant induces layer-by-layer growth by suppressing the hopping me
chanism of diffusion over the terraces and promoting atomic exchange,
both at the terraces and across the steps. This mechanism may be rathe
r general: and has profound implications for the synthesis of artifici
al materials.