Short-range lateral interactions and depolarization of Na atoms on Cu surfaces

The interaction between Na atoms adsorbed on copper surfaces is investigated by means of first-principles simulations based on density-functional theory. In particular, the range of adatom-adatom distances characteristic of coverages of about 0.04-0.08 ML, which have been extensively studied by He-3 scattering experiments, is addressed. We found that the dipole-dipole repulsion effectively reproduces the interaction energy for Na-Na distances larger than those within the c(2 x 2) structure. Here, the remaining short-range term is well characterized by an exponential decay and is shown to be independent of the details of the adsorption configuration and of the surface orientation. Depolarization effects of Na adatoms at short and intermediate distances are also discussed, and are shown to play an important role in the dipole-dipole repulsion, though they are not usually accounted for in molecular dynamics simulations of this system.