MINIMUM ENERGY 2D PATTERNS OF ATOMS ADSORBED ON A HEXAGONAL LATTICE

We obtain the lowest energy 2D arrangements of atoms adsorbed on a hex agonal lattice, assuming rational coverage and a repulsive dipolar ads orbate-adsorbate interaction. To this end we exhaustively explore the ordered arrangements compatible with the coverage, including those tha t have multiatomic unit cells. For some coverages (theta=1/3, 1/4 and 1/7) we find a well defined ground state, and for others a nearly infi nite degeneracy related to the possibility of creating dense arrays of linear defects with a negligible energy cost. We compare our results with some experimental determinations of surface structures in alkali overlayers on fcc (111) and hcp (0001) metal faces. Except for those s ystems that form islands, we have found agreement between our predicte d ground states and experiment. Furthermore, no ordered structures wit h the coverages of our near degenerate states have been observed.