LONG-RANGE INTERACTION BETWEEN ADATOMS AT THE CU(111) SURFACE IMAGED BY SCANNING-TUNNELING-MICROSCOPY

We have used an ultra-high vacuum variable temperature scanning tunnel ling microscope to study the Cu(111) surface at temperatures from 90 K to 300 K. After the sample is heated to 900 K, adatoms enriched at th e surface. Around these adatoms ring-formed standing-wave features can be seen in the local density of states (LDOS). When more than one of the adatoms were imaged it became evident that the adatoms preferred l ateral distances in which they shared LDOS standing-wave maximas. This means that adatoms were positioned at multiples of half the Fermi wav elength (15 Angstrom) from each other. We ascribe the interaction that gave these results to the surface state electrons which form the LDOS standing waves. Furthermore the interaction was long-ranged (at least in the order of 80 Angstrom), oscillatory with the pair distance, and present at high temperatures since the adatoms stick to the surface a bove room temperature.