  A combined low-temperature scanning tunneling microscopy and density
functional theory study on the binding and diffusion of copper monomers,
dimers, and trimers adsorbed on Cu(111) is presented. Whereas atoms in trimers
are found in fcc sites only, monomers as well as atoms in dimers can occupy the
stable fcc as well as the metastable hcp site. In fact the dimer fcc-hcp
configuration was found to be only 1.3 meV less favorable with respect to the
fcc-fcc configuration. This enables a confined intra-cell dimer motion, which
at temperatures below 5 K is dominated by thermally assisted tunneling.
