Electronic structure and stability of ring clusters in the Si(111)-(7 × 7) Co surface

The ring-cluster structure of the Si(111)-(7 × 7) Co surface is investigated by comparing local-density approximation, spin-dependent pseudofunction theoretical results with data. In the ring-cluster model, each surface unit cell contains a single Co atom substituting for a Si surface atom; this Co atom is then covered by three ''bridging'' and three ''capping'' Si adatoms, yielding C3 point-group symmetry with respect to the Co site. We confirm the energetic stability of the ring-cluster structure, and determine atomic coordinates, including the height, 0.46, that the Co atom lies above the surface plane. Scanning-tunneling-microscopy (STM) images are calculated from the energy-selected electronic-charge densities, and show the threefold (Si capping adatoms) and onefold (Co atoms) structures in the surface unit cell that are experimentally observed for empty and filled states, respectively. The surface density of states is computed to be that of an insulator, in agreement with the tunneling spectra. The predicted surface magnetic moments are small, and the surface electronic structure is calculated for future comparison with photoemission measurements. The relatively small computed barrier height of 0.4 eV per adatom pair between the two degenerate orientations of an isolated cluster suggests that the cluster geometry may switch relatively rapidly at room temperature between the two orientations, giving rise to the toroidal C6-symmetric appearance of isolated clusters in STM images.