  The interaction of cobalt atoms with silicon (111) surface has been
investigated by means of scanning tunneling microscopy (STM) and low-energy
electron diffraction (LEED). Besides the Co silicide islands, we have
successfully distinguished two inequivalent Co-induced
$\sqrt{13}\times\sqrt{13}$ reconstructions on Si (111) surface. Our
high-resolution STM images provide some structural properties of the two
different $\sqrt{13}\times\sqrt{13}$ derived phases. Both of the two phases
seem to form islands with single domain. The new findings will help us to
understand the early stage of Co silicide formations.
