  Using scanning tunneling microscopy together with the first-principles
density functional theory calculations we study structural properties of the
Si(111)5x2-Au surface covered by Ag adatoms. The STM topography data show that
a submonolayer coverage of Ag does not lead to a well-ordered adatom chain
structure with a periodicity 5x4, as it was observed for Si, Pb and In adatoms.
Instead of that, we observe Ag adatoms located in different nonequivalent
positions within the unit cell, which confirms the electronic nature of
stabilization of the Si(111)5x2-Au surface. Moreover, the DFT calculations give
five different structural models of the Si(111)5x2-Au/Ag surface. All the
models are almost degenerate in energy, which further supports the above
scenario.
