Self-doping of gold chains on silicon: a new structural model for Si(111)-(5 x 2)-Au

A new structural model for the Si(111)-(5 x 2)-Au reconstruction is proposed and analyzed using first-principles calculations. The basic model consists of a "double honeycomb chain" decorated by Si adatoms. The 5 x 1 periodicity of the honeycomb chains is doubled by the presence of a half-occupied row of Si atoms that partially rebonds the chains. Additional adatoms supply electrons that dope the parent band structure and stabilize the period doubling; the optimal doping corresponds to one adatom per four 5 x 2 cells, in agreement with experiment. All the main features observed in scanning tunneling microscopy and photoemission are well reproduced.