  Pt deposited onto a Ge(001) surface gives rise to the spontaneous formation
of atomic nanowires on a mixed Pt-Ge surface after high temperature annealing.
We study possible structures of the mixed surface and the nanowires by total
energy (density functional theory) calculations. Experimental scanning
tunneling microscopy images are compared to the calculated local densities of
states. On the basis of this comparison and the stability of the structures, we
conclude that the formation of nanowires is driven by an increased
concentration of Pt atoms in the Ge surface layers. Surprisingly, the atomic
nanowires consist of Ge instead of Pt atoms.
