  High-index surfaces of silicon with adsorbed gold can reconstruct to form
highly ordered linear step arrays. These steps take the form of a narrow strip
of graphitic silicon. In some cases - specifically, for Si(553)-Au and
Si(557)-Au - a large fraction of the silicon atoms at the exposed edge of this
strip are known to be spin-polarized and charge-ordered along the edge. The
periodicity of this charge ordering is always commensurate with the structural
periodicity along the step edge and hence leads to highly ordered arrays of
local magnetic moments that can be regarded as "spin chains". Here, we
demonstrate theoretically as well as experimentally that the closely related
Si(775)-Au surface has - despite its very similar overall structure - zero spin
polarization at its step edge. Using a combination of density-functional theory
and scanning tunneling microscopy, we propose an electron-counting model that
accounts for these differences. The model also predicts that unintentional
defects and intentional dopants can create local spin moments at Si(hhk)-Au
step edges. We analyze in detail one of these predictions and verify it
experimentally. This finding opens the door to using techniques of surface
chemistry and atom manipulation to create and control silicon spin chains.
