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In recent years, it has become possible to create well-ordered semiconductor surfaces with metallic surface states by using self-assembly of metal atoms. Since these states lie in the band gap of the semiconductor, they completely de-couple from the substrate. The surface structures can be tailored from two-dimensional triangular lattices to nearly one-dimensional atomic chains, which may be considered as the ultimate nanowires. The dimensionality can be varied systematically between 2D and 1D by using vicinal surfaces with variable step spacing. Angle-resolved photoemission and scanning tunneling spectroscopy reveal surprising features, such as a fractional band filling, nanoscale phase separation into doped and undoped chain segments, and a spin-splitting at a non-magnetic surface. |