  Stabilization of the Si(553) surface by Au adsorption results in two
different atomically defined chain types, one of Au atoms and one of Si. At low
temperature these chains develop two- and threefold periodicity, respectively,
previously attributed to Peierls instabilities. Here we report evidence from
scanning tunneling microscopy that rules out this interpretation. The x3
superstructure of the Si chains vanishes for low tunneling bias, i.e., close
the Fermi level. In addition, the Au chains remain metallic despite their
period doubling. Both observations are inconsistent with a Peierls mechanism.
On the contrary, our results are in excellent, detailed agreement with the
Si(553)-Au ground state predicted by density-functional theory, where the x2
periodicity of the Au chain is an inherent structural feature and every third
Si atom is spin-polarized.
