Si(557)-Ag: A metallic quasi-one-dimensional system

Abstract

A quasi-one-dimensional (1D) system was fabricated by adsorbing one-fifth of a monolayer of Ag onto a Si(557) surface. Structural investigations performed with electron diffraction and scanning tunneling microscopy provided evidence of a midterrace row of Si adatoms with $2\ifmmode\times\else\texttimes\fi{}$ periodicity along $[1\overline{1}0]$, parallel to the step edges. A similar feature is also found in the Si(557)-Au system. Period doubling along $[1\overline{1}0]$ is also found in the Si(557)-Au system. However, the lattice gas of extra Si adatoms, that is a distinctive feature of Si(557)-Au [J. N. Crain et al., Phys. Rev. Lett. 90, 176805 (2003)], is absent in Si(557)-Ag. Inverse photoemission studies of the Si(557)-Ag energy bands revealed a Fermi level crossing parallel to the step edges along $\overline{\ensuremath{\Gamma}K}$ at $0.5\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{\ensuremath{-}1}$. In the orthogonal direction $(\overline{\ensuremath{\Gamma}{M}^{\ensuremath{'}}})$, the dispersion of this band is flat, indicating that the adlayer system has a quasi-1D symmetry. Another state with anisotropic dispersion is free-electron-like along $\overline{\ensuremath{\Gamma}K}$ but, once again, flat along $\overline{\ensuremath{\Gamma}{M}^{\ensuremath{'}}}$. This is likely to be an image state that is in resonance with the bulk bands of Si and also perturbed by the quasi-1D surface reconstruction.