Atomic chain ordering with ultra-long periods: Pb/Si(5Â 5Â 7)

These one-dimensional periodicities can be explained assuming that split-off states from surface bands are completely filled by two electrons per Pb atom with corresponding gap opening. This behavior is reminiscent of the formation of charge density waves with tunable wavelengths as a function of excess Pb concentration, and indicates strong electron correlation in this strongly anisotropic 2d system. The alternative, simple band filling within a rigid band model is expected to destabilize the (2 2 3) facet structure upon further adsorption of Pb, which has not been observed.