Screened Coulomb interaction at Si(111)2×1

We perform a model calculation of the contribution of surface states to the static electronic screening at Si(111)2×1. Surface states are found to enhance screening at short distances, and to give a negative contribution at intermediate distances, which vanishes at long distances. The amount of deviation from substrate screening (almost a factor of 3 at short distances) is mainly due to excitonic effects, while the qualitative behavior is shown to be caused by the reduced dimension of the surface. The results are in agreement with experimental findings.