Screening properties of surface states at Si(111) 2 × 1

We calculate the contribution of surface states to the screening of the Coulomb interaction at the Si(111) 2 × 1 surface. Local-field and exchange effects are included according to the many-body approach of Hanke and Sham. Surface states are of little importance for the screening of the interaction between two charges if their separation is large. However, they turn out to give an effect of the order of magnitude of 50% of substrate screening at small distances.