EFFECTS OF COULOMB INTERACTIONS ON THE ELECTRONIC-STRUCTURE OF THE SI(001) SURFACE

The electronic structure of the Si(001) surface is obtained with a tig ht-binding method developed in the slab geometry. The effects of local Coulomb interactions on the stability of antiferromagnetic solutions and on the width of the gap in the surface states at the Si(001) surfa ce reconstructed with symmetric dimers are first visualized with a two -orbital model Hamiltonian. It allows us to study the effect of the co upling between the dangling and the bonding orbitals. The calculation performed with the tight-binding method supports the results of the mo del and provides an explanation of the semiconducting character of the reconstructed Si(001) surface with symmetric dimers. For realistic pa rameters which include the atomic values of the interorbital exchange, an antiferromagnetic order of (2 x 2) symmetry is stable. It is argue d that even if the long-range antiferromagnetic order is destabilized by quantum fluctuations in quasi-two-dimensional surface states, the s hort-range antiferromagnetic order induced in the dangling orbitals by Coulomb interactions is sufficient to explain the semiconducting beha vior of symmetric dimers.