Many-body calculation of the surface-state energies for Si(111)2×1

The surface-state excitation energies for the Si(111)2×{}1 surface have been calculated in the GW approximation. The energy position and the dispersion of the occupied and empty surface states are in excellent agreement with photoemission and inverse-photoemission experiments. The calculated quasiparticle surface-state band gap, 0.62 eV, is 0.15 eV larger than the measured onset energy for electron-hole pair excitations. The possibility that excitonic effects are responsible for this difference is examined.