U parameter of the mott-hubbard insulator 6H-SiC(0001)-(sqrt.

The 6H-SiC(0001)-(sqrt[3]xsqrt[3])R30 degrees surface exhibits one half-filled localized dangling-bond orbital per surface unit cell. Its electronic structure can accurately be described as a Mott-Hubbard insulator. We investigate its spectrum by a spin-polarized ab initio quasiparticle calculation. The resulting band structure shows one occupied and one empty surface band, separated by a direct band gap of 1.95 eV. Since the band gap in the spectrum of the Hubbard model is directly given by the on-site Coulomb-interaction parameter U of the dangling-bond orbital, our results allow for a reliable determination of U = 1.95 eV.