ELECTRON CORRELATIONS ON A POTASSIUM-COVERED GAAS(110) SURFACE - ABINITIO CALCULATIONS OF THE HUBBARD CORRELATION-ENERGY

Experimentally it was found that the GaAs(110) surface with alkali-met al adsorbates is non-metallic at coverages up to about one monolayer. In contrast, density-functional-theory (DFT) calculations reveal a par tially occupied surface band, which seems to indicate a metallic chara cter of the surface. This apparent contradiction could be solved by th e formation of a Mott-Hubbard insulating state. Therefore we performed DFT calculations for GaAs(110) with K (and Na) adsorbates in order to evaluate the electron correlation energy U and the crystal-field effe cts. The results show that alkali adatoms donate their electrons to su bstrate surface states. These states consist of well localized Ga dang ling orbitals and therefore provide an adequate basis for a Hubbard Ha miltonian. We find U of the same order as the hopping integral t, whic h brings the surface in the Mott-Hubbard regime.