Electron correlations on a potassium-covered GaAs(110) surface: ab-initio calculations of the Hubbard correlation energy

Abstract

Experimentally it was found that the GaAs(110) surface with alkali-metal adsorbates is non-metallic at coverages up to about one monolayer. In contrast, density-functional-theory (DFT) calculations reveal a partially occupied surface band, which seems to indicate a metallic character of the surface. This apparent contradiction could be solved by the 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 effects. The results show that alkali adatoms donate their electrons to substrate surface states. These states consist of well localized Ga dangling orbitals and therefore provide an adequate basis for a Hubbard Hamiltonian. We find U of the same order as the hopping integral t, which brings the surface in the Mott-Hubbard regime.