Bound bipolaron at the surface: The negative-Ubehavior of GaAs(110) with adsorbed alkali metals

Using density-functional theory we investigate the electron-lattice coupling of Ga dangling bonds at GaAs(110)/Na and GaAs(110)/K. We find that the electron capture at the bond causes a significant local surface unrelaxation. The lattice distortion even overcompensates the electron-electron repulsion, leading to the formation of a two-electron bound state (surface bipolaron), which manifests the negative-U behavior of the Ga orbital. The electron pairing favors the clustering of adatoms. The influence of the pairing on scanning tunneling microscope images and surface core-level spectra is discussed.