EVIDENCE OF A BIPOLARONIC, INSULATING STATE OF NA SUBMONOLAYER ON GAAS(110)

We present electron-energy-loss-spectroscopy results for the Na/GaAs(1 10) ultrathin interface. The data do not show the loss features common ly found for Cs, K, and Rb overlayers, which are considered to be the fingerprints of a Mott-Hubbard insulating state of the interface. In p hotoemission we observe a distinct Na-induced surface state, appearing at a coverage of about 0.25 ML, which disappears approaching the comp lete monolayer. We interpret our results on the basis of the bipolaron model which suggests that insulating character of the interface origi nates from the ''negative-U'' behavior of the surface state.