Observation of a quasi-1D Mott-Hubbard insulator: The re-entrant Na/Si(111)-3 x 1 surface

We have investigated the origin of a semiconducting phase of the re-entrant Na/Si(111)-3 x 1 surface formed by adding extra adatoms on the earlier 3 x 1 phase at 1/3 monolayers (MLs). The additional Na adatoms are found to form quasi-one-dimensional (1D) atomic chains in our scanning tunnelling microscopy images while keeping the surface semiconducting. The unique features in our valence band and in high-resolution electron-energy-loss spectra suggest that the re-entrant 3 x 1 surface is a Mott-Hubbard type insulator. We thus report a novel quasi-1D insulator-insulator transition where electron-electron correlation plays a decisive role.