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

Abstract

We have investigated the origin of a semiconducting phase of there-entrant Na/Si(111)-3 × 1 surface formed by addingextra adatoms on the earlier 3 × 1 phase at 1/3 monolayers(MLs). The additional Na adatoms are found to formquasi–one-dimensional (1D) atomic chains in our scanningtunnelling microscopy images while keeping the surfacesemiconducting. The unique features in our valence band and inhigh-resolution electron-energy-loss spectra suggest that there-entrant 3 × 1 surface is a Mott-Hubbard–type insulator.We thus report a novel quasi-1D insulator-insulator transitionwhere electron-electron correlation plays a decisive role.