Li adsorption at extremely low coverages (10−3ML and below) on the metallic Si(111)-(7 x 7) surface was studied by β-NMR experiments (measurement of T1 -times). Instead of increasing linearly with the sample temperature, as expected for a metallic system, the relaxation rate α = 1∕T1 was almost constant in between 50K and 300K sample temperature and rises considerably above. Comparison with T1 -times around 900K (observed with 6Li-NMR) excludes adsorbate diffusion as the cause of the relaxation rate. Thus the almost temperature independent relaxation rate below 300K points to an extremely localized and thus narrow band (width about 10meV ) which pins the Fermi energy. It was responsible for the metallicity of the (7 x 7) -reconstruction. Because of the steeply rising relaxation rate beyond 300K this narrow band was located energetically within a gap (approximately 100–500meV wide) in between a lower filled and an upper empty (Hubbard) band. Due to its extremely narrow width it could hardly be detected in photo electron experiments. In dynamical mean field theories based on Hubbard Hamiltonians this kind of density of states was typical for correlated electron systems close to a Mott–Hubbard metal-insulator transition.

Metallic Si(111)-(7 x 7) Reconstruction - a Surface Close to a Mott-Hubbard Metal-Insulator Transition. R.Schillinger, C.Bromberger, H.J.Jänsch, H.Kleine, O.Kühlert, C.Weindel, D.Fick: Physical Review B, 2005, 72[11], 115314 (11pp)