Li adsorption at extremely low coverages on the “metallic” Si(111)–(7 x 7) surface was experimentally studied by β-NMR experiments. 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 Arrhenius like above. In order to understand this behaviour in a transparent way a closed form analysis was presented using rectangular density of states distributions. 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. Because of the steeply rising relaxation rate beyond 300K it was located energetically within a gap (about 380meV wide) in between a lower filled and an upper empty (Hubbard) band. 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.

The Si(111)–(7 x 7) Reconstruction - a Surface Close to a Mott–Hubbard Metal–Insulator Transition? D.Fick, C.Bromberger, H.J.Jänsch, O.Kühlert, R.Schillinger, C.Weindel: Surface Science, 2006, 600[18], 3835-8