The Si(1 1 1)-(7 × 7) reconstruction: A surface close to a Mott-Hubbard metal-insulator transition?

Li adsorption at extremely low coverages on the “metallic” Si(1 1 1)-(7 × 7) surface has been experimentally studied recently by β-NMR experiments. Instead of increasing linearly with the sample temperature, as expected for a metallic system, the relaxation rate α = 1/T₁ is almost constant in between 50 K and 300 K sample temperature and rises Arrhenius like above. In order to understand this behaviour in a transparent way a closed form analysis is presented using rectangular density of states distributions. The almost temperature independent relaxation rate below 300 K points to an extremely localized and thus narrow band (width about 10 meV) which pins the Fermi energy. Because of the steeply rising relaxation rate beyond 300 K it is located energetically within a gap (about 380 meV 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 is typical for correlated electron systems close to a Mott-Hubbard metal-insulator transition.