Item

Abstract

Li adsorption at extremely low coverages (10(-3) ML and below) on the metallic Si(111)-(7x7) surface has been studied by beta-NMR experiments (measurement of T-1-times). Instead of increasing linearly with the sample temperature, as expected for a metallic system, the relaxation rate alpha=1/T-1 is almost constant in between 50 K and 300 K sample temperature and rises considerably above. Comparison with T-1-times around 900 K (observed with Li-6-NMR) excludes adsorbate diffusion as the cause of the relaxation rate. Thus 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. It is responsible for the metallicity of the (7x7)-reconstruction. Because of the steeply rising relaxation rate beyond 300 K this narrow band is located energetically within a gap (approximately 100-500 meV wide) in between a lower filled and an upper empty (Hubbard) band. Due to its extremely narrow width it can hardly be detected in photo electron experiments. 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.