Li adsorption at extremely low coverages (${10}^{-3}$ ML and below) on the metallic $\mathrm{Si}\left(111\right)\text{−}(7\times 7)$ 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 $50\mathrm{K}$ and $300\mathrm{K}$ sample temperature and rises considerably above. Comparison with $T_1$-times around $900\mathrm{K}$ (observed with ${}^6\mathrm{Li}$-NMR) excludes adsorbate diffusion as the cause of the relaxation rate. Thus the almost temperature independent relaxation rate below $300\mathrm{K}$ points to an extremely localized and thus narrow band (width about $10\mathrm{meV}$) which pins the Fermi energy. It is responsible for the metallicity of the $(7\times 7)$-reconstruction. Because of the steeply rising relaxation rate beyond $300\mathrm{K}$ this narrow band is located energetically within a gap (approximately $100–500\mathrm{meV}$ 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.

• Received 7 January 2005

DOI:https://doi.org/10.1103/PhysRevB.72.115314