Electron Correlations In The Clean And Hydrogen-Covered Si(111)-(7 X 7) Surface At Extremely Low Li Coverages

The observation of "Korringa" nuclear spin relaxation (spin-lattice relaxation) of Li-8 probe atoms, adsorbed at extremely low coverages (below 10(-5) ML) on the Si(111)-(7 x 7) surface points to the existence of a highly correlated two-dimensional electron gas at this surface ("metallic" surface). The observed large relaxation rates, as compared to Li adsorption on metals, is interpreted as an interplay of longer correlation times and a reduction of electronic density of states. Surprisingly, relaxation rates, linear in temperature and independent of magnetic field were observed for Li adsorbed on a hydrogen-covered Sill 111-(7 x 7) surface also. Various explanations for this surprising and puzzling result are discussed.