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 relaxat ion) 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 cor related 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 reductio n of electronic density of states. Surprisingly, relaxation rates, linear i n temperature and independent of magnetic field were observed for Li adsorb ed on a hydrogen-covered Sill 111-(7 x 7) surface also. Various explanation s for this surprising and puzzling result are discussed.