Electron localization in(7×7)reconstructed and hydrogen-covered Si(111) surfaces as seen by NMR on adsorbed Li

MetallicSi(111)−(7×7)-reconstruction: A surface close to a Mott-Hubbard metal-insulator transition

“…As compared to the previous experiments 12,38 it is now possible to extend the present ones to much higher magnetic fields and to much lower temperatures. Whereas high magnetic fields suppress considerably diffusive contributions to the T 1 -times, low temperatures proved to enhance spectroscopic details in the past.…”

“…Details of the load lock concept, of the wet chemical treatment and of the transfer to UHV can be found elsewhere. 12,45 The highly ordered wet chemically hydrogenated surface is termed "hydrogen terminated" in accordance with the literature. 45,46 Heating was available by electron bombardment from the rear, cooling by contact to a liquid helium reservoir, instead of a LN 2 one as in previous experiments.…”

“…45,46 Heating was available by electron bombardment from the rear, cooling by contact to a liquid helium reservoir, instead of a LN 2 one as in previous experiments. 12,18,38 The lowest temperature reached at the Si crystal was 50 K.…”

“…6 Among the various reconstructions of the Si͑111͒-surface the ͑7 ϫ 7͒ one is not only by far the oldest example but also the best investigated one. [7][8][9][10][11][12][13] ͑For further, in particular older references, see Ref. 9͒ Nevertheless there exists up to now no clear-cut picture as to the detailed reasons which causes its metallicity.…”

“…We therefore resumed our ␤-NMR experiments on extremely dilute 8 Li adsorbates on the Si͑111͒-͑7 ϫ 7͒ surface. 12,38 The total Li coverage amounted to 10 −3 monolayer ͑ML͒ at most. That is in the average one Li atom per 20 ͑7 ϫ 7͒ unit cells.…”