Origin of the successive metal-insulator-metal transitions induced by K and Cs adsorption on the Si(111)7x7 surface

Adsorption of K and Cs on the Si(111)7x7 surface has been studied in situ u sing a new photoemission method. Explicit results concerning behavior of su rface states of the Si(111)7x7 surface and alkali-metal induced states near both the VBM and E-F are obtained. The metal-to-insulator transition has b een revealed at low K and Cs coverage Theta < 0.15ML. The transition is pro vided by disappearance of S-1 surface state associated with Si "adatom" dan gling bonds. No interaction between adsorbed atoms and Si "rest-atom" dangl ing bonds was found. A model taking into account local interactions between 6 alkali-metal atoms and 12 Si "adatom" dangling bonds is developed for 2D system. The important role in this interaction can be caused metal cluster formation in the 7x7 unit mesh through specific "Negative-U" effect. At in creasing coverage the results indicate metallization via adsorbed layer, wh ich can be related to Mott transition.