MOTT INSULATOR MODEL OF THE SI(111)-(2 * 1) SURFACE.

THEORETICAL CLUSTER CALCULATIONS FOR THE SI(111)-(2 * 1) SURFACE LEAD TO A DESCRIPTION IN WHICH THERE IS A SINGLY OCCUPIED DANGLING BOND ON EACH SURFACE ATOM. THESE ORBITALS ARE WEAKLY COUPLED, FORMING A NONDEGENERATE, NONMETALLIC TWO-DIMENSIONAL MOTT INSULATOR, A DESCRIPTION INVOLVING VERY STRONG ELECTRON CORRELATION EFFECTS. STRONG EXPERIMENTAL SUPPORT FOR THIS MOTT INSULATOR DESCRIPTION IS PROVIDED BY (I) THE MAGNITUDE OF SI(2P) CORE LEVEL SHIFTS, (II) THE DISPERSION OF THE DANGLING BOND ENERGY BANDS, AND (III) THE ABSOLUTE IONIZATION POTENTIAL FROM THE DANGLING BOND LEVELS.