title: Triangular Mott-Hubbard insulator phases of Sn/Si(111) and Sn/Ge(111) surfaces.
abstract: The ground state of Sn/Si(111) and Sn/Ge(111) surface alpha phases is reexamined theoretically, based on ab initio calculations where correlations are approximately included through the orbital dependence of the Coulomb interaction (in the local density+Hubbard U approximation). The effect of correlations is to destabilize the vertical buckling in Sn/Ge(111) and to make the surface magnetic, with a metal-insulator transition for both systems. This signals the onset of a stable narrow gap Mott-Hubbard insulating state, in agreement with very recent experiments. Antiferromagnetic exchange is proposed to be responsible for the observed Gamma-point photoemission intensity, as well as for the partial metallization observed above 60 K in Sn/Si(111). Extrinsic metallization of Sn/Si(111) by, e.g., alkali doping, could lead to a novel 2D triangular superconducting state of this and similar surfaces.
authors.affiliation: CNISM and Dipartimento di Fisica, Università degli Studi di L'Aquila, I-67010 Coppito (L'Aquila), Italy.
authors.forename: G
authors.initials: G
authors.lastname: Profeta
authors.forename: E
authors.initials: E
authors.lastname: Tosatti