METALLIC CHARGE-DENSITY WAVES AND SURFACE MOTT INSULATORS FOR ADLAYERSTRUCTURES ON SEMICONDUCTORS - EXTENDED HUBBARD MODELING

Motivated by the recent experimental evidence of commensurate surface CDW in Pb/Ge(111) and Sn/Ge(111) root 3-adlayer structures, as well as by the insulating states found on K/Si(111):B and SiC(0001), we have investigated the role of electron-electron interactions, and also of e lectron-phonon coupling, on the narrow surface state band originating from the dangling bond orbitals of the adsorbate. We model the problem by an extended two-dimensional Hubbard model at half-filling on a tri angular lattice. We include an on-site Hubbard repulsion U and a neare st-neighbor term V, plus a long-range Coulomb tail. The electron-phono n interaction is treated in the deformation potential approximation. W e have explored the phase diagram of the model, including the possibil ity of commensurate (3 x 3) phases, using mainly the Hartree-Fock appr oximation. For U larger than the bandwidth we find magnetic insulators , possibly corresponding to the situation in SiC and in K/Si. For smal ler U,the inter-site repulsion V can stabilize metallic CDW phases, re miniscent of the (3 x 3) structures of Sn/Ge, and possibly of Pb/Ge. ( C) 1998 Elsevier Science B.V. All rights reserved.