SYMMETRY OF THE SUPERCONDUCTING ORDER-PARAMETER IN THE ANDERSON LATTICE MODEL WITH NEAREST-NEIGHBOR ATTRACTIVE INTERACTION

We study an extended version of the Anderson lattice model including a density-density attractive interaction between correlated electrons o n nearest-neighbor sites. Using a perturbative expansion around the at omic limit for the normal phase and the broken-symmetry Hartree-Fock s cheme for the attractive potential, we examine which of the possible s ymmetries of the superconducting order parameter leads to the highest transition temperatures, in the case of nearest-neighbor hopping on a square lattice. We find that in a wide range of electron densities aro und the half filling, the increase of the on-site Coulomb S-repulsion U changes the pairing symmetry from d-wave into p- or into anisotropic s-wave, depending on the parameter regime. For large values of U comp ared to the bandwidth, the superconducting phase is suppress-ed, regar dless of the symmetry of the order parameter. The effects introduced b y variations in the hybridization coupling are also analyzed.