The role of local repulsive interactions on superconductor quantum critical points

The study of superconductivity in multi-band systems is relevant for a variety of materials of actual interest. High T-c cuprates, the new Fe-based materials and heavy fermion superconductors are multi-band systems and this feature must be taken into account for a proper description of their properties. In this paper we study superconductivity in two-band systems. One of the bands is a wide band of conduction electrons. The other is a narrow band of f or d character with an attractive interaction among quasi-particles in neighboring sites and a local repulsive interaction U. These bands are coupled by hybridization, which can be controlled by external pressure allowing to probe the phase diagram of these systems. First, we consider the case of weak repulsive interactions for which a mean-field approximation is satisfactory. We obtain the zero temperature phase diagram for weak and strong attractive interactions. We show that in the presence of U the crossover from the BCS to the Bose-Einstein Condensation (BEC) regime at strong attractive couplings remains smooth. Next we consider the case of strong repulsive U and introduce a Hubbard-I approximation to deal with this interaction. We obtain zero temperature phase diagrams as functions of hybridization and the strength of the repulsive and attractive interactions. We consider the cases of extended s-wave and d-wave symmetry of the order parameter. (C) 2012 Elsevier B.V. All rights reserved.