SCREENING EFFECTS IN SUPERCONDUCTORS

The partition function of the Hubbard model with local attraction and long-range Coulomb repulsion between electrons is written as a functio nal integral with an action A involving a pairing field Delta and a lo cal potential V. After integration over V and over fluctuations in \De lta\(2), the final form of A involves a Josephson coupling between the local phases of Delta and a ''kinetic energy'' term, representing the screened Coulomb interaction between charge fluctuations. The competi tion between Josephson coupling and charging energy allows to understa nd the relation between T-C and composition in high-T-C materials, in particular superlattices, alloys and bulk systems of low doping.