SIGN REVERSAL OF THE ORDER-PARAMETER IN S-WAVE SUPERCONDUCTORS

We show that in a superconductor where two or more bands cross the Fer mi level it is possible, in the framework of the conventional (s wave) BCS theory, that the sign of the superconducting gap is different on the different sheets of the Fermi surface. At least one of two conditi ons has to be satisfied: (1) The interband pairing interaction is weak er than the Coulomb pseudopotential, while the intraband one is strong er, or (2) there is strong interband scattering by magnetic impurities . In the case of YBa2Cu3O7 we shall argue that the first condition is possibly satisfied, and the second one very likely satisfied. In many aspects such a sign-reversal s wave superconductor is similar to a d w ave superconductor, and thus demands revising recent experiments aimed to distinguish between the s and d wave superconductivity in this com pound.