Chernogolovka Solid State Phys Inst Chernogolovka 142432 Russia
Carnegie Inst Washington Geophys Lab Washington Dc 20015
Max Planck Inst Festkorperforsch D-70569 Stuttgart Germany

We show that in a superconductor where two or more bands cross the Fermi 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 conditions has to be satisfied: (1) The interband pairing interaction is weaker than the Coulomb pseudopotential, while the intraband one is stronger, 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 wave superconductor, and thus demands revising recent experiments aimed to distinguish between the s and d wave superconductivity in this compound.
 

Physica C, 243 153-159, 1995.