ON THE NATURE OF THE ORDER-PARAMETER IN HTSC AND INFLUENCE OF IMPURITIES

The previously proposed model, based on the extended saddle point singularities in the electron spectrum, weak screening of the Coulomb interaction and phonon-mediated interaction between electrons, is supplemented by a small short-range repulsion of Hund's or spin-fluctuation origin. This permits one to explain the change of sign of the order parameter in a natural way and to resolve the seeming conflicts between existing experimental data. The influence of impurity scattering on T-c, the order parameter at T=0 and the density of states, including the energy gap, are studied for the same model. A simple argument shows that the small-angle scattering due to ionized impurities does not change the superconducting parameters, and all changes come from the large-angle scattering. For the vicinity of the singularity the ''out scattering'' has a small probability, since the density of states beyond the singularity is low. Therefore, the suppression of the gap in this vicinity is weak, and the same is true for T-c. On the other hand, the scattering of electrons distant from the singularity into the singular region is much larger, due to the enhanced density of states. Since the order parameter beyond the singularity is still defined by its value in the singular region, it scales with this value and varies only slightly. However, the energy gap in this region, which does not coincide with the order parameter, increases rapidly with impurity concentration so that the gap becomes isotropic in the strong scattering limit. These conclusions agree with preliminary experimental observations.