Gap anisotropy in superconductors with paramagnetic impurities

The effect of magnetic impurities with localized moments on the anisotropy in the gap and, hence, of the critical temperature is studied by numerical solution of the Eliashberg equations with a simple model for the anistropy in the interaction. Quantitative results are given for lead for the transition temperature as a function of normal and spin-flip lifetimes. A significantly higher critical concentration of spin-flip scatterers is needed to suppress superconductivity in the anisotropic system than in the isotropic one with the same mean electron-phonon interaction; for concentrations between that critical for the anisotropic and that critical for the isotropic system superconductivity persists because the anisotropy is found to have increased above that in the pure material. This anisotropy increase is studied in detail