Impurities in two band model of superconductor order parameter anisotropy

The impurity concentration dependence of the superconducting transition temperature Tc is studied within the framework of a two-band model of order parameter anisotropy. Both intraband and interband electron scattering by nonmagnetic and magnetic impurities are taken into account. It is demonstrated that at various values of model parameters, both types of impurity concentration dependence of Tc are possible: weak dependence, typical of the model with s-wave order parameter, and strong suppression of Tc by impurities with critical impurity concentration at which Tc=0. It is found that in some cases there is a possibility of a rise of critical temperature with increasing impurity concentration. The obtained results are consistent with existing experimental data for the high-temperature superconductors.