Scattering of electrons by impurities within the framework of two band   model of order parameter anisotropy
  The impurity concentration dependence of superconducting transition temperature $T_c$ is studied within the framework of 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 $T_c$ are possible: weak dependence typical of the model with s-wave order parameter and strong suppression of $T_c$ by impurities with critical impurity concentration at which $T_c=0$. 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. 
P.I. Arseev, N.K. Fedorov, S.O. Loiko