BCS superconductivity in mixed valence and heavy fermion superconductors*

We consider competition of Kondo effect and s-wave superconductivity in heavy fermion and mixed valence superconductors, using the slave boson $1/N$ approach for the periodic Anderson model. Similar to the well known results for single-impurity Kondo effect in superconductors, we have found that re-entrant behavior of the superconducting transition temperature, $T_c$, should be observed in heavy fermion superconductors as a function of model parameters or concentration of impurities. Suppression of $T_c$ in mixed valence superconductors is much weaker, without re-entrant behavior of $T_c$. Our results have most validity in the low-temperature regime.