Nonmagnetic Localized Impurity States in Dilute Superconducting Alloys

The effect of magnetic-element impurities in a virtual nonmagnetic state on the superconducting transition temperature $T_c$ of dilute alloys is discussed. It is found that, in general, one-body-type interactions between conduction ($s$) electrons of the host metal and the $d$ electrons of the impurity atoms tend to decrease $T_c$, whereas the two-body type of interactions between $s$ and $d$ electrons can enhance $T_c$ under certain conditions. It appears that the additional attractive electronic interaction, in contradistinction to the phonon-induced mechanism, between electrons of the BCS pairs is a consequence of a proper cognizance of the instantaneous two-body interaction between conduction and impurity electrons.