Nonmagnetic localized impurity states in dilute superconducting alloys

The effect of magnetic-element impurities in a virtual nonmagnetic state on the superconducting transition temperature Tc 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 Tc, whereas the two-body type of interactions between s and d electrons can enhance Tc 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