We analyse here the role electrons on Anderson-U impurities play in superconductivity in a metal alloy. We find that phonon coupling at impurities counteracts the traditional effects which dominate T_c suppression in the non-magnetic limit. In some cases, we findt hat non-magnetic impurities can enhance T_c. In the Kondo limit, a Fermi liquid analysis reveals that it is the enhancement in the density of states arising from the Kondo resonancethat counteracts pair-weakening.
Categories: cond-mat.str-el cond-mat.supr-con