Theory of magnetic impurities in superconductors. I

The Nagaoka Green's functions equations for a superconductor are solved exactly and the question of bound states within the gap is answered unambiguously. It is shown that there are always two bound states symmetrically located with respect to the center of the gap. For ferromagnetic coupling,γ>0, and for antiferromagnetic coupling, butT _K ?T _c0 orT _K ?T _c0, these bound states are very close to the gap edges. ForT _K ～T _c 0, however, the bound states move towards the center of the gap. It is pointed out that in the latter case important deviations from the Abrikosov-Gorkov theory, i.e. manifestations of the Kondo effect, should occur which could be tested experimentally by investigating the density of states.