Conditions for Bound States in a Superconductor with a Magnetic Impurity

Abstract

Using analytic continuation techniques recently developed for the normal-metal Kondo effect, we examine the structure of Suhl's scattering amplitudes in a superconductor. It is found that there are no bound states (poles in the energy gap) unless the Kondo coupling is stronger than the Cooper pairing force, i.e., $T_K>T_{c0\mathrm{}}$. The positions of the bound-state poles are given as a function of coupling strength and temperature. The poles always stay on the real axis.