Strong-coupling limit of the Kondo problem

We examine the low-temperature properties of a single magnetic impurity in a metal, for an arbitrary ratio of the Kondo coupling to the bandwidth, and arbitrary band filling, using the slave-boson technique. For sufficiently strong coupling, we find that true bound states of conduction electrons and the magnetic impurity appear. Then, the corrections to χ and γ take small negative values. In the regime where the conduction band is nearly empty, we find a zero-temperature phase transition between a singlet binding phase and a free-local-moment phase.