Strong Coupling Approach to the Supersymmetric Kondo Model
  We carry out the strong coupling expansion for the SU(N) Kondo model where the impurity spin is represented by a L-shaped Young tableau. Using second order perturbation theory around the strong coupling fixed point it is shown that when the antisymmetric component of the Young-tableau contains more than N/2 entries, the strong-coupling fixed point becomes unstable to a two-stage Kondo effect. By comparing the strong coupling results obtained here with the result using a supersymmetric large N expansion, we are also able to confirm the validity of the the supersymmetric formalism for mixed symmetry Kondo models. 
P. Coleman, C. Pepin and A. M. Tsvelik