1/(N-1) expansion based on a perturbation theory in U for the Anderson   model with N-fold degeneracy
  We study low-energy properties of the N-fold degenerate Anderson model. Using a scaling that takes u=(N-1) U as an independent variable in place of the Coulomb interaction U, the perturbation series in U is reorganized as an expansion in powers of 1/(N-1). We calculate the renormalized parameters, which characterize the Kondo state, to the next leading order in the 1/(N-1) expansion at half-filling. The results, especially the Wilson ratio, agree very closely with the exact numerical renormalization group results at N=4. This ensures the applicability of our approach to N > 4, and we present highly reliable results for nonequilibrium Kondo transport through a quantum dot. 
Akira Oguri, Rui Sakano, and Tatsuya Fujii
cond-mat.mes-hall cond-mat.str-el