A new perturbational approach to spectral and thermal properties of strongly correlated electron systems is presented: The Anderson model is reexamined for $U\to\infty$\,, and it is shown that an expansion of Green's functions with respect to the hybridization $V$ built on Feynman diagrams obeying standard rules is possible. The local correlations of the unperturbed system (the atomic limit) are included exactly through a two-particle vertex. No auxiliary particles are introduced into the theory. As an example and test the small energy scale and many-body resonance of the Kondo problem are reproduced analytically.
Categories: cond-mat hep-th