In contrast with the conventional approach where effective models
describe the vicinity of the low-energy renormalization group fixed
points(3,5), our effective model very accurately describes *both
certain low- and high-energy
properties of the original Kondo model*: compare for example
our discussion of the dynamical spin-spin correlation function
in Fig. 2.3. It also yields
thermodynamic quantities that are in excellent agreement with
much more expensive numerical methods (see Fig. 3.3). The nontrivial
behavior of the effective hybridization function encodes the
quasiparticle interaction, which leads to e.g. the correct Wilson
ratio for small magnetic fields (with 5% accuracy). Notice,
however, that our effective model does
not allow the correct evaluation of higher-order correlation functions
beyond the low-energy limit,

In conclusion, our approach describes many aspects of the complicated many-body Kondo physics for not too large magnetic fields within a simple noninteracting model. Therefore one can very easily and intuitively understand certain properties of the Kondo model, e.g. the dependence of correlation functions on a local magnetic field (Fig. 2.3). One main prospect of our approach is to look at other correlation functions using this effective model, in particular the -matrix for applications in the framework of DMFT calculations. Future prospects also include cluster problems and the single impurity Anderson model. Work along these lines is in progress.

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