2-IMPURITY ANDERSON MODEL - RESULTS OF A PERTURBATIVE EXPANSION IN U

We discuss the results of a study of the two-impurity Anderson model c arried out by using a perturbative expansion in U, which is known to p rovide a power expansion of the exact solution in the one-impurity cas e. The results presented concern the uniform and staggered susceptibil ities, the instantaneous spin-spin correlations [S1.S2], the impurity self-energy and spectral density, the linear coefficient of the impuri ty specific heat (gamma), and the Wilson's ratio. The calculations are performed up to second order in the interaction, and for the nearly ' 'symmetric'' case. Most of the quantities investigated display 2k(F)R oscillations as a function of the impurity separation R, with a series of regions of ferromagnetic and antiferromagnetic correlations betwee n the spins, which are quite independent of the value of the interacti on U. The uniform susceptibility and the linear coefficient of the spe cific heat are enhanced (suppressed) in correspondence with regions of ferromagnetic (antiferromagnetic) correlations. Moreover, we find tha t the oscillations in the static susceptibilities tend to increase con siderably with increasing U, and conjecture that this increase might l ead to large oscillations in the Kondo regime. These oscillations woul d be the hallmark of strong Ruderman-Kittel-Kasuya-Yosida interactions in a Fermi-liquid (Kondo) ground state. We also find that the Wilson ratio has a rich nonuniversal behavior, confirming previous investigat ions on the two-impurity Kondo model. The relevance of these results a nd the connection with the literature is discussed.