Mean-Field Analysis of Two Antiferromagnetically Coupled Anderson Impurities

We have solved, near T = 0, a model describing two magnetic impurities coupled to a band of itinerant electrons and also antiferromagnetically to each other, using an auxiliary boson mean field theory which is exact in a large N limit. Depending upon parameters we find two possible ground states; one in which the Kondo effect occurs and one in which it does not. As the direct antiferromagnetic interaction is varied relative to the Kondo coupling at T = 0, a phase transition occurs which may be of first or second order. We compute the low temperature behavior of the specific heat and the uniform and staggered susceptibility. The nature of the Kondo phase and of the phase transition differ significantly from the results found in a recent numerical renormalization group calculation of a similar model with N = 2.