Abstract
The phase transition temperatureT c , the specific heat discontinuity ΔC, and the critical magnetic field of a superconductor containing intermediate valence impurities are calculated. The impurities are described by the Anderson model with orbital degeneracy in theU → ∞ limit when two active ionic configurations, corresponding to4f 0 and4f 1 (Ce impurities), are present. The properties of the superconducting alloy are expressed in terms of thet-matrix for the scattering off the impurities. It is assumed that no correlation between the impurities exists. The transition temperature decreases approximately exponentially with the impurity concentration as for the spin-fluctuation limit and the strong coupling Kondo limit. The critical field deviation functionD(T/T c ) is the same as for the BCS theory. This indicates pair weakening rather than pair breaking, which is consistent with the picture that the mixed valence problem is driven by charge fluctuations, while spin fluctuations play only a secondary role.
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