Schrieffer-Wolff transformation for the Anderson Hamiltonian in a superconductor.

A generalized Schrieffer-Wolff transformation is introduced to relate the Anderson impurity model in a superconductor to a Kondo-like Hamiltonian. The effective exchange interactions coupling a magnetic impurity to a Bardeen-Cooper-Schrieffer (BCS) spectrum of electronic excitations are expressed in terms of the superconducting coherence factors. New exchange mechanisms originate from the particle-hole coherence unique to superconductors. These interactions are relevant for large asymmetry in the magnetic limit; they vanish for nonmagnetic impurities and in the symmetric Anderson model, for which the ``normal'' Schrieffer-Wolff coupling constant is recovered. The magnetically induced equal-spin Cooper-pairing correlations are suggested to be relevant for superconducting valence fluctuators and within heavy-fermion superconductivity.