ANOMALOUS RESONANCE OF THE SYMMETRICAL SINGLE-IMPURITY ANDERSON MODELIN THE PRESENCE OF PAIRING FLUCTUATIONS

We consider the symmetric single-impurity Anderson model in the presen ce of pairing fluctuations. In the isotropic limit, the degrees of fre edom of the local impurity are separated into hybridizing and nonhybri dizing modes. The self-energy for the hybridizing modes can be obtaine d exactly, leading to two subbands centered at +/- U/2. For the nonhyb ridizing modes, the second order perturbation yields a singular resona nce of the marginal Fermi liquid form. By multiplicative renormalizati on, the self-energy is derived exactly, showing that the resonance is pinned at the Fermi level, while its strength is weakened by renormali zation.