CONSERVING SLAVE-BOSON APPROXIMATIONS FOR THE ANDERSON MODEL BEYOND NCA

We derive a general scheme to construct conserving slave boson approxi mations for the single-impurity Anderson model beyond the noncrossing approximation (NCA). The pseudofermion and slave boson spectral functi ons are computed in a conserving T-matrix approximation which includes the maximum number of impurity spin flips in each order of the hybrid ization. In a perturbative evaluation, the singlet channel of the cond uction electron-pseudofermion T-matrix has a pole which is renormalize d by selfconsistency. As a result, the exponents of the infra-red powe rlaw behavior of the pseudoparticle spectral functions are modified w. r.t. NCA and depend on the impurity occupation number. We present resu lts for the exponents in the Kondo regime which are consistent with ex act values given by the x-ray emission and absorption exponents.