Numerical renormalization group calculation of near-gap peaks in   spectral functions of the Anderson model with superconducting leads
  We use the numerical renormalization group method (NRG) to investigate a single-impurity Anderson model with a coupling of the impurity to a superconducting host. Analysis of the energy flow shows, in contrast to previous belief, that NRG iterations can be performed up to a large number of sites, corresponding to energy differences far below the superconducting gap. This allows us to calculate the impurity spectral function very accurately for frequencies near the gap edge, and to resolve, in a certain parameter regime, sharp peaks in the spectral function close to the gap edge. 
Theresa Hecht, Andreas Weichselbaum, Jan von Delft and Ralf Bulla