Renormalization-group methods are applied to the Anderson model for a localized level coupled to a Fermi system in which the density of states varies like ${\left|\epsilon \right|}^r$ near the Fermi energy ($\epsilon =0\mathrm{}$). This model with $r=1\mathrm{} or 2$ may describe magnetic impurities in unconventional superconductors and certain semiconductors. The pseudogap suppresses mixed valence in favor of local-moment behavior. However, it also reduces the exchange coupling on entry to the local-moment regime, thereby narrowing the range of parameters within which the impurity spin becomes Kondo screened.

• Received 26 July 1996

DOI:https://doi.org/10.1103/PhysRevB.54.R15614