Stabilization of local moments in gapless Fermi systems

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 /epsilon/(r) near the Fermi energy (epsilon=0). This model with r=1 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.