KONDO EFFECT IN FERMI SYSTEMS WITH A GAP - A RENORMALIZATION-GROUP STUDY

We present the results of a Wilson renormalization-group study of the single-impurity Kondo and Anderson models in a system with a gap in th e conduction-electron spectrum. The behavior of the impurity susceptib ility and the zero-frequency response function, T << S-z;S-z >> are di scussed in the cases with and without particle-hole symmetry. For the case of no particle-hole symmetry, we find a transition, as the bandga p Delta increases, from a singlet ground state (when Delta much less t han T-K, where T-K is the Kondo temperature) to a doublet ground state (when a Delta much greater than T-K). But there is no such transition for the case with particle-hole symmetry: the ground state is always a doublet. In addition, for the asymmetric Anderson model the correlat ion functions, <(S) over right arrow.<(sigma)over right arrow>(O)>, < n(d) >, and < n(d)(2-n(d))> are computed.