Quantum Monte Carlo and Numerical Renormalization Group Studies of Magnetic Impurities in Nonmetallic Systems

We study magnetic properties of a single-impurity Anderson model in the symmetric case, in which the density of states for conduction electrons vanishes in a finite energy gap containing the Fermi energy. A quantum Monte Carlo simulation at finite temperatures and low-energy excitations calculated by a numerical renormalization group method reveal that at low temperatures the impurity magnetic susceptibility follows Curie's law. This behavior is consistent that the ground state is a doublet. The low-temperature Curie constant decreases monotonically with decreasing energy gap and the critical point is the zero gap. In the narrow gap limit, the impurity g-value of the doublet state is proportional to the gap width. This situation is due to the special feature of the symmetric case.