KONDO INSULATOR AND CHARGE-TRANSFER INSULATOR IN LATTICE ANDERSON MODEL

Properties of the heavy-fermion valence-fluctuating compounds leading to insulating ground states (called Kondo insulators) are investigated theoretically by using the U-infinite degenerate lattice Anderson model in the auxiliary boson representation. The model is treated within the approximation including terms of the leading order in the expansion w.r.t. inverse of the spin-orbit degeneracy (1/N-expansion) under the strict local constraints guaranteeing equivalence of the bosonic version to the original U-infinite model. The results are compared with the experimental observations on the possible candidates for Kondo insulators. Differences between Kondo insulators and charge transfer type insulators now paid much attention as the mother systems for the high-T(c) superconductors are briefly discussed on a unified basis within the model described above.