INSULATING STATES OF CORRELATED ELECTRONS

The insulating states of three model systems, the two-chain Hubbard mo del, the two-layer two-dimensional Hubbard model and the two-dimension al symmetric periodic Anderson model are discussed. All of these syste ms at half-filling have a charge gap but they can exhibit various type s of spin gaps ranging from a vanishing spin gap in systems with stron g antiferromagaetic correlations to a spin gap which is equal to the c harge gap in the band-insulator regime. The interchain, interplane, or Anderson model f-d hybridization plays a key role in determining the spin-to-charge gap ratio and the nature of the insulating state.