Abstract
The insulating states of three model systems, the two-chain Hubbard model, the two-layer two-dimensional Hubbard model, and the two-dimensional symmetric periodic Anderson model are discussed. All of these systems at half-filling have a charge gap but they can exhibit various types of spin gaps ranging from a vanishing spin gap in systems with strong antiferromagnetic correlations to a spin gap which is equal to the charge 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.
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