Magnetic transition in a correlated band insulator

The effect of on-site electron-electron repulsion U in a band insulator is explored for a bilayer Hubbard Hamiltonian with opposite sign hopping in the two sheets. The ground-state phase diagram is determined at half-filling in the plane of U and the interplanar hybridization V through a computation of the antiferromagnetic (AF) structure factor, local moments, single-particle and spin wave spectra, and spin correlations. Unlike the case of the ionic Hubbard model, no evidence is found for a metallic phase intervening between the Mott and band insulators. Instead, upon increase of U at large V, the behavior of the local moments and of single-particle spectra give quantitative evidence of a crossover to a Mott insulator state preceding the onset of magnetic order. Our conclusions generalize those of single-site dynamical mean-field theory, and show that including interlayer correlations results in an increase of the single-particle gap with U. DOI: 10.1103/PhysRevB.87.12514