Exciton Excitations in a One-Dimensional Band Insulator with Hubbard Interactions

We study the properties of exciton excitations in the Hubbard model with alternating potential and dimerized hopping terms at half-filling. With increasing Coulomb repulsion, we find that a spin triplet exciton band develops below the band gap. At certain critical interaction, the excitation gap of this exciton band vanishes and a new phase with a dimerized ground state emerges. The value of this critical Coulomb interaction is determined perturbatively.