Metal-insulator transitions in the half-filled ionic Hubbard model

We study electronic phase transitions in the half-filled ionic Hubbard model with an on-site Coulomb repulsion U and an ionic energy Delta by using the coherent potential approximation. For a fixed and finite Delta two transitions from the band insulator via a metallic state to a Mott insulator are found with increasing U. The values of the critical correlation-driven metal-insulator transitions U(c1)(Delta) and U(c2)(Delta) are estimated. Our results are in reasonable agreement with the ones obtained by single-site dynamical mean-field theory and determinant quantum Monte Carlo simulation.