Correlated Insulated Phase Suggests Bond Order between Band and Mott Insulators in Two Dimensions

We investigate the ground state phase diagram of the half-filled repulsive Hubbard model in two dimensions in the presence of a staggered potential Δ, the so-called ionic Hubbard model, using cluster dynamical mean-field theory. We find that for large Coulomb repulsion, U≫Δ, the system is a Mott insulator (MI). For weak to intermediate values of Δ, on decreasing U, the Mott gap closes at a critical value U_c1(Δ) beyond which a correlated insulating phase with possible bond order is found. Further, this phase undergoes a first-order transition to a band insulator (BI) at U_c2(Δ) with a finite charge gap at the transition. For large Δ, there is a direct first-order transition from a MI to a BI with a single metallic point at the phase boundary.