Novel Phase 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 $\Delta$, the so-called ionic Hubbard model, using cluster dynamical mean field theory. We find that for large Coulomb repulsion, $U >>\Delta$, the system is a Mott insulator (MI). For weak to intermediate values of $\Delta$, on decreasing U, the Mott gap closes at a critical value $U_{c1}(\Delta)$ beyond which a correlated insulating phase suggesting bond order (BO) is found. Further, this phase undergoes a first-order transition to a band insulator (BI) at $U_{c2}(\Delta)$ with a finite charge gap at the transition. For large $\Delta$, there is a direct first-order transition from a MI to a BI with a single metallic point at the phase boundary.