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 $Δ$, the so-called ionic Hubbard model, using cluster dynamical mean field theory. We find that for large Coulomb repulsion, $U\gg Δ$, 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 (BO) 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.

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