Bond-located spin density wave phase in the two-dimensional (2D) ionic Hubbard model

We investigate the quantum phase transitions in the half-filled ionic Hubbard model on a two-dimensional (2D) square lattice using the variational cluster approach (VCA). We present explicit evidence for the tendency toward a novel intermediate phase in this model. This phase is characterized by bond-located magnetization. For weak Coulomb repulsion U, the system is a band insulator, and then undergoes a transition to the intermediate phase at the first-phase boundary U=Uc1. As U is increased beyond the second transition point Uc2, there occurs a Mott insulator accompanied by a long-range antiferromagnetic (AF) order. The bond-located spin density wave competes with the antiferromagnetism while the charge-density modulation exists all the way due to the staggered potential Δ.