  The interaction-driven insulator-to-metal transition has been reported in the
ionic Hubbard model (IHM) for moderate interaction $U$, while its metallic
phase only occupies a narrow region in the phase diagram. To explore the
enlargement of the metallic regime, we extend the ionic Hubbard model to two
coupled layers and study the interplay of interlayer hybridization $V$ and two
types of intralayer staggered potentials $\Delta$: one with the same (in-phase)
and the other with a $\pi$-phase shift (anti-phase) potential between layers.
Our determinant Quantum Monte Carlo (DQMC) simulations at lowest accessible
temperatures demonstrate that the interaction-driven metallic phase between
Mott and band insulators expands in the $\Delta-V$ phase diagram of bilayer IHM
only for in-phase ionic potentials; while anti-phase potential always induces
an insulator with charge density order. This implies possible further extension
of the ionic Hubbard model from the bilayer case here to realistic
three-dimensional model.
