Band-insulator-metal-Mott-insulator transition in the half-filled t-t′ ionic Hubbard chain

We investigate the ground-state phase diagram of the half-filled t-t(') repulsive Hubbard model in the presence of a staggered ionic potential Delta using the continuum-limit bosonization approach. We find that with increasing on-site repulsion U, depending on the value of the next-nearest-neighbor hopping amplitude t('), the model shows three different versions of the ground-state phase diagram. For t '< t(c)(*'), the ground-state phase diagram consists of the following three insulating phases: band insulator at U < U-c, ferroelectric insulator at U-c < U < U-s, and correlated (Mott) insulator at U > U-c. For t(')>t(c ') there is only one transition from a spin-gapped metallic phase at U < U-c to a ferroelectric insulator at U > U-c. Finally, for intermediate values of the next-nearest-neighbor hopping amplitude t(c)(*')< t '< t(c)(') we find that with increasing on-site repulsion, at U-c1 the model undergoes a second-order commensurate-incommensurate-type transition from a band insulator into a metallic state and at larger U-c2 there is a Kosterlitz-Thouless-type transition from a metal to a ferroelectric insulator.