Quantum phase diagram of the generalized ionic Hubbard model for ABn chains

We investigate the ground-state phase diagram of the Hubbard model for the AB(N-1) chain with filling 1/N, where N is the number of atoms per unit cell. In the strong-coupling limit, a charge transition takes place from a band insulator (BI) to a correlated insulator for increasing on-site repulsion U and positive on-site energy difference Delta (energy at A sites lower than at B sites). In the weak-coupling limit, a bosonization analysis suggests that for N > 2 the physics is qualitatively similar to the case N=2 which has already been studied: an intermediate phase emerges, which corresponds to a bond-ordered ferroelectric insulator (FI) with spontaneously broken inversion symmetry. We have determined the quantum phase diagram for the cases N=3 and N=4 from the crossings of energy levels of appropriate excited states, which correspond to jumps in the charge and spin Berry phases, and from the change of sign of the localization parameter z(L)(c). From these techniques we find that, quantitatively, the BI and FI phases are broader for N > 2 than when N=2, in agreement with the bosonization analysis. Calculations of the Drude weight and z(L)(c) indicate that the system is insulating for all parameters, with the possible exception of the boundary between the BI and FI phases.