Minimal charge gap in the ionic Hubbard model

TL;DR

This paper investigates the ionic Hubbard model at zero temperature using mean-field theory, revealing a persistent charge gap at the insulator transition, an intermediate phase for strong interactions, and detailed phase diagrams for specific lattice structures.

Contribution

It introduces a mean-field analysis showing the charge gap remains open at the transition and identifies an intermediate phase in the ionic Hubbard model.

Findings

Charge gap does not close at the insulator transition.

An intermediate phase exists for strong on-site repulsions.

Phase diagrams are computed for specific lattice configurations.

Abstract

We study the ionic Hubbard model at temperature T=0 within the mean-field approximation, and show that the charge gap does not close completely at the ionic-band insulator to antiferromagnetic insulator transition, contrary to previous expectations. Furthermore, we find an intermediate phase for on-site repulsions $U>U_c$ for different lattices, and calculate the phase diagram for the ionic Hubbard model with alternating U, corresponding to a Cu-O lattice.