Modelling one-dimensional insulating materials with the ionic Hubbard model

The single-particle spectral-weight function of the ionic Hubbard model (IHM) at half-filling shows an abrupt change of regime at a critical value of the coupling constant (Hubbard U). Specifically, this function jumps at the Fermi points kF = ± π/2 from a two-peak to a four-peak structure accompanied by a (non-vanishing) minimum of the single-particle charge gap. This jump separates a weak-coupling regime, the band insulating phase, from a strong-coupling regime which evolves gradually into the Mott-Hubbard phase. We take advantage of this critical behaviour to model several quasi-one-dimensional materials in terms of the IHM instead of the simpler one-band Hubbard model. For instance, the two regimes are physically realized in the angle-resolved photoelectron spectra of (TaSe4)2I, and the blue-bronze K0.3MoO3, respectively