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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 k F = ±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 strongcoupling 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 angleresolved photoelectron spectra of (TaSe 4 ) 2 I, and the blue-bronze K 0.3 MoO 3 , respectively.

The evolution of gaps in the one-electron density of states for the half-filled Hubbard model on a triangular lattice is studied as a function of both the temperature and the coupling constant ͑Hubbard U) using quantum Monte Carlo. The formation of gaps ͑or pseudogaps͒ at finite temperature allows us to distinguish between three regimes: ͑1͒ A strong-coupling Mott-Hubbard regime, characterized by a gap, which persists even at high temperatures; ͑2͒ a weak-coupling paramagnetic regime, characterized by the absence of a pseudogap at any finite temperature; and ͑3͒ an intermediate-coupling spin-density-wave regime, characterized by a pseudogap, which appears when U is increased beyond a critical ͑temperature-dependent͒ value. The behavior of the ͱ3 ϫͱ3 adlayer structures on fourth-group semiconductor surfaces is briefly commented upon in the light of the above discussion.

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