We have studied a three-band Hubbard model describing the CuO 2 layers in the high-T c superconductors via Quantum Monte Carlo simulations. For a Hubbard interaction U d = 6 on the copper sites (in units of the Cu-O hybridization t Cu − O ), we see the formation of a correlation gap depending on the size of the charge-transfer energy Δ = e p − e d . For inverse temperatures as low as β = 10 we study the behavior of the charge-transfer gap as a function of the parameters Δ, U p and of the O-O hopping matrix element t o - o .
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