Anisotropy of the energy gap in the insulating phase of the U-t-t′ Hubbard model

We apply a diagrammatic expansion method around the atomic limit (U much greater than t) for the U-t-t' Hubbard model at half filling and finite temperature by means of a continued fraction representation of the one-particle Green's function. From the analysis of the spectral function A(k,omega) we find an energy dispersion relation with a (cos k(x) - cos k(y))(2) modulation of the energy gap in the insulating phase. This anisotropy is compared with experimental ARPES results on insulating cuprates.