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 >> 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(\vec{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. 
Ph. Brune and A. P. Kampf