  Using cluster perturbation theory, it is shown that the spectral weight and
pseudogap observed at the Fermi energy in recent Angle Resolved Photoemission
Spectroscopy (ARPES) of both electron and hole-doped high-temperature
superconductors find their natural explanation within the t-t'-t''-U Hubbard
model in two dimensions. The value of the interaction U needed to explain the
experiments for electron-doped systems at optimal doping is in the weak to
intermediate coupling regime where the t-J model is inappropriate. At strong
coupling, short-range correlations suffice to create a pseudogap but at weak
coupling long correlation lengths associated with the antiferromagnetic wave
vector are necessary.
