Spectral Weight Function and Charge-Excitation Gap of the Half-Filled 2D Hubbard Model

Using the combination of the quantum Monte-Carlo and the maximum entropy methods, we study the singleparticle spectral weight function for the half-fiUed two-dimensional Hubbard model including the next-nearest-neighbor hopping. In both the weak- and strong-coupling regimes, it is found that a gap persists as the next-nearest-neighbor hopping is turned on. This indicates that a well-defined chargeexcitation gap is a universal nature of the model, irrespective of its band structure.