  The single-particle spectral properties near the Mott transition in the
two-dimensional Hubbard model with next-nearest-neighbor hopping are
investigated by using cluster perturbation theory. Complicated spectral
features of this model are simply interpreted, by considering how the
next-nearest-neighbor hopping shifts the spectral weights of the
two-dimensional Hubbard model. Various anomalous features observed in
hole-doped and electron-doped cuprate high-temperature superconductors are
explained in a unified manner as properties near the Mott transition in a
two-dimensional system whose spectral weights are shifted by
next-nearest-neighbor hopping.
