  Single-particle excitation spectra of the two-dimensional Hubbard model on
the square lattice near half filling and at zero temperature are investigated
on the basis of the self-consistent projection operator method. The method
guarantees a high accuracy of the spectra with high energy and high momentum
resolutions. It takes into account long-range intersite correlations as well as
the strong on-site correlations. Effects of nonlocal excitations are clarified
by comparing the results with those of the single-site approximation. The
calculated spectra verify the quantum Monte-Carlo results for finite
temperatures. The spectra at the Fermi level yield a hole-like Fermi surface in
the underdoped region and an electron-like Fermi surface in the overdoped
region. From a numerical analysis of the momentum dependent effective mass and
self-energy, it is concluded that a marginal Fermi-liquid like state persists
even at finite doping concentrations in the strongly correlated region because
a van Hove singularity is pinned to the Fermi surface. It is also found that a
kink structure appears in the quasiparticle energy band in the same region. The
kink is shown to be caused by a mixing between the quasiparticle band and an
excitation band with strong short-range antiferromagnetic correlations. These
results suggest an explanation for some of the unusual properties of the normal
state in high-$T_{\rm c}$ cuprates.
