  A precursor effect on the Fermi surface in the two-dimensional Hubbard model
at finite temperatures near the antiferromagnetic instability is studied using
three different itinerant approaches: the second order perturbation theory, the
paramagnon theory (PT), and the two-particle self-consistent (TPSC) approach.
In general, at finite temperature, the Fermi surface of the interacting
electron systems is not sharply defined due to the broadening effects of the
self-energy. In order to take account of those effects we consider the
single-particle spectral function $A({\bf k},0)$ at the Fermi level, to
describe the counterpart of the Fermi surface at T=0. We find that the Fermi
surface is destroyed close to the pseudogap regime due to the spin-fluctuation
effects in both PT and TPSC approaches. Moreover, the top of the effective
valence band is located around ${\bf k}=(\pi/2,\pi/2)$ in agreement with
earlier investigations on the single-hole motion in the antiferromagnetic
background. A crossover behavior from the Fermi-liquid regime to the pseudogap
regime is observed in the electron concentration dependence of the spectral
function and the self-energy.
