Remnant Fermi surface in a pseudogap regime of the two-dimensional Hubbardmodel at finite temperature

A precursor effect on the Fermi surface in the two-dimensional Hubbard mode l at finite temperatures near the antiferromagnetic instability is studied using three different itinerant approaches: the second order perturbation t heory, the paramagnon theory (PT), and the two-particle self-consistent (TP SC) approach. In general, at finite temperature: the Fermi surface of the i nteracting electron systems is not sharply defined due to the broadening ef fects of the self-energy. In order to take account of those effects we cons ider the single-particle spectral function A(k, 0) at the Fermi level, to d escribe the counterpart of the Fermi surface at T = 0. We find that the Fer mi surface is destroyed close to the pseudogap regime due to the spin-fluct uation effects in both PT and TPSC approaches. Moreover; the top of the eff ective valence band is located around k = (pi /2, pi /2) in agreement with earlier investigations on the single-hole motion in the antiferromagnetic b ackground. A crossover behavior from the Fermi-liquid regime to the pseudog ap regime is observed in the electron concentration dependence of the spect ral function and the self-energy.