Incipient antiferromagnetism and low-energy excitations in the half-filled two-dimensional Hubbard model

We present single-particle and thermodynamic properties of the half-filled single-band Hubbard model in 2D calculated in the self-consistent fluctuation exchange approximation. The low-energy excitations at moderate temperatures and small $U$ are quasiparticles with a short lifetime. As the temperature is lowered, coupling to evolving spin fluctuations leads to the extinction of these quasiparticles, signaled by a weak pseudogap in the density of states and by a positive slope in Re $\Sigma ({\bf k}_F, \varepsilon)$ and a local maximum in $|$Im $\Sigma ({\bf k}_F, \varepsilon)|$ at $\varepsilon = 0$. We explain these results using a simple spin-fluctuation model.