Starting from the Hubbard model in the weak-coupling limit, we derive a spin-fermion model where the collective spin excitations are described by a non-linear sigma model. This result is used to compute the fermion spectral function $A({ \bf k},\omega)$ in the low-temperature regime where the antiferromagnetic (AF) coherence length is exponentially large (``renormalized classical'' regime). At the Fermi level, $A({ \bf k}_F,\omega)$ exhibits two peaks around $\pm\Delta_0$ (with $\Delta_0$ the mean-field gap), which are precursors of the zero-temperature AF bands, separated by a pseudogap.
Categories: cond-mat.str-el