Attractive Hubbard model and single-particle pseudogap due to classical pairing fluctuations in two dimensions

It is shown that in the two-dimensional attractive Hubbard model, the mean-field phase transition is replaced by a renormalized classical regime of fluctuations where a pseudogap opens up in the single-particle spectral weight. It is argued that this pseudogap and precursors of the ordered state quasiparticles can occur only in strongly anisotropic quasi two-dimensional materials. This precursor phenomenon differs from preformed local pairs. Furthermore, while critical antiferromagnetic fluctuations would also lead to a pseudogap in the repulsive model, there are some important differences between the superconducting and magnetic pseudogap.