Attractive Hubbard model and single-particle pseudogap caused by classicalpairing 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 fl uctuations where a pseudogap opens up in the single-particle spectral weigh t. It is argued that this pseudogap and precursors of the ordered state qua siparticles can occur only in strongly anisotropic quasi-two-dimensional ma terials. This precursor phenomenon differs from preformed local pairs. Furt her, while critical antiferromagnetic fluctuations would also lead to a pse udogap in the repulsive model, there are some important differences with th r superconducting case. (C) 1998 Elsevier Science Ltd. All rights reserved.