The attractive Hubbard model in 2-D: Is it capable of describing a pseudogap and preformed pairs?

Deviations from Fermi liquid behavior are well documented in the normal sta te of the cuprate superconductors, and some of these differences seem to be related to pretransitional features appearing at temperatures above T-c. T he observation of a pseudogap, e.g., in ARPES experiments, is a familiar ex ample of this physics. One potential explanation for this behavior involves preformed pairs with finite lifetimes existing in the normal state above T -c. In this way, two characteristic temperatures can be established. A high er one T* at which pairs begin to form and the actual T-c at which a phase- coherent superconducting phase is established. In order to test these ideas we have investigated the negative U Hubbard model in two dimensions in the fully self-consistent ladder approximation at low electron densities. In t he nonself-consistent version of this theory the system always shows an ins tability toward Bose-condensation of infinite lifetime pairs. In contrast t o this, pairs obtain a finite lifetime due to pair-pair interaction and the sharp two-particle bound state is strongly lifetime broadened when self-co nsistency is applied. A quasiparticle scattering rate which varies linearly with temperature is also found. The fully self-consistent calculation we w ere able to perform employed a (k) over right arrow-averaged approximation in which the self-energy loses its (k) over right arrow-dispersion due to a (k) over right arrow-average. This approximation is found to preserve the essential physics.