Pair fluctuation effects above T_c

We explore the occurrence of pairing effects above Tc in the 2D attractive Hubbard model. The presence of pairs above Tc goes beyond the BCS approximation, in which pair formation and condensation occur at the same temperature. Using the fully self-conserving T-matrix formalism, which is valid above Tc, we find that (1) the distribution function, n(κ), shows a 10% change of the weight from "below" to "above" κF with respect to the free case, and (2) the phase shift, δφ(ω, κ), shows Θ-like behavior as a function of ω for large momentum κ along the diagonal of the Brillouin zone. Our calculations have been carried out for an interaction of U/t = -4.0 and a temperature of T t = 0.125, where t is the hopping matrix element between nearest neighbors. We conclude that for such an interaction value the Fermi surface is not a well-defined quantity. © 1994.