Pair fluctuation effects above $T_{\rm c}$

Details Pair fluctuation effects above $T_{\rm c}$ Pair fluctuation effects above $T_{\rm c}$

1994-06-03

arxiv.org/abs/cond-mat/9406018v1

Physics

Condensed Matter

2 pages (2 figures available upon request from odr@zurich.ibm.com), LaTeX using espcrc2.sty (Elsevier-style file), for Grenobl

Scientific paper

We explore the occurrence of pairing effects above $T_{\rm c}$ in the 2D attractive Hubbard model. The presence of pairs above $T_{\rm c}$ 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 $T_{\rm c}$, we find that (1) the distribution function, $n(k)$, shows a 10\% change of the weight from ``below'' to ``above'' $k_{\rm F}$ with respect to the free case, and (2) the phase shift, $\delta\phi(\omega,k)$, shows $\Theta$-like behavior as a function of $\omega$ for large momentum $k$ 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.

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