We explore the effect of fluctuations for T greater than or equal to Tc in the two-dimensional negative Hubbard model within the framework of the self-consistent T-matrix formalism, which goes beyond the BCS approximation and includes pair fluctuations. We enter the regime in which correlations are important, namely U/t = -4.0, where t denotes the hopping matrix element between nearest neighbors and U is the strength of the on-site interaction. Our results show the following: (i) Compared to the free case, the distribution function, n(k), reveals considerable renormalization. Thus we conclude that the Fermi surface is blurred for correlated electron systems. (ii) There is a pseudogap in the density of states, N(omega), around the chemical potential, mu. (iii) The real part of the T-matrix, at zero frequency and zero momentum, vs. T/t exhibits a divergence at a particular temperature. This shows that more
approximate T-matrix calculations, i.e. extended to T=0, are meaningless.
By: J. J. Rodriguez-Nunez (Univ. de Neuchatel, Switz.), S. Schafroth (Univ. Zurich, Switz.), R. Micnas, T. Schneider, H. Beck (Univ. de Neuchatel, Switz.) and M. H. Pedersen (Univ. Zurich, Switz.)
Published in: RZ2609 in 1994
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