Mean field theory of short-range order in strongly correlated low dimensional electronic systems

Mean field approach, although a generally reliable tool that captures major short-range correlations, often fails in symmetric low dimensional strongly correlated electronic systems like those described by the Hubbard model. In these situations a symmetry is almost broken. The problem is linked to the restoration of the symmetry due to strong fluctuations (both quantum and thermal) on all scales. The restoration of symmetry in statistical models of scalar order parameter fields was treated recently successfully on the Gaussian approximation level by symmetrization of the correlators. Here the idea is extended to fermionic systems in which the order parameter is composite. Furthermore, the precision of the correlators can be improved perturbatively. Such a scheme (based on covariant Gaussian approximation) is demonstrated on the one dimensional (1D) and 2D one band Hubbard models by comparison of the correlator with exact diagonalization and MC simulations, respectively.