Abstract:
The dual fermion approach is a novel approach to describe nonlocal effects in strongly correlated systems with local interactions. It is based on a fully renormalized diagrammatic expansion around the well-established dynamical mean field theory (DMFT) and allows to treat non-local correlations beyond the mean-field level. This approach is generalized to facilitate the calculation of two-particle Green functions and susceptibilities [1]. The Bethe-Salpeter equations in terms of dual fermions are formulated for the full vertex in the particle-hole and particle-particle channels and an approximation for practical calculations is introduced. The method is applied to the two-dimensional Hubbard model. It is shown that the theory converges in the opposite limits of weak and strong coupling. Results for the half-filled and doped case are presented. At half filling, we calculate the spin-spin susceptibility and find a suppression of the Neel temperature as compared to DMFT due to the incorporation of the fluctuations.

[1] Phys. Rev. B 77, 195105 (2008).