Dual fermion approach to susceptibility of correlated lattice fermions

In this paper, we show how the two-particle Green function can be obtained within the framework of the dual fermion approach. This facilitates the calculation of the susceptibility in strongly correlated systems where long-ranged nonlocal correlations cannot be neglected. We formulate the Bethe-Salpeter equations for the full vertex in the particle-particle and particle-hole channels and introduce an approximation for practical calculations. The scheme is applied to the two-dimensional Hubbard model at half-filling. The spin-spin susceptibility is found to strongly increase for the wave vector q=(pi,pi), which indicates the antiferromagnetic instability. We find a suppression of the critical temperature compared to the mean-field result due to the incorporation of the nonlocal spin fluctuations.