SYMMETRY-BREAKING IN THE VERTEX-RENORMALIZED LINKED-CLUSTER EXPANSION FOR STRONGLY CORRELATED FERMIONS

We have shown that mean-field-type symmetry-breaking approximations for correlated fermions can be constructed by performing vertex renormalization (VR) in the linked-cluster expansion (LCE), just as for the case of lattice spin models. Using the diagrammatic technique for Hubbard operators, we have constructed explicitly a partial VR in the second order of the LCE for the half-filled Hubbard model around the atomic limit. We obtain a crossover from large-U Heisenberg mean-field behavior to zero Neel temperature at U = 0.