Superconducting instabilities in the tt' Hubbard model in the large-N limit

Considering Hubbard models with infinite U, N degrees of freedom per site, and hopping terms between nearest and second-nearest neighbours on a square lattice, and using a formulation with X-operators, the leading contributions at large N to the anomalous self-energy and the linearized Eliashberg equation are calculated. Some of these terms have the form of Fock-like terms with an effective two-particle interaction, others have a different structure and have not been discussed previously. Calculating transition temperatures, we find that the new terms found in our 1/N expansion compensate the other terms nearly perfectly suppressing superconducting instabilities to extremely small temperatures.