Three-site interactions and superconductivity of itinerant spins: A high-temperature expansion.

Abstract

We consider hard-core fermions on a single-band lattice additionally with nearest-neighbor two-site and three-site two-body interactions. We derive exact high-temperature expansions of the superconducting susceptibilities and correlation functions for different pair symmetries. These results can serve as a useful test of approximate theories. As an example, a mean-field treatment of the nearest-neighbor interactions is found to underestimate the susceptibility enhancement. In addition, the three-site interaction appears in the exact expansion to be as important as the two-site interaction, even in the case of nearly localized spins.