Ring-exchange mechanism for triplet superconductivity in a two-chain Hubbard model : Possible relevance to Bechgaard salts

The density-matrix renormalization group method is used to study the ground state of the two-chain zigzag-bond Hubbard model at quarter filling. We show that, with a proper choice of the signs of hopping integrals, the ring-exchange mechanism yields ferromagnetic spin correlations between interchain neighboring sites and produces an attractive interaction between electrons as well as long-range pair correlations in the spin-triplet channel, thereby leading the system to triplet superconductivity. We argue that this mechanism may have possible relevance to the superconductivity observed in Bechgaard salts.