Theory of triangular lattice quasi-one-dimensional charge-transfer solids

Recent investigations of the magnetic properties and the discovery of superconductivity in quasi-one-dimensional triangular lattice organic charge-transfer solids have indicated the severe limitations of the effective 12-filled band Hubbard model for these and related systems. We present computational studies within the 14-filled band Hubbard model for these highly anisotropic systems. Individual organic monomer molecules, and not their dimers, constitute the sites of the Hamiltonian within our theory. We find enhancement of the long-range component of superconducting pairing correlations by the Hubbard repulsive interaction U for band parameters corresponding to κ-(BEDT-TTF)₂CF3SO₃, which is superconducting under moderate pressure. We find significantly weaker superconducting pairing at realistic values of U in κ-(BEDT-TTF)2B(CN)4, and we ascribe the experimentally observed transition to a spin-gapped insulator to the formation of a paired-electron crystal. We make the testable prediction that the spin gap will be accompanied by charge ordering and period doubling in two lattice directions. The weaker tendency to superconductivity in κ-(BEDT-TTF)2B(CN)4 compared to κ-(BEDT-TTF)₂CF₃SO₃ is consistent with the more one-dimensional character of the former. Pressure-induced superconductivity is, however, conceivable. The overall results support a valence bond theory of superconductivity we have proposed recently.