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
1/2-filled band Hubbard model for these and related systems. Our computational
studies of these materials within a 1/4-filled band Hubbard model in which the
organic monomer molecules, and not their dimers, constitute the sites of the
Hamiltonian are able to reproduce the experimental results. We ascribe the spin
gap transition in kappa-(BEDT-TTF)_2B(CN)_4 to the formation of a
two-dimensional paired-electron crystal and make the testable prediction that
the spin gap will be accompanied by charge-ordering and period doubling in two
directions. We find enhancement of the long-range component of superconducting
pairing correlations by the Hubbard repulsive interaction for band parameters
corresponding to kappa-(BEDT-TTF)_2CF_3SO_3. The overall results strongly
support a valence bond theory of superconductivity we have proposed recently.Comment: 8 pages, 7 figure
