Coulomb repulsion in (TMTSF)2X and (TMTTF)2X

On the basis of studies of transport properties of (TMTSF)2 X, (TMTTF)2X and their binary alloys the authors discuss the role of on-site Coulomb repulsion relative to the transfer integrals. In TMTTF-salts U/ta are believed to be large, resulting in a Hubbard gap, whereas U/ta in TMTSF-salts are weak or of intermediate size. They suggest that the difference in U/t a is a result of difference in U rather than difference in t a and moreover that U is closely related to the interchain banding (tb)