Pairing correlations on t-U-J ladders

We find that the pairing correlations on the usual t-U Hubbard ladder an si gnificantly enhanced by the addition of a nearest-neighbor exchange interac tion J. Likewise, these correlations are also enhanced for the t-J model wh en the on-site Coulomb interaction is reduced from infinity. Moreover, the pairing correlations are larger on a t-U-J ladder than on a t-J(eff) ladder in which J(eff) has been adjusted so that the two models have the same spi n gap at half filling. This enhancement of the pairing correlations is asso ciated with an increase in the pair-binding energy and the pair mobility in the t-U-J model and points to the importance of the charge-transfer nature of the cuprate systems.