Pairing Correlations on t-U-J Ladders

We find that the pairing correlations on the usual t-U Hubbard ladder are significantly enhanced by the addition of a nearest-neighbor exchange interaction J. Likewise, these correlations are also enhanced for the t-J model when 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 spin gap at half filling. This enhancement of the pairing correlations is associated 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.