Enhanced pairing in the repulsive Hubbard model with next-nearest-neighbor hopping

We study the behavior of pair susceptibilities in the two-dimensional repulsive Hubbard model with next-nearest-neighbor hopping $t_2$ by Monte Carlo simulations. In a certain range of band filling, we find that the d-wave pair susceptibilities can be enhanced by a Hubbard repulsion U with respect to the noninteracting case, unlike the case with $t_2$=0. We discuss the connection of this model to ${\mathrm{CuO}}_2$ planes in the oxide superconductors and the possible implication of our results for these materials.