Binding of holes in two-dimensional lattices with different boundary conditions

Using exact diagonalization we compute the binding energy of two holes for the t-J model in lattices with different boundary conditions. We find that with periodic, antiperiodic, mixed, and other boundary conditions there is always negative binding energy in a certain range of J, for all the sizes studied. This is different from the behavior obtained in the one-dimensional case, suggesting that binding might survive in the thermodynamic limit in two dimensions.