Spin Gap and Superconductivity in the One-Dimensional t-J Model with   Coulomb Repulsion
  The one-dimensional t-J model with density-density repulsive interactions is investigated using exact diagonalization and quantum Monte Carlo methods. A short-range repulsion pushes phase separation to larger values of J/t, and leads to a widened precursor region in which a spin gap and strengthened superconducting correlations appear. The correlation exponent is calculated. On the contrary, a long-range repulsion of $1/r$-form suppresses superconductivity in the precursor region. 
M.Troyer, H. Tsunetsugu, T.M. Rice, J. Riera and E. Dagotto