Spin gap and superconductivity in the one-dimensionalt-Jmodel with Coulomb repulsion

The one-dimensional t-J model with density-density repulsive interactions is investigated with the use of 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 form 1/r suppresses superconductivity in the precursor region.