eta-superconductivity in the one-dimensional Penson-Kolb model

Using exact Lanczos diagonalizations we have studied in detail the transition into the eta-superconducting state in the Penson-Kolb model. We have shown that the transition occurs at any band filling and the critical value W-c, varies between W-c similar or equal to 1.8t at half-filling to W-c similar or equal to -2t for two particles on the lattice, The transition corresponds to abrupt and drastic change in the ground-state structure. After the transition the contribution of the one-electron band to the ground-state energy is almost suppressed.