SUPERCONDUCTING GROUND-STATE IN A MODEL WITH BOND-CHARGE INTERACTION

An electronic Hubbard-type model with bond-charge interaction and on-site repulsion U is studied in one and two dimensions. Evidence is provided in favor of superconductivity, without phase separation, for U<Uc(n), although no explicit attraction between particles is present in the Hamiltonian. The presence of a bound state for two particles in vacuum in D=1 and 2 (but not for D=3) suggests that, at low density and low dimensionality, superconductivity is due to condensation of preexistent pairs (dimers). The one-dimensional (1D) phase diagram is studied analytically as well as numerically and shows a transiiton between a Luttinger liquid and a strong-coupling phase with diverging superconducting susceptibility. Estimates of critical exponents are given at low density and at quarter filling for several values of the parameters. The 2D model at quarter filling is analyzed by Lanczos diagonalization on a 4×4 cluster. The numerical results are consistent with the phase diagram obtained by BCS mean-field theory and is qualitatively similar to the 1D case: a ground state with spin gap and off-diagonal long-range order at U<Uc(n); and a Fermi liquid above Uc with no sign of phase separation.