SELF-CONSISTENT-FIELD STUDIES OF THE NEGATIVE-U HUBBARD-MODEL

The phase boundary between normal and superconducting phases of the negative-U Hubbard model is obtained within a self-consistent weak-coupling approach known as the ''pseudopotential parquet'' approximation. The shortcomings of simpler self-consistent approaches, including the Hartree-Fock and fluctuation-exchange approximations, are outlined. For intermediate values of U, pseudopotential parquet predicts (i) a maximum in the superconducting transition temperature T(c) at an electron filling [n] of approximately 0.92; and (ii) a sharp decrease in T(c) for values of [n] closer to 1. The scale of transition temperatures is consistent with results from quantum simulations.