J. Luo et Ne. Bickers, SELF-CONSISTENT-FIELD STUDIES OF THE NEGATIVE-U HUBBARD-MODEL, Physical review. B, Condensed matter, 48(21), 1993, pp. 15983-15991
The phase boundary between normal and superconducting phases of the ne
gative-U Hubbard model is obtained within a self-consistent weak-coupl
ing approach known as the ''pseudopotential parquet'' approximation. T
he shortcomings of simpler self-consistent approaches, including the H
artree-Fock and fluctuation-exchange approximations, are outlined. For
intermediate values of U, pseudopotential parquet predicts (i) a maxi
mum in the superconducting transition temperature T(c) at an electron
filling [n] of approximately 0.92; and (ii) a sharp decrease in T(c) f
or values of [n] closer to 1. The scale of transition temperatures is
consistent with results from quantum simulations.