Critical temperature for the two-dimensional attractive Hubbard model

The critical temperature for the attractive Hubbard model on a square lattice is determined from the analysis of two independent quantities, the helicity modulus ${\rho }_s$ and the pairing correlation function $P_s.$ These quantities have been calculated through quantum Monte Carlo simulations for lattices up to $18\times 18,$ and for several densities, in the intermediate-coupling regime. Imposing the universal-jump condition for an accurately calculated ${\rho }_s,$ together with thorough finite-size scaling analyses (in the spirit of the phenomenological renormalization group) of $P_s,$ suggests that $T_c$ is considerably higher than hitherto assumed.