Two-dimensional negative-UHubbard model

Quantum Monte Carlo simulations are used to explore the phase diagram of the negative-U Hubbard model. As the system is doped away from half filling, 〈n〉=1, the s-wave pair-field correlations are enhanced and the charge-density-wave correlations suppressed. There is no indication of phase separation. Away from half filling, the pair-field correlations are consistent with Kosterlitz-Thouless scaling and a finite ${\mathit{T}}_{\mathit{c}}$ which peaks near 〈n〉=1 but vanishes at exactly half filling.