Fig. 4
Temperature dependence of ε(T) = 1 − λd(T) for the 2D Hubbard model. Here, ε(T) is normalized to its value at T/t = 0.2. a ε(T) for 〈n〉 = 0.93 and b for 〈n〉 = 0.85. The inset in a shows Monte-Carlo results for the susceptibility of a 2D xy model, which has a fixed amplitude with only a phase degree of freedom that can fluctuate. The dashed curves in a show the linear Curie–Weiss behavior, while in b, they show the BCS Cooper pair fluctuation result \({\mathrm{ln}}\left( {T/T_c^{MF}} \right)\)
