FIG. 2. Color online The difference Wd−WN in the spectral weight between the superconducting state with d-wave symmetry and the normal state at zero temperature vs doping dotted blue curve . The dashed green curve shows the same quantity for s-wave symmetry, and the pink points indicate the slope with respect to T2 of the spectral weight near Tc. All three quantities are always negative and show an enhancement near half-filling due to the van Hove singularity. In fact, the pink points are almost a perfect inverted image of the density of states see the minus sign in Eq. 8 , except for the small density regime near half-filling, where the van Hove singularity makes the Sommerfeld expansion invalid.
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