FLEX Study on the compressibility of the two-dimensional Hubbard model

We investigate, on the basis of the fluctuation exchange (FLEX) approximation, a hole-number dependence of the compressibility in the two-dimensional t-t′-U Hubbard model. We choose a value of U near the critical Coulomb interaction of the antiferromagnetic transition, where the shape of the renormalized Fermi surface is deformed showing the tendency of nesting. It is found that, near half-filling, the charge susceptibility χ_c = ∂n/∂μ is larger about three times than the value given by the renormalized RPA, χ_c = Π₀/(1+UΠ₀), where Π₀ is the polarization function calculated by FLEX approximation.