Fig. 3: Coefficients of the momentum-expansion of the static susceptibility as a function of the coupling strength α = EB/EF at T = Ti.
a The momentum rigidity c2(α) for d-wave (solid line) and s-wave pairing interaction (dashed line). In the anisotropic d-wave case c2 becomes negative at intermediate coupling, α ~ 0.7 indicating that the homogeneous d-wave SC is unstable. Inset: c2(μi), the sign change of the momentum rigidity occurs around the same range in which μi turns from positive to negative values. The momentum rigidity for the isotropic s-wave case remains positive regardless the coupling strength. b cn(α) coefficients, n = 2, 4, 6, for the d-wave pairing. The positive value of c6 allows to recover the stability of the action. The computation of the higher order coefficients allows to define the finite momentum of the critical mode and the relative instability temperature. We use here the same set of parameters of Fig. 2 and plot the results in dimensionless units i.e., cn ≡ cnΛn/2.
