Superconductivity near an Ising nematic quantum critical point in two dimensions

Abstract: Near a two-dimensional Ising-type nematic quantum critical point, the quantum fluctuations of the nematic order parameter are coupled to the electrons, leading to non-Fermi liquid behavior and unconventional superconductivity. The interplay between these two effects has been extensively studied through the Eliashberg equations for the superconducting gap. However, previous studies often rely on various approximations that may introduce uncertainties in the results. Here, we revisit this problem without these approximations and examine how their removal changes the outcomes. We numerically solve four self-consistent Eliashberg integral equations of the mass renormalization $A_{1}(p)$, the chemical potential renormalization $A_{2}(p)$, the pairing function $\Phi(p)$, and the nematic self-energy (polarization) function $\Pi(q)$ using the iteration method. Our calculations retain the explicit non-linearity and the full momentum dependence of these equations. We find that discarding some commonly used approximations allows for a more accurate determination of the superconducting gap $\Delta = \Phi/A_{1}$ and the critical temperature $T_{c}$. The Eliashberg equations have two different convergent gap solutions: an extended $s$-wave gap and a $d_{x^{2}-y^{2}}$-wave gap. The latter is fragile, whereas the former is robust against small perturbations.