Superconductivity in correlated organic materials: gap symmetry and critical temperatures

Despite considerable efforts, no consensus about the superconducting gap symmetry in κ-(ET)₂X was found to date. Several theoretical studies proposed gap symmetries that range from s- to d-wave. A recent RPA spin fluctuation theory study proposed an exotic eight-node gap (extended s+d_x²-y²) for a large family of κ-(ET)₂X superconductors. In addition, it was found that those materials lie close to the phase boundary to d_xy symmetry and might therefore allow to switch symmetries by tuning the hopping.
In an effort to tackle this issue, we present calculations of the gap function by using the extended Two-Particle Self-Consistent approach and Eliashberg theory. With this method we can both, study gap symmetries and critical temperatures.
Remarkably, we find a shift of the phase boundary compared to RPA results. Namely, materials with relatively low in-plane anisotropy probably realize the four-node d_xy gap symmetry while the rest exhibit extended s+d_x²-y² solutions. We will discuss these results in comparison with experimental evidence.