Superconductivity in correlated BEDT-TTF molecular conductors: critical temperatures and gap symmetries

TL;DR

This study uses an advanced theoretical approach to analyze superconductivity in BEDT-TTF molecular conductors, revealing the sensitive dependence of gap symmetry and critical temperature on material parameters and suggesting experimental tunability.

Contribution

It extends the Two-Particle Self-Consistent approach to multi-site models, providing a more accurate description of superconducting gap symmetries and critical temperatures in BEDT-TTF systems compared to previous methods.

Findings

Phase boundary between d_xy and s+d_x^2-y^2 gap symmetries is within realistic parameters.

Critical temperatures are predicted and compared with experimental data.

Switching between pairing symmetries can be achieved via pressure, doping, or disorder.

Abstract

Starting from an {\it ab initio}-derived two-site dimer Hubbard hamiltonian on a triangular lattice, we calculate the superconducting gap functions and critical temperatures for representative $\kappa$-(BEDT-TTF)$_2$X superconductors by solving the linearized Eliashberg equation using the Two-Particle Self-Consistent approach (TPSC) extended to multi-site problems. Such an extension allows for the inclusion of molecule degrees of freedom in the description of these systems. We present both, benchmarking results for the half-filled dimer model as well as detailed investigations for the 3/4-filled molecule model. Remarkably, we find in the latter model that the phase boundary between the two most competing gap symmetries discussed in the context of these materials -d$_{xy}$ and the recently proposed eight-node gap s+d$_{x^2-y^2}$ symmetry- is located within the regime of realistic model parameters and is especially sensitive to the degree of in-plane anisotropy in the materials as well as to the value of the on-site Hubbard repulsion. We show that these results provide a more complete and accurate description of the superconducting properties of $\kappa$-(BEDT-TTF)$_2$X than previous Random Phase Approximation (RPA) calculations and, in particular, we discuss predicted critical temperatures in comparison to experiments. Finally, our findings suggest that it may be even easier to experimentally switch between the two pairing symmetries as previously anticipated by invoking pressure, chemical doping or disorder effects.