Near-degeneracy of extended $s + d_{x^2-y^2}$ and $d_{xy}$ order parameters in quasi-two-dimensional organic superconductors

TL;DR

This study uses ab initio calculations and a spin-fluctuation model to analyze the superconducting pairing symmetry in quasi-two-dimensional organic superconductors, revealing a near-degeneracy between two order parameters and explaining experimental contradictions.

Contribution

It introduces a detailed ab initio approach to determine pairing symmetry and identifies a phase transition between extended s+d and d_{xy} symmetries in organic superconductors.

Findings

Many materials are close to the phase transition line.

The mixed order parameter explains experimental contradictions.

Positioning of materials in the phase diagram correlates with observed behaviors.

Abstract

The symmetry of the superconducting order parameter in quasi-two-dimensional BEDT-TTF organic superconductors is a subject of ongoing debate. We report ab initio density functional theory calculations for a number of organic superconductors containing $\kappa$-type layers. Using projective Wannier functions we derive parameters of a common low-energy Hamiltonian based on individual BEDT-TTF molecular orbitals. In a random phase approximation spin-fluctuation approach we investigate the evolution of the superconducting pairing symmetry within this model and point out a phase-transition between extended $s + d_{x^2-y^2}$ and $d_{xy}$ symmetry. We discuss the origin of the mixed order parameter and the relation between the realistic molecule description and the widely used dimer approximation. Based on our ab initio calculations we position the investigated materials in the obtained molecule model phase diagram and simulate scanning tunneling spectroscopy experiments for selected cases. Our calculations show that many $\kappa$-type materials lie close to the phase transition line between the two pairing symmetry types found in our calculation, possibly explaining the multitude of contradictory experiments in this field.