Mixed order parameters, accidental nodes and broken time reversal symmetry in organic superconductors: a group theoretical analysis

TL;DR

This paper uses group theory to analyze the symmetry and phase transitions of various organic superconductors, revealing complex order parameters and the effects of structural distortions on their superconducting states.

Contribution

It provides a detailed group theoretical framework to understand the order parameters and phase transitions in layered organic superconductors, including effects of distortions and specific pairing symmetries.

Findings

Highly frustrated organic superconductors undergo two superconducting transitions.

Symmetry of the order parameter varies with crystal distortion.

Certain materials exhibit mixed order parameters like d+id or d_{xy}+s.

Abstract

We present a group theoretic analysis of several classes of organic superconductors. We argue that highly frustrated half-filled layered organic superconductors, such as kappa-(ET)_2Cu2(CN)_3 (where ET is BEDT-TTF) and beta'-[Pd(dmit)_2]_2X, undergo two superconducting phase transitions, the first from the normal state to a d-wave superconductor and the second to a d+id state. We show that the monoclinic distortion of kappa-(ET)_2Cu(NCS)_2 means that the symmetry of its superconducting order parameter is different from that of orthorhombic kappa-(ET)_2Cu[N(CN)_2]Br. We propose that quarter filled layered organic superconductors, e.g., theta-(ET)_2I_3, have d_{xy}+s order parameters.