Charge fluctuations and superconductivity in organic conductors: the case of $\beta"$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$

TL;DR

This study uses $^{13}$C NMR to investigate the normal and superconducting states of an organic charge-transfer salt, revealing charge order and gap nodes, and suggesting charge fluctuations drive pairing without antiferromagnetic fluctuations.

Contribution

It provides the first detailed NMR analysis of $eta''$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$, highlighting charge order and proposing charge fluctuations as the pairing mechanism.

Findings

Normal state is a charge-ordered metal with stripe configuration.

Superconducting state shows gap nodes and singlet pairing.

No evidence of low-energy antiferromagnetic spin fluctuations above $T_c$.

Abstract

A $^{13}$C NMR study of the normal and superconducting states of the all-organic charge-transfer salt $\beta"$-(BEDT-TTF)$_2$SF$_5$CH$_2$CF$_2$SO$_3$ is presented. We find that the normal state is a charge-ordered metal configured as vertical stripes, produced by a combination of 1/4-filling, correlations, and a polar counterion sublattice. The NMR properties associated with the superconducting state are consistent with gap nodes and singlet pairing, and therefore similar to other organic superconductors. Quite distinct, however, is the absence of evidence for low-energy antiferromagnetic spin fluctuations for $T>T_c=4.5$ K. Both aspects are discussed in the context of a proposal that the pairing in this compound is driven by charge fluctuations.