Anisotropic Fully-Gapped Superconductivity Possibly Mediated by Charge Fluctuations in a Nondimeric Organic Complex

TL;DR

This study reveals that a nondimeric organic superconductor exhibits fully gapped, anisotropic superconductivity likely mediated by charge fluctuations, with evidence from ultrasonic, quantum oscillation, and heat capacity measurements.

Contribution

It provides the first evidence of anisotropic fully-gapped superconductivity possibly driven by charge fluctuations in a nondimeric organic complex.

Findings

Charge disproportionation occurs below 8 K, just above T_c.

Fermi surface variation and mass enhancement are linked to charge disproportionation.

Superconducting gap is fully gapped but anisotropic with fourfold symmetry.

Abstract

We investigate low-temperature electronic properties of the nondimeric organic superconductor $\beta^{\prime\prime}$-(BEDT-TTF)$_4$[(H$_3$O)Ga(C$_2$O$_4$)$_3$]PhNO$_2$. By examining ultrasonic properties, charge disproportionation (CD) without magnetic field dependence is detected below $T_{\rm CD}$$\sim$8~K just above the superconducting critical temperature $T_{\rm c}$$\sim$6~K. From quantum oscillations in high fields, we find variation in the Fermi surface and mass enhancement induced by the CD. Heat capacity studies elucidate that the superconducting gap function is fully gapped in the Fermi surface, but anisotropic with fourfold symmetry. We point out that the pairing mechanism of the superconductivity is possibly dominated by charge fluctuations.