Upper critical field study in the organic superconductor $\beta''$-(ET)$_{2}$SF$_{5}$CH$_{2}$CF$_{2}$SO$_{3}$ : Possibility of Fulde-Ferrell-Larkin-Ovchinnikov state

TL;DR

This study investigates the upper critical magnetic field in an organic superconductor, revealing potential evidence for the Fulde-Ferrell-Larkin-Ovchinnikov state through critical field measurements and phase transition signatures.

Contribution

It provides the first measurements of the upper critical field in this organic superconductor and suggests the possible existence of the FFLO state based on observed phase transition features.

Findings

Upper critical fields exceed the Pauli limit at low temperatures.

A kink indicating a phase transition appears below 1.23 K.

Evidence suggests the possible realization of the FFLO state.

Abstract

We report upper critical field measurements in the metal-free-all-organic superconductor $\beta''$-(ET)$_{2}$SF$_{5}$CH$_{2}$CF$_{2}$SO$_{3}$ obtained from measuring the in-plane penetration depth using the tunnel diode oscillator technique. For magnetic field applied parallel to the conducting planes the low temperature upper critical fields are found to exceed the Pauli limiting field calculated by using a semi-empirical method. Furthermore, we found a signature that could be the phase transition between the superconducting vortex state and the Fulde-Ferrell-Larkin-Ovchinnikov state in the form of a kink just below the upper critical field and only at temperatures below 1.23 K.