The Fulde-Ferrell-Larkin-Ovchinnikov State in the Organic Superconductor k-(BEDT-TTF)2Cu(NCS)2 as Observed in Magnetic Torque Experiments

TL;DR

This study observes the FFLO state in an organic superconductor through magnetic torque experiments, revealing a phase transition and an upturn in the critical field near the Pauli limit.

Contribution

First experimental evidence of the FFLO state in k-(BEDT-TTF)2Cu(NCS)2 using magnetic torque measurements under high magnetic fields.

Findings

Crossover from second- to first-order transition at 21 T

Upturn in upper critical field beyond Pauli limit

Phase separation into low- and high-field superconducting states

Abstract

We present magnetic-torque experiments on the organic superconductor k-(BEDT-TTF)2Cu(NCS)2 for magnetic fields applied parallel to the 2D superconducting layers. The experiments show a crossover from a second-order to a first-order transition when the upper critical field reaches 21 T. Beyond this field, which we interpret as the Pauli limit for superconductivity, the upper critical field line shows a pro-nounced upturn and a phase transition line separates the superconducting state into a low- and a high-field phase. We interpret the data in the framework of a Fulde-Ferrell-Larkin-Ovchinnikov state.