Magnetic-Torque Evidence for the Fulde-Ferrell-Larkin-Ovchinnikov State for In-Plane Magnetic Fields in the Organic Superconductor k-(BEDT-TTF)2Cu(NCS)2

TL;DR

This study provides magnetic-torque evidence for the existence of the FFLO state in the organic superconductor k-(BEDT-TTF)2Cu(NCS)2 under high in-plane magnetic fields, revealing a high-field phase beyond the Pauli limit.

Contribution

It offers detailed magnetic-torque measurements confirming the FFLO state in k-(BEDT-TTF)2Cu(NCS)2, advancing understanding of high-field superconducting phases in organic materials.

Findings

Superconductivity persists above the Pauli limit of 21 T.

A high magnetic field phase with an upturned critical field line was observed.

Evidence supports the presence of an FFLO state in the material.

Abstract

We present magnetic-torque measurements of the organic superconductor k-(BEDT-TTF)2Cu(NCS)2 for in-plane magnetic fields up to 32 T. In this layered two-dimensional compound the superconductivity can persist even in fields above the Pauli limit of about 21 T. There, a pronounced upturn of the upper-critical-field line occurs and the superconducting phase-transition line splits and forms an additional high magnetic field phase. k-(BEDT-TTF)2Cu(NCS)2 is a spin-singlet superconductor, therefore, such a superconducting high-field phase beyond the Pauli limit can originate only from Cooper pairing with finite center-of-mass momentum. The measurements are discussed in connection with a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, in accordance with earlier specific-heat observations. The torque experiments allow us to investigate the high magnetic-field phase diagram and the FFLO state of k-(BEDT-TTF)2Cu(NCS)2 in great detail.