Calorimetric Evidence for a Fulde-Ferrell-Larkin-Ovchinnikov Superconducting State in the Layered Organic Superconductor \kappa$-(BEDT-TTF)$_2$Cu(NCS)$_2$

TL;DR

This study provides calorimetric evidence indicating the existence of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconducting state in the layered organic superconductor -(BEDT-TTF)$_2$Cu(NCS)$_2$ under high magnetic fields, revealing complex phase behavior.

Contribution

The paper presents the first calorimetric evidence for an FFLO state in a layered organic superconductor, demonstrating a second first-order transition within the superconducting phase.

Findings

Superconducting transition becomes first order above 21 T in parallel magnetic fields.

An additional first-order transition line appears within the superconducting phase.

Evidence supports the realization of an FFLO state separating homogeneous superconductivity from the normal state.

Abstract

The specific heat of the layered organic superconductor $\kappa$-% (BEDT-TTF)$_2$Cu(NCS)$_2$, where BEDT-TTF is bisethylenedithio-% tetrathiafulvalene, has been studied in magnetic fields up to 28 T applied perpendicular and parallel to the superconducting layers. In parallel fields above 21 T, the superconducting transition becomes first order, which signals that the Pauli-limiting field is reached. Instead of saturating at this field value, the upper critical field increases sharply and a second first-order transition line appears within the superconducting phase. Our results give strong evidence that the phase, which separates the homogeneous superconducting state from the normal state is a realization of a Fulde-Ferrell-Larkin-Ovchinnikov state.