Calorimetric Measurements of Magnetic-Field-Induced Inhomogeneous Superconductivity Above The Paramagnetic Limit

TL;DR

This study provides the first thermodynamic evidence for the magnetic-field-induced FFLO superconducting state above the paramagnetic limit in an organic superconductor, confirming its unique entropy and spin domain characteristics.

Contribution

It presents the first calorimetric and magnetocaloric observations of the FFLO state, establishing its thermodynamic properties and phase transition characteristics in a specific organic superconductor.

Findings

First thermodynamic evidence of FFLO phase transition at $H_p$

High-field superconducting state shows bulk paramagnetic ordering

Results align with recent NMR and magnetic penetration depth studies

Abstract

We report the first magneto-caloric and calorimetric observations of a magnetic-field-induced phase transition within a superconducting state to the long-sought exotic "FFLO" superconducting state first predicted over 50 years ago. Through the combination of bulk thermodynamic calorimetric and magnetocaloric measurements in the organic superconductor $\kappa$ - (BEDT-TTF)$_2$Cu(NCS)$_2$, as a function of temperature, magnetic field strength, and magnetic field orientation, we establish for the first time that this field-induced first-order phase transition at the paramagnetic limit $H_p$ for traditional superconductivity is to a higher entropy superconducting phase uniquely characteristic of the FFLO state. We also establish that this high-field superconducting state displays the bulk paramagnetic ordering of spin domains required of the FFLO state. These results rule out the alternate possibility of spin-density wave (SDW) ordering in the high field superconducting phase. The phase diagram determined from our measurements --- including the observation of a phase transition into the FFLO phase at $H_p$ --- is in good agreement with recent NMR results and our own earlier tunnel-diode magnetic penetration depth experiments, but is in disagreement with the only previous calorimetric report.