Thermodynamic evidence for the Fulde-Ferrell-Larkin-Ovchinnikov state in the KFe2As2 superconductor

TL;DR

This study provides thermodynamic evidence for the existence of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state in the superconductor KFe2As2 by analyzing its magnetic phase diagram near the upper critical field.

Contribution

The paper presents experimental evidence of the FFLO state in KFe2As2 using magnetic torque and specific heat measurements with precise field orientation control.

Findings

Observation of a double transition in magnetic torque and specific heat.

Upturn of the upper critical field line beyond the Pauli limit.

Confirmation of the FFLO state in KFe2As2 superconductor.

Abstract

We have investigated the magnetic phase diagram near the upper critical field of KFe2As2 by magnetic torque and specific heat experiments, using a high-resolution piezo-rotary positioner to precisely control the parallel orientation of the magnetic field with respect to the FeAs layers. We observe a clear double transition when the field is oriented strictly in-plane, and a characteristic upturn of the upper critical field line well beyond the Pauli limit at 4.7 T. This provides firm evidence that an FFLO state is realized in this iron-based KFe2As2 superconductor.