Nonmagnetic disorder effect to the FFLO phase in layered organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂

Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductivity, which can be stabilized even above the Pauli limit, has been attracted much interest because of the exotic superconductivity beyond the framework of the BCS theory. There are two necessary conditions of the FFLO superconductivity: the quenched orbital effect and clean limit superconductivity. In the layered organic superconductors, the former condition is satisfied in magnetic fields parallel to the conducting planes. The later condition, suggesting that the FFLO superconductivity is unstable to the disorder of the electronic system, has not been systematically studied so far.

In this study, we have performed the resistance and magnetic torque measurements to reveal the disorder effects on the FFLO phase of a highly two dimensional organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂ (κ-NCS, T_c = 10.2 K). Non-magnetic disorders are introduced into the Cu(NCS)₂ molecules (not BEDT-TTF conducting layers) by X-ray irradiation, which is known as an effective method to control the local random potential quantitatively.

We have precisely measured the magnetic field dependence of the resistance and the magnetic torque. In the parallel field, both T_c and the irreversibility field of the torque curves giving the lower limit of H_c2 decrease with increasing the irradiation time. From these results, the detailed high-field superconducting phase diagram of the disorder introduced κ-NCS is determined