Phase Boundary in a Superconducting State of κ-(BEDT-TTF)₂Cu(NCS)₂: Evidence of the Fulde–Ferrell–Larkin–Ovchinnikov Phase

Abstract

The angular dependences of the magnetoresistance and magnetic torque have been investigated for the layered organic superconductor κ-(BEDT-TTF)2Cu(NCS)2 with a Tc of 9.3 K. The diamagnetic signal perpendicular to the layers below 6 K is strongly suppressed in magnetic fields parallel to the layers above a characteristic field Hkink, which approximately corresponds to the Pauli paramagnetic limit ∼23 T and is significantly lower than the upper critical field Hc2. The Hkink field slightly decreases with increasing temperature and coincides with Hc2 at 6 K. These results show the presence of an inhomogeneous superconducting phase, that is, the Fulde–Ferrell–Larkin–Ovchinnikov (FFLO) phase, between the Hkink and Hc2 fields. The presence of a tricritical point at T ≈ 6 K is consistent with conventional FFLO theories.