Magnetic torque evidence for the Fulde-Ferrell-Larkin-Ovchinnikov state in the layered organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂

We present magnetic-torque measurements of the organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂ for inplane magnetic fields up to 32 T. In this layered two-dimensional compound the superconductivity can persist even in fields above the Pauli limit of about 21 T. There, a pronounced upturn of the upper-critical-field line occurs and the superconducting phase-transition line splits and forms an additional high-magnetic-field phase. κ-(BEDT-TTF)₂Cu(NCS)₂ is a spin-singlet superconductor; therefore, such a superconducting high-field phase beyond the Pauli limit can originate only from Cooper pairing with finite center-of-mass momentum. The measurements are discussed in connection with a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, in accordance with earlier specific-heat observations. The torque experiments allow us to investigate the high-magnetic-field phase diagram and the FFLO state of κ-(BEDT-TTF)₂Cu(NCS)₂ in great detail.