The Fulde–Ferrell–Larkin–Ovchinnikov state in the organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂ as observed in magnetic-torque experiments

We present magnetic-torque experiments on the organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂ for magnetic fields applied parallel to the 2D superconducting layers. The experiments show a crossover from a second-order to a first-order transition when the upper critical field reaches 21 T. Beyond this field, which we interpret as the Pauli limit for superconductivity, the upper critical field line shows a pronounced upturn and a phase transition line separates the superconducting state into a low- and a high-field phase. We interpret the data in the framework of a Fulde–Ferrell–Larkin–Ovchinnikov state.