Evidence for an Fulde-Ferrell-Larkin-Ovchinnikov state in a layered organic superconductor

We present specific heat and torque-magnetization data of the layered organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂ in high magnetic fields up to 28 T and at low temperatures down to 1.5 K. The magnetic field was aligned perfectly parallel to the superconducting planes of the sample. In that orientation and at low temperatures the upper critical field, B_c2, gets close to the Pauli limit which is about 21 T. But instead of the expected saturation at this field value we observe an upturn of B_c2 towards low temperatures. This comes along with a change of the order of the phase transition from second order to first order. In addition, another first-order phase transition appears below B_c2 in the superconducting state. These features strongly point to the formation of a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase in the state above the Pauli limit. Our results are in line with the behaviour predicted for FFLO states in two-dimensional systems.