Evidence for an FFLO state in layered organic superconductors

In the so-called FFLO state, named after Fulde, Ferrell, Larkin, and Ovchinnikov, the superconducting state can survive even at high magnetic fields above the Pauli paramagnetic limit. The quasi-two-dimensional (2D) organic superconductors have been suggested as good candidates for exhibiting the FFLO state. When applying the magnetic field exactly parallel to the conducting layers the orbital pair breaking is greatly suppressed and the Pauli limit is reached. We performed high-resolution specific-heat and torque-magnetization experiments in magnetic fields up to 28 T for different 2D organic superconductors. In a very narrow region close to parallel orientation we observe additional anomalies below the upper critical field signalling the existence of an additional superconducting phase. The specific-heat data for kappa-(BEDT-TTF)2Cu(NCS)2 with Tc = 9.1 K show that the superconducting transition becomes first order for fields above 21 T indicating that the Pauli limit is reached. Below about 3 K, the upper critical field increases sharply and a second first-order transition appears within the superconducting phase. Our results give strong evidence for the realization of the FFLO state in organic superconductors.