The specific heat of the layered organic superconductor -BEDT-TTF 2 CuNCS 2 , where BEDT-TTF is bisethylenedithio-tetrathiafulvalene, has been studied in magnetic fields up to 28 T applied perpendicular and parallel to the superconducting layers. In parallel fields above 21 T, the superconducting transition becomes first order, which signals that the Pauli-limiting field is reached. Instead of saturating at this field value, the upper-critical-field increases sharply and a second first-order transition line appears within the superconducting phase. Our results give strong evidence that the phase, which separates the homogeneous superconducting state from the normal state is a realization of a Fulde-Ferrell-LarkinOvchinnikov state.

We report on specific heat ͑C p ͒, transport, Hall probe, and penetration depth measurements performed on Fe͑Se 0.5 Te 0.5 ͒ single crystals ͑T c ϳ 14 K͒. The thermodynamic upper critical field H c2 lines has been deduced from C p measurements up to 28 T for both H ʈ c and H ʈ ab, and compared to the lines deduced from transport measurements ͑up to 55 T in pulsed magnetic fields͒. We show that this thermodynamic H c2 line presents a very strong downward curvature for T → T c which is not visible in transport measurements. This temperature dependence associated to an upward curvature of the field dependence of the Sommerfeld coefficient confirms that H c2 is limited by paramagnetic effects. Surprisingly this paramagnetic limit is visible here up to T / T c ϳ 0.99 ͑for H ʈ ab͒ which is the consequence of a very small value of the coherence length c ͑0͒ϳ4 Å ͓and ab ͑0͒ϳ15 Å͔, confirming the strong renormalization of the effective mass ͑as compared to DMFT calculations͒ previously observed in ARPES measurements ͓A. Phys. Rev. Lett. 104, 097002 ͑2010͔͒. H c1 measurements lead to ab ͑0͒ = 430Ϯ 50 nm and c ͑0͒ = 1600Ϯ 200 nm and the corresponding anisotropy is approximatively temperature independent ͑ϳ4͒, being close to the anisotropy of H c2 for T → T c . The temperature dependence of both ͑ϰT 2 ͒ and the electronic contribution to the specific heat confirm the nonconventional coupling mechanism in this system.

We present magnetic-torque measurements of the organic superconductor κ-(BEDT-TTF) 2 Cu(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) 2 Cu(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) 2 Cu(NCS) 2 in great detail.