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We present upper critical field data for κ-(BEDT-TTF)2Cu(NCS)2 with the magnetic field close to parallel and parallel to the conducting layers. We show that we can eliminate the effect of vortex dynamics in these layered materials if the layers are oriented within 0.3 degrees of parallel to the applied magnetic field. Eliminating vortex effects leaves one remaining feature in the data that corresponds to the Pauli paramagnetic limit (Hp). We propose a semi-empirical method to calculate the Hp in quasi 2D superconductors. This method takes into account the energy gap of each of the quasi 2D superconductors, which is calculated from specific heat data, and the influence of many body effects. The calculated Pauli paramagnetic limits are then compared to critical field data for the title compound and other organic conductors. Many of the examined quasi 2D superconductors, including the above organic superconductors and CeCoIn5, exhibit upper critical fields that exceed their calculated Hp suggesting unconventional superconductivity. We show that the high field low temperature state in κ-(BEDT-TTF)2Cu(NCS)2 is consistent with the Fulde Ferrell Larkin Ovchinnikov state.

We report the phase diagram of λ-(BETS)2GaCl4 from rf penetration depth measurements with a tunnel diode oscillator in a pulsed magnetic field. We examined four samples with 1100 field sweeps in a range of angles with the magnetic field parallel and perpendicular to the conducting planes. In the parallel direction, Hc2 appears to include a tricritical point at 1.6 K and 10 T with a phase line that increases to 11 T as the temperature is decreased to 500 mK. The second phase line forms a clearly defined high field low temperature region satisfying several of the conditions of the FuldeFerrell-Larkin-Ovchinnikov (FFLO) state. We show remarkably good fits of Hc2 to WHH in the reentrant α > 1, λso = 0 regime. We also note a sharp angle dependence of the phase diagram about the field parallel orientation that characterizes Pauli paramagnetic limiting and further supports the possibility of FFLO behavior. Unrelated to the FFLO study, at fields and temperatures below Hc2 and Tc, we find rich structure in the penetration depth data that we attribute to impurities at the surface altering the superconducting properties while maintaining the same crystallographic axes as Hc2.

High temperature superconducting (HTS) tape can be cut and stacked to generate large magnetic fields at cryogenic temperatures after inducing persistent currents in the superconducting layers. A field of 17.7 T was trapped between two stacks of HTS tape at 8 K with no external mechanical reinforcement. 17.6 T could be sustained when warming the stack up to 14 K. A new type of hybrid stack was used consisting of a 12 mm square insert stack embedded inside a larger 34.4 mm diameter stack made from different tape. The magnetic field generated is the largest for any trapped field magnet reported and 30% greater than previously achieved in a stack of HTS tapes. Such stacks are being considered for superconducting motors as rotor field poles where the cryogenic penalty is justified by the increased power to weight ratio. The sample reported can be considered the strongest permanent magnet ever created.

We report upper critical-field measurements in the metal-free-all-organic superconductor ␤Љ-͑ET͒ 2 SF 5 CH 2 CF 2 SO 3 obtained from measuring the in-plane penetration depth using the tunnel diode oscillator technique. For magnetic field applied parallel to the conducting planes the low-temperature upper critical fields are found to exceed the Pauli limiting field calculated by using a semiempirical method. Furthermore, we found a signature that could be the phase transition between the superconducting vortex state and the Fulde-FerrellLarkin-Ovchinnikov state in the form of a kink just below the upper critical field and only at temperatures below 1.23 K.In this Rapid Communication we report the upper critical field H c2 versus temperature phase diagram of ␤Љ-͑ET͒ 2 SF 5 CH 2 CF 2 SO 3 over temperatures ranging from 0.45 to 4.0 K and find an enhancement of H c2 over the Pauli limiting field H p at temperatures below 1.23 K. An additional lower-field phase transition that appears as a kink just below H c2 and only at temperatures below 1.23 K has been observed. We suggest that this lower-field phase transition could be a signature of the superconducting vortex state ͑SCVS͒-to-Fulde-Ferrell-Larkin-Ovchinnikov ͑FFLO͒ phase transition.Since the first discovery of organic superconductors in 1980, 1 their unique features, such as low dimensionality and cleanness due to self-organizing, have drawn a lot of interest. 2 Due to these features, several theoretical studies have suggested organic superconductors as possible candidates for the FFLO state. [3][4][5] The FFLO state is an additional superconducting state that stabilizes above H p when the superconducting order parameter lowers its free energy by becoming spatially inhomogeneous in a high magnetic field. It was predicted by Fulde et al. 6 and independently by Larkin et al. 7 in 1964. Subsequent theoretical studies have shown that heavy fermion and quasitwo-dimensional organic superconductors are possible candidates for the FFLO state. [3][4][5] In the past decade several heavy fermion materials, such as CeRu 2 ͑Ref. 8͒ and UPd 2 Al 3 , 9 were suggested to have a FFLO state, but these observations were later found to be due to flux motion 10 or to be inconsistent with theoretical models. 11 A more unambiguous observation of the FFLO state was reported in CeCoIn 5 by measuring the specific heat and magnetization 12 and was confirmed by subsequent specific-heat 13 and penetration depth measurements. 14 In organic superconductors there have been several reports of the possible existence of the FFLO state. [15][16][17][18] In the metal-free-all-organic superconductor ␤Љ-͑ET͒ 2 SF 5 CH 2 CF 2 SO 3 the enhancement of H c2 above the value of H p has been observed at low temperatures. 19 This observation raised the possibility of the existence of the FFLO state. The material also satisfies two important conditions necessary to become a candidate for the FFLO state. One is that the material be in the Pauli limit, which means that the orbital limiting field H orb is much gre...