Large-scale superconducting electric devices for power industry depend critically on wires with high critical current densities at temperatures where cryogenic losses are tolerable. This restricts choice to two high-temperature cuprate superconductors, (Bi,Pb)2Sr2Ca2Cu3Ox and YBa2Cu3Ox, and possibly to MgB2, recently discovered to superconduct at 39 K. Crystal structure and material anisotropy place fundamental restrictions on their properties, especially in polycrystalline form. So far, power applications have followed a largely empirical, twin-track approach of conductor development and construction of prototype devices. The feasibility of superconducting power cables, magnetic energy-storage devices, transformers, fault current limiters and motors, largely using (Bi,Pb)2Sr2Ca2Cu3Ox conductor, is proven. Widespread applications now depend significantly on cost-effective resolution of fundamental materials and fabrication issues, which control the production of low-cost, high-performance conductors of these remarkable compounds.

[7,8] These discoveries have generated much interest in the mechanisms and manifestations of unconventional superconductivity in the family of doped quaternary layered oxypnictides LOMPn (L = La, Pr, Ce, Sm; M = Mn, Fe, Co, Ni; Pn=P, As) [9,10], because many features of these materials clearly set them apart from other superconductors. First, ab-initio calculations indicate that superconductivity originates from the d-orbitals of what would normally be expected to be pairbreaking magnetic Fe ions, suggesting that new non-phonon pairing mechanisms are responsible for the high T c [11,12]. Second, F-doped LaFeAsO is a semimetal, which exhibits strong ferromagnetic and antiferromagnetic fluctuations and a possible spin density wave instability around 150K in the parent undoped LaFeAsO [5,[13][14][15][16]. And third, superconductivity emerges on several disconnected pieces of the Fermi surface [11,12,17,18], thus exhibiting the multi-gap pairing, which has recently attracted so much attention in MgB 2 [19].Given the importance of magnetic correlations in the doped oxypnictides, transport measurements at very high magnetic fields are vital to probe the mechanisms of superconductivity. Indeed, first measurements of the upper critical field B c2 at low fields B < 7T have yielded a slope B c2 / (T c ) = dB c2 /dT º 2T/K near T c , for both La and Sm based oxypnictides [2][3][4][5][6]. From the conventional one-band Werthamer-Helfand-Hohenberg (WHH) theory [20] such slopes already imply rather high values B c2 (0) = 0.69T c B c2/ º 36T for LaFeAsO 0.89 F 0.1 , B c2 (0) º 59.3T for SmFeAsO 0.89 F 0.1 , and B c2 (0) º 72 T for PrFeAsO 0.89 F 0.1 , all well above B c2 of Nb 3 Sn. However, studies of the high-field superconductivity in MgB 2 alloys have shown that the upward curvature of B c2 (T) resulting from the multiband effects can significantly increase B c2 (0) as compared to the WHH one-band extrapolation (see, e.g., the review [21] and references therein). To address these important issues, we have performed high-field dc transport measurements on LaFeAsO 0.89 F 0.1 samples up to 45T. We show that B c2 (T) indeed exhibits two-gap behavior similar to that in MgB 2 , and B c2 (0) values exceed the WHH extrapolation by the factor ~ 2. Moreover, the observed B c2 (0) also exceeds the BCS paramagnetic limitPolycrystalline LaFeAsO 0.89 F 0.11 samples were made by solid state synthesis [4]. A sample ~ 3 x 1 x 0.5 mm was used for our four probe transport measurements in the 45T Hybrid magnet at the NHMFL, supplemented by low field measurements in a 9T superconducting magnet. Our low-field data agreed well with the earlier data taken at ORNL on the same sample [4], indicating its good temporal and atmospheric stability. The 45T Hybrid magnet was swept only from 11.5T to 45T due to the constant 11.5T background of the outsert magnet while lower fields were swept from 0T to 9T in a PPMS with resistivity measured in AC mode using a 5mA excitation current, whereas the high field resistance R(B) was measured by a Keithley nanovo...

The discovery of superconductivity at 39 K in MgB 2 1 raises many issues. One of the central questions is whether this new superconductor resembles a hightemperature-cuprate superconductor or a lowtemperature metallic superconductor in terms of its current carrying characteristics in applied magnetic fields. In spite of the very high transition temperatures of the cuprate superconductors, their performance in magnetic fields has several drawbacks 2 . Their large anisotropy restricts high bulk current densities to much less than the full magnetic field-temperature (H-T) space over which superconductivity is found. Further, weak coupling across grain boundaries makes transport current densities in untextured polycrystalline forms low and strongly magnetic field sensitive 3,4 . These studies of MgB 2 address both issues. In spite of the multi-phase, untextured, nano-scale sub-divided nature of our samples, supercurrents flow throughout without the strong sensitivity to weak magnetic fields characteristic of Josephson-coupled grains 3 . Magnetization measurements over nearly all of the superconducting H-T plane show good temperature scaling of the flux pinning force, suggestive of a current density determine d by flux pinning. At least two length scales are suggested by the magnetization and magneto optical (MO) analysis but the cause of this seems to be phase inhomogeneity, porosity, and minority insulating phase such as MgO rather than by weakly coupled grain boundaries. Our results suggest that polycrystalline ceramics of this new class of superconductor will not be compromised by the weak link problems of the high temperature superconductors, a conclusion with enormous significance for applications if higher temperature analogs of this compound can be discovered.The principal samples were synthesized by direct reaction of bright Mg flakes (Aldrich Chemical) and sub-micron amorphous B powder (Callery Chemical). Starting materials were lightly mixed in half-gram batches, and pressed into pellets. These pellets were placed on Ta foil, which was in turn placed on Al 2 O 3 boats, and fired in a tube furnace under a mixed gas of 95% Ar 5% H 2 for 1 hour at 600 C, 1 hour at 800 C, and 1 hour at 900 C, and then lightly ground. The resulting powders were pressed into pellets and then hot pressed at 10 kbar at temperatures between 650 and 800 °C for periods between 1 and 5.5 hours. Disks ~4 mm in diameter and ~1 mm thick were cut from these pellets for property characterization. As noted later, this process cannot yet be considered optimum.Magnetization properties were examined in SQUID and vibrating sample magnetometers (VSM) in applied fields up to 14 T from 4.2 to above T c . Figure 1 shows onset T c values of 37-38 K for the above samples and for commercial MgB 2 powder (99.5%, ~2 µm diameter by examination, CERAC). Sample 1 and the commercial powder show smooth transitions with some temperature dependence of the zerofield cooled (ZFC) shielded moment, while sample 3 exhibits a step, indicative of non-uniformity in su...

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