Critical fields and anisotropy of NdFeAsO0.82F0.18 single crystals

By using flux method, we have grown single crystals of NdFeAsO0.82F0.18 at ambient pressures. Resistive measurements reveal a surprising discovery that the anisotropy Gamma=(m(c)/m(ab))(1/2) is below 5, which is much smaller than the theoretically calculated value. The data measured up to 400 K show a curved feature, which prevents a conjectured linear behavior for an unconventional metal. The upper critical fields determined based on the Werthamer-Helfand-Hohenberg formula [N. R. Werthamer , Phys. Rev. 147, 295 (1966)] are H-c2(ab)(T=0 K)approximate to 304 T and H-c2(c)(T=0 K)approximate to 62-70 T. These high values indicate very encouraging applications of the new superconductors. (C) 2008 American Institute of Physics.