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 Γ=(mc∕mab)1∕2 is below 5, which is much smaller than the theoretically calculated value. The data measured up to 400K 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 et al., Phys. Rev. 147, 295 (1966)] are Hc2ab(T=0K)≈304T and Hc2c(T=0K)≈62–70T. These high values indicate very encouraging applications of the new superconductors.