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We present measurements of the resistivity and the upper critical field H_c2of Nd(O_0.7F_0.3)FeAs single crystals in strong DC and pulsed magnetic fieldsup to 45 T and 60 T, respectively. We found that the field scale of H_c2 iscomparable to ~100 T of high T_c cuprates. H_c2(T) parallel to the c-axisexhibits a pronounced upward curvature similar to what was extracted fromearlier measurements on polycrystalline samples. Thus this behavior is indeedan intrinsic feature of oxypnictides, rather than manifestation of vortexlattice melting or granularity. The orientational dependence of H_c2 showsdeviations from the one-band Ginzburg-Landau scaling. The mass anisotropydecreases as T decreases, from 9.2 at 44K to 5 at 34K. Spin dependentmagnetoresistance and nonlinearities in the Hall coefficient suggestcontribution to the conductivity from electron-electron interactions modifiedby disorder reminiscent that of diluted magnetic semiconductors. The Ohmicresistivity measured below T_c but above the irreversibility field exhibits aclear Arrhenius thermally activated behavior over 4-5 decades. The activationenergy has very different field dependencies for H||ab and H\perp ab. Wediscuss to what extent different pairing scenarios can manifest themselves inthe observed behavior of H_{c2}, using the two-band model of superconductivity.The results indicate the importance of paramagnetic effects on H_c2(T),whichmay significantly reduce H_c2(0) as compared toH_c2(0)~200-300 T based onextrapolations of H_c2(T) near T_c down to low temperatures.

Upper critical fields and thermally-activated transport of Nd(O_0.7F_0.3)FeAs single crystal