Upper critical fields and thermally-activated transport of NdFeAsO _0.7F_0.3 single crystal

We present detailed measurements of the longitudinal resistivity ρxx (T,H) and the upper critical field Hc2 of NdFeAsO0.7 F0.3 single crystals in strong dc and pulsed magnetic fields up to 45 and 60 T, respectively. We found that the field scale of Hc2 is comparable to Hc2 ∼100 T of high-Tc cuprates. Hc2(T) parallel to the c axis exhibits a pronounced upward curvature similar to what was extracted from earlier measurements on polycrystalline LaFeAs(O,F), NdFeAs(O,F), and SmFeAs(O,F) samples. Thus, this behavior of H⊥c2(T) is indeed an intrinsic feature of oxypnictides rather than manifestation of vortex lattice melting or granularity. The orientational dependence of Hc2 (θ) as a function of the angle θ between H and the c axis shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy parameter γ(T) = (mc/mab) 1/2 = H⊥c2/H⊥c2 obtained from these measurements decreases as temperature decreases from γ ≃ 9.2 at 44 K to γ ≃ 5 at 34 K, where ∥ and ⊥ correspond to H parallel and perpendicular to the ab planes, respectively. Spin-dependent magnetoresistance and nonlinearities in the Hall coefficient suggest contribution to the conductivity from electron-electron interactions modified by disorder reminiscent of that in diluted magnetic semiconductors. The Ohmic resistivity ρxx (T,H) measured below Tc but above the irreversibility field exhibits a clear Arrhenius thermally-activated behavior ρ= ρ0 exp [- Ea (T,H)/T] over 4-5 decades of ρxx. The activation energy Ea (T,H) has very different field dependencies for H∥ab and H ⊥ ab varying from 4× 103 K at H=0.2 T to ∼200 K at H=35 T. We discuss to what extent different pairing scenarios suggested in the literature can manifest themselves in the observed behavior of Hc2, using the two-band model of superconductivity in oxypnictides. The results indicate the importance of paramagnetic effects on Hc2(T) in oxypnictides, which may significantly reduce Hc2(0) as compared to Hc2(0)∼200-300 T based on extrapolations of Hc2(T) near Tc down to low temperatures.