Abstract and subjects
We present detailed measurements of the longitudinal resistivity rho(xx)(T,H) and the upper critical field H-c2 of NdFeAsO0.7F0.3 single crystals in strong dc and pulsed magnetic fields up to 45 and 60 T, respectively. We found that the field scale of H-c2 is comparable to H-c2 similar to 100 T of high-T-c cuprates. H-c2(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(perpendicular to)(T) is indeed an intrinsic feature of oxypnictides rather than manifestation of vortex lattice melting or granularity. The orientational dependence of H-c2(theta) as a function of the angle theta between H and the c axis shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy parameter gamma(T)=(m(c)/m(ab))(1/2)=H-c2(parallel to)/H-c2(perpendicular to) obtained from these measurements decreases as temperature decreases from gamma similar or equal to 9.2 at 44 K to gamma similar or equal to 5 at 34 K, where parallel to and perpendicular to 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 rho(xx)(T,H) measured below T-c but above the irreversibility field exhibits a clear Arrhenius thermally-activated behavior rho=rho(0) exp[-E-a(T,H)/T] over 4-5 decades of rho(xx). The activation energy E-a(T,H) has very different field dependencies for H parallel to ab and H perpendicular to ab varying from 4x10(3) K at H=0.2 T to similar 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 H-c2, using the two-band model of superconductivity in oxypnictides. The results indicate the importance of paramagnetic effects on H-c2(T) in oxypnictides, which may significantly reduce H-c2(0) as compared to H-c2(0)similar to 200-300 T based on extrapolations of H-c2(T) near T-c down to low temperatures.