Upper critical fields and thermally-activated transport ofNdFeAsO0.7F0.3single crystal

Abstract

We present detailed measurements of the longitudinal resistivity rho(xx)(T,H) and the upper critical field H(c2) of NdFeAsO(0.7)F(0.3) 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.