Upper critical fields and thermally-activated transport of Nd(O0.7F0.3)FeAs single crystal
Abstract
We present measurements of the resistivity and the upper critical field Hc2 of Nd(O0.7F0.3)FeAs single crystals in strong DC and pulsed magnetic fields up to 45 T and 60 T, respectively. We found that the field scale of Hc2 is comparable to ~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 samples. Thus this behavior is indeed an intrinsic feature of oxypnictides, rather than manifestation of vortex lattice melting or granularity. The orientational dependence of Hc2 shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy decreases as T decreases, from 9.2 at 44K to 5 at 34K. Spin dependent magnetoresistance and nonlinearities in the Hall coefficient suggest contribution to the conductivity from electron-electron interactions modified by disorder reminiscent that of diluted magnetic semiconductors. The Ohmic resistivity measured below Tc but above the irreversibility field exhibits a clear Arrhenius thermally activated behavior over 4-5 decades. The activation energy has very different field dependencies for H||ab and H ab. We discuss to what extent different pairing scenarios can manifest themselves in the observed behavior of Hc2, using the two-band model of superconductivity. The results indicate the importance of paramagnetic effects on Hc2(T),which may significantly reduce Hc2(0) as compared toHc2(0)~200-300 T based on extrapolations of Hc2(T) near Tc down to low temperatures.