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

Abstract

We present detailed measurements of the longitudinal resistivity ${\ensuremath{\rho}}_{xx}(T,H)$ and the upper critical field ${H}_{c2}$ of ${\text{NdFeAsO}}_{0.7}{\text{F}}_{0.3}$ 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 ${H}_{c2}\ensuremath{\sim}100\text{ }\text{T}$ 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 ${H}_{c2}^{\ensuremath{\perp}}(T)$ is indeed an intrinsic feature of oxypnictides rather than manifestation of vortex lattice melting or granularity. The orientational dependence of ${H}_{c2}(\ensuremath{\theta})$ as a function of the angle $\ensuremath{\theta}$ between $H$ and the $c$ axis shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy parameter $\ensuremath{\gamma}(T)={({m}_{c}/{m}_{ab})}^{1/2}={H}_{c2}^{\ensuremath{\parallel}}/{H}_{c2}^{\ensuremath{\perp}}$ obtained from these measurements decreases as temperature decreases from $\ensuremath{\gamma}\ensuremath{\simeq}9.2$ at 44 K to $\ensuremath{\gamma}\ensuremath{\simeq}5$ at 34 K, where $\ensuremath{\parallel}$ and $\ensuremath{\perp}$ 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 ${\ensuremath{\rho}}_{xx}(T,H)$ measured below ${T}_{c}$ but above the irreversibility field exhibits a clear Arrhenius thermally-activated behavior $\ensuremath{\rho}={\ensuremath{\rho}}_{0}\text{ }\text{exp}[\ensuremath{-}{E}_{a}(T,H)/T]$ over 4--5 decades of ${\ensuremath{\rho}}_{xx}$. The activation energy ${E}_{a}(T,H)$ has very different field dependencies for $H\ensuremath{\parallel}ab$ and $H\ensuremath{\perp}ab$ varying from $4\ifmmode\times\else\texttimes\fi{}{10}^{3}\text{ }\text{K}$ at $H=0.2\text{ }\text{T}$ to $\ensuremath{\sim}200\text{ }\text{K}$ at $H=35\text{ }\text{T}$. 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 ${H}_{c2}(0)\ensuremath{\sim}200--300\text{ }\text{T}$ based on extrapolations of ${H}_{c2}(T)$ near ${T}_{c}$ down to low temperatures.