Upper critical fields of NdFeAsO$_{0.7}$F$_{0.3}$ single crystal

We present measurements of the resistivity and the upper critical field $H_{c2}$ of NdFeAs O$_{0.7}$F$_{0.3}$ single crystals in strong DC and pulsed magnetic fields up to 45 T and 60 T, respectively. We found that the field scale of $H_{c2}$ is comparable to $\sim100$~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 samples. Thus this behavior is indeed an intrinsic feature of oxypnictides, rather than manifestation of vortex lattice melting or granularity. The orientational dependence of $H_{c2}$ shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy decreases as $T$ decreases, from 9.2 at 44 K to 5 at 34 K. We discuss to what extent different pairing scenarios can manifest themselves in the observed behavior of $H_{c2}$, using the two-band model of superconductivity. The results indicate the importance of paramagnetic effects on $H_{c2}(T)$, which may significantly reduce $H_{c2}(0)$ as compared to $H_{c2}(0)\sim200-300$ T based on extrapolations of $H_{c2}(T)$ near $T_c$ down to low temperatures.