Resistivity and Upper Critical Field in KFe2As2 Single Crystals

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We report resistivity measurements performed on KFe2As2 single crystals down to $T = 0.3$ K and in magnetic fields up to 17.5 T. The in-plane resistivity vs $T$ curve has a convex shape down to ${\sim}50$ K and shows a $T^{2}$ dependence below ${\sim}45$ K. The ratio of the $c$-axis to in-plane resistivities is ${\sim}10$ at room temperature and ${\sim}40$ at 4.2 K. The superconducting upper critical field $B_{\text{c2}}(T)$ has been determined from the resistivity data: $B_{\text{c2}}^{ab}(0) = 4.47$ T and $B_{\text{c2}}^{c}(0) = 1.25$ T. The anisotropy parameter $\Gamma = B_{\text{c2}}^{ab}/B_{\text{c2}}^{c}$ increases with $T$ and is 6.8 at $T = T_{\text{c}}$. The strong curvature of the $B_{\text{c2}}^{ab}(T)$ curve indicates the existence of strong spin paramagnetic effects.
2009-06-15