Determination of anisotropic Hc2 up to 60 T in Ba0.55K0.45Fe2As2 single crystals

The radio frequency penetration depth was measured in the superconductor (Ba0.55K0.45)Fe2As2 under pulsed magnetic fields extending to 60 T and down to 14 K. Using these data we are able to infer a H-c2(T), H-T phase diagram, for applied fields parallel and perpendicular to the crystallographic c axis. The upper critical-field curvature is different for the respective orientations, but they each remains positive down to 14 K. The upper critical-field anisotropy is moderate, approximate to 3.5 close to T-c, and drops with the decrease in temperature, reaching approximate to 1.2 at 14 K. These data and analysis indicate that (i) (Ba0.55K0.45)Fe2As2 anisotropy diminishes with temperature and has an unusual temperature dependence and (ii) H-c2(T=0) for this compound may easily approach fields of 75 T.