Determination of anisotropic H_c2 up to 60 T in Ba_0.55K_0.45Fe₂As₂ single crystals

The radio frequency penetration depth was measured in the superconductor (Ba_0.55K_0.45)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, ≈3.5 close to T_c, and drops with the decrease in temperature, reaching ≈1.2 at 14 K. These data and analysis indicate that (i) (Ba_0.55K_0.45)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.