Anisotropic Hc2 of K0.8Fe1.76Se2 determined up to 60 T

The anisotropic upper critical field, H-c2(T), curves for K0.8Fe1.76Se2 are determined over a wide range of temperatures down to 1.5 K and magnetic fields up to 60 T. Anisotropic initial slopes of H-c2 similar to -1.4 T/K and -4.6 T/K for magnetic field applied along c axis and ab plane, respectively, were observed. Whereas the c axis H-c2(c) (T) increases quasilinearly with decreasing temperature, the ab plane H-c2(ab) (T) shows a flattening, starting near 25 K above 30 T. This leads to a nonmonotonic temperature dependence of the anisotropy parameter gamma(H) H-c2(ab)/H-c2(c). The anisotropy parameter is similar to 2 near T-c similar to 32 K and rises to a maximum gamma(H) similar to 3.6 around 27 K. For lower temperatures, gamma(H) decreases with T in a linear fashion, dropping to gamma(H) similar to 2.5 by T similar to 18 K. Despite the apparent differences between the K0.8Fe1.76Se2 and (Ba0.55K0.45)Fe2As2 or Ba(Fe0.926Co0.074)(2)As-2, in terms of the magnetic state and proximity to an insulating state, the H-c2(T) curves are remarkably similar.