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

The anisotropic upper critical field, Hc2(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 Hc2 ∼ -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 Hc2c (T) increases quasilinearly with decreasing temperature, the ab plane Hc2ab(T) shows a flattening, starting near 25 K above 30 T. This leads to a nonmonotonic temperature dependence of the anisotropy parameter γH≡ Hc2ab/Hc2c. The anisotropy parameter is ∼2 near Tc∼32 K and rises to a maximum γH∼ 3.6 around 27 K. For lower temperatures, γH decreases with T in a linear fashion, dropping to γH∼2.5 by T∼18 K. Despite the apparent differences between the K0.8Fe1.76Se2 and (Ba0.55K0.45)Fe2As2 or Ba(Fe0.926Co0.074)2As2, in terms of the magnetic state and proximity to an insulating state, the Hc2(T) curves are remarkably similar. [ABSTRACT FROM AUTHOR]