Weak anisotropy of the superconducting upper critical field in Fe _1.11Te_0.6Se_0.4 single crystals

We have determined the resistive upper critical field Hc₂ for single crystals of the superconductor Fe_1.11Te_0.6Se _0.4 using pulsed magnetic fields of up to 60 T. A rather high zero-temperature upper critical field of μ0 Hc₂ (0) 47 T is obtained in spite of the relatively low superconducting transition temperature (Tc 14K). Moreover, Hc2 follows an unusual temperature dependence, becoming almost independent of the magnetic field orientation as the temperature T→0. We suggest that the isotropic superconductivity in Fe _1.11Te_0.6Se_0.4 is a consequence of its three-dimensional Fermi-surface topology. An analogous result was obtained for (Ba,K) Fe₂ As₂, indicating that all layered iron-based superconductors exhibit generic behavior that is significantly different from that of the "high- Tc " cuprates.