Weak anisotropy of the superconducting upper critical field in Fe1.11Te0.6Se0.4 single crystals

We have determined the resistive upper critical field H-c2 for single crystals of the superconductor Fe1.11Te0.6Se0.4 using pulsed magnetic fields of up to 60 T. A rather high zero-temperature upper critical field of mu H-0(c2)(0)approximate to 47 T is obtained in spite of the relatively low superconducting transition temperature (T-c approximate to 14 K). 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 Fe1.11Te0.6Se0.4 is a consequence of its three-dimensional Fermi-surface topology. An analogous result was obtained for (Ba, K)Fe2As2, indicating that all layered iron-based superconductors exhibit generic behavior that is significantly different from that of the high-Tc cuprates.