We have determined the resistive upper critical field $H_{\text{c}2}$ for single crystals of the superconductor ${\text{Fe}}_{1.11}{\text{Te}}_{0.6}{\text{Se}}_{0.4}$ using pulsed magnetic fields of up to 60 T. A rather high zero-temperature upper critical field of ${\mu }_0{H}_{\text{c}2}\left(0\right)\approx 47\text{T}$ is obtained in spite of the relatively low superconducting transition temperature $\left(T_{\text{c}}\approx 14\text{K}\right)$. Moreover, $H_{\text{c}2}$ follows an unusual temperature dependence, becoming almost independent of the magnetic field orientation as the temperature $T\to 0$. We suggest that the isotropic superconductivity in ${\text{Fe}}_{1.11}{\text{Te}}_{0.6}{\text{Se}}_{0.4}$ is a consequence of its three-dimensional Fermi-surface topology. An analogous result was obtained for $\left(\text{Ba},\text{K}\right){\text{Fe}}_2{\text{As}}_2$, indicating that all layered iron-based superconductors exhibit generic behavior that is significantly different from that of the “high-$T_{\text{c}}$” cuprates.

• Received 19 October 2009

DOI:https://doi.org/10.1103/PhysRevB.81.020509