Codoping -- A way to enhance the upper critical field in iron-arsenic superconductors*

Technological key features of iron-based superconductors are the high critical temperature $T_{c}$ of up to 55 K and the high tolerance against magnetic fields, which led so far to upper critical fields in the range of 75 T. Furthermore, the small $H_{c2}$-anisotropy between field applied along the $c$-direction and in the \textit{ab}-plane, in particular for the FeSe and \textit{AE}Fe$_{2}$As$_{2}$ (\textit{AE} = Ca, Sr, Ba) materials, is a prerequisite for several technical applications. Currently, different approaches (chemical substitutions, processing) are discussed how to increase $H_{c2}$ further. Here, we show a feasibility study for codoping of polycrystalline Sr- or BaFe$_{2}$As$_{2}$ samples, namely the simultaneous substitution of K on the Sr/Ba layer and of Co on the FeAs layer. The upper critical field was investigated by magnetoresistance in high pulsed magnetic fields up to 64~T. We find, that the extrapolated critical field $H_{c2}(T\to $0) is enhanced by 15{\%} for Ba$_{0.55}$K$_{0.45}$Fe$_{1.95}$Co$_{0.05}$As$_{2}$ in comparison to Ba$_{0.55}$K$_{0.45}$Fe$_{2}$As$_{2}$, although $T_{c}$ is almost identical in both materials. These results suggest that codoping is a promising route for the systematic optimization of iron-arsenic based superconductors for high-magnetic field and high-current applications.