Upper critical field of the 122-type iron pnictide superconductors

The upper critical fields ($H_{c2}$) of the single crystals $(\mathrm{S}\mathrm{r},\mathrm{N}\mathrm{a})\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ and $\mathrm{B}{\mathrm{a}}_{0.55}{\mathrm{K}}_{0.45}\mathrm{F}{\mathrm{e}}_2\mathrm{A}{\mathrm{s}}_2$ were determined by means of measuring the electrical resistivity, ${\rho }_{xx}({\mu }_{0}H)$, using the facilities of pulsed magnetic field at Los Alamos. In general, these compounds possess a very large upper critical field ($H_{c2}(0)$) with a weak anisotropic effect. The detailed curvature of $H_{c2}(T_c)$ may depend on the magnetic field orientation and the sample compositions. We argue that such a difference mainly results from the multi-band effect, which might be modified via doping.