Enhancement of the upper critical field in codoped iron-arsenic high-temperature superconductors

We present the first study of codoped iron-arsenide superconductors of the 122 family (Sr/Ba)_1-xK_xFe_2-yCo_yAs₂ with the purpose to increase the upper critical field H_c2 compared to single doped Sr/BaFe₂As₂ materials. H_c2 was investigated by measuring the magnetoresistance in high pulsed magnetic fields up to 64 T. We find, that H_c2 extrapolated to T = 0 is indeed enhanced significantly to approx 90 T for polycrystalline samples of Ba0:55K0:45Fe1:95Co0:05As₂ compared to approx 75 T for Ba_0.55K_0.45Fe₂As₂ and BaFe_1.8Co_0.2As₂ single crystals. Codoping thus is a promising way for the systematic optimization of iron-arsenic based superconductors for magnetic-field and high-current applications.