Superconductivity at 38 K in the Iron Arsenide (Ba_1-xK_x)Fe₂As₂

The ternary iron arsenide BaFe₂As₂ becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity at T_c=38  K in (Ba_1-xK_x)Fe₂As₂ with x≈0.4. The parent compound BaFe₂As₂ crystallizes in the tetragonal ThCr₂Si₂-type structure, which consists of (FeAs)^δ- iron arsenide layers separated by Ba²⁺ ions. BaFe₂As₂ is a poor metal and exhibits a spin density wave anomaly at 140 K. By substituting Ba²⁺ for K⁺ ions we have introduced holes in the (FeAs)^- layers, which suppress the anomaly and induce superconductivity. The T_c of 38 K in (Ba_0.6K_0.4)Fe₂As₂ is the highest in hole doped iron arsenide superconductors so far. Therefore, we were able to expand this class of superconductors by oxygen-free compounds with the ThCr₂Si₂-type structure.