Superconductivity at 38 K in the Iron Arsenide(Ba1−xKx)Fe2As2

The ternary iron arsenide ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have discovered bulk superconductivity at $T_c=38\mathrm{K}$ in $({\mathrm{Ba}}_{1-x}{\mathrm{K}}_x){\mathrm{Fe}}_2{\mathrm{As}}_2$ with $x\approx 0.4$. The parent compound ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ crystallizes in the tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure, which consists of $(\mathrm{FeAs}{)}^{\delta -}$ iron arsenide layers separated by ${\mathrm{Ba}}^{2+}$ ions. ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ is a poor metal and exhibits a spin density wave anomaly at 140 K. By substituting ${\mathrm{Ba}}^{2+}$ for ${\mathrm{K}}^{+}$ ions we have introduced holes in the $(\mathrm{FeAs}{)}^{-}$ layers, which suppress the anomaly and induce superconductivity. The $T_c$ of 38 K in $({\mathrm{Ba}}_{0.6}{\mathrm{K}}_{0.4}){\mathrm{Fe}}_2{\mathrm{As}}_2$ 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 ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure.