Superconductivity at 38 K in the iron arsenide (Ba1-xKx)Fe2As2

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Abstract
The ternary iron arsenide BaFe2As2 becomes superconducting by hole doping, which was achieved by partial substitution of the barium site with potassium. We have found bulk superconductivity up to T_C = 38 K in (Ba1-xKx)Fe2As2 with x = 0.4. The parent compounds BaFe2As2 and KFe2As2 both form the tetragonal ThCr2Si2-type structure, which consists of (FeAs)- iron arsenide layers separated by barium or potassium ions. BaFe2As2 is a poor metal and exhibits a SDW anomaly at 140 K. BaFe2As2 is a relatively good metal and shows no anomaly. By substituting Ba2+ for K+ ions we have introduced holes in the (FeAs)- layers, which suppress the SDW anomaly and induce superconductivity. This scenario is very similar to the recently discovered arsenide-oxide superconductors, where hole doping of LaFeAsO results in a T_C of 25 K. A T_C of 38 K in (Ba1-xKx)Fe2As2 is the highest observed critical temperature 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. Our results suggest, that superconductivity in these systems evolves essentially from the (FeAs)- layers and may occur in other related compounds.

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