Single-crystalline films of superconducting Ba1- xKxFe2As2 and Sr1- xKxFe2As2 were grown at temperatures below 350 °C under reduced As flux by molecular beam epitaxy. We performed a systematic study of the doping dependence of Tc in Ba 1-xKxFe2As2 for x = 0.0 to 1.0 and Sr1-xKxFe2As2 for x = 0.0 to 0.5. The highest Tcon (Tcend) so far attained is 38.3 K (35.5 K) at x ∼ 0.3 for Ba1-xKxFe2As 2, and 33.4 K (31.0 K) at x ∼ 0.4 for Sr1- xKxFe2As2. The optimum K-doping level inducing the highest Tc is different in the two systems. The implication of the results is briefly discussed.
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