Inverse Iron Isotope Effect on the Transition Temperature of the (Ba,K)Fe2As2 Superconductor

We report that the (Ba,K)Fe₂As₂ superconductor (transition temperature, T_c∼38K) has an inverse iron isotope coefficient α_Fe=-0.18(3) (where T_c∼M^-αFe and M is the iron isotope mass); i.e., the sample containing the large iron isotope mass depicts a higher T_c. Systematic inverse shifts in T_c were clearly observed between the samples using three types of Fe isotopes (Fe54, natural Fe, and Fe57). This indicates the first evidence of the inverse isotope effect in high-T_c superconductors. This anomalous mass dependence on T_c implies an exotic coupling mechanism in Fe-based superconductors.