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 similar to 38 K) has an inverse iron isotope coefficient alpha(Fe)=-0.18(3) (where T-c similar to M-alpha Fe 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 (Fe-54, natural Fe, and Fe-57). 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.