  We report that (Ba,K)Fe2As2 superconductor (a transition temperature, Tc = 38
K) shows inverse Iron isotope effect (-0.18) (the sample including the larger
atomic weight of Fe depicts higher Tc). Measurements of both temperature
dependent magnetization and resistivity reveal a clear inverse shift by
systematic studies on Tc 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-Tc superconductors. This atomic mass dependence on Tc implies the exotic
coupling mechanism.
