Absence of an appreciable iron isotope effect on the transition temperature of the optimally doped SmFeAsO_(1-y) superconductor

We report the iron (Fe) isotope effect on the transition temperature (Tc) in the oxygen-deficient SmFeAsO_{1-y}, a 50 K-class Fe-based superconductor. For the optimally-doped samples with Tc = 54 K, change of the Fe average atomic mass (MFe) causes a negligibly small shift in Tc, with the Fe isotope coefficient (alphaFe) as small as -0.024 \pm 0.015. This result contrasts with the finite, inverse isotope shift observed in optimally-doped (Ba,K)Fe2As2, indicating that the contribution of the electron-phonon interaction markedly differs between these two Fe-based high-Tc superconductors.