Absence of an Appreciable Iron Isotope Effect on the Transition Temperature of the Optimally Doped SmFeAsO1-y Superconductor

We report the iron (Fe) isotope effect on the transition temperature (T-c) in oxygen-deficient SmFeAsO1-y, a 50-K-class, Fe-based superconductor. For the optimally doped samples with T-c = 54 K, a change of the average atomic mass of Fe (M-Fe) causes a negligibly small shift in T-c, with the Fe isotope coefficient (alpha(Fe)) as small as -0.024 +/- 0.015 (where alpha(Fe) = -dlnT(c)/dlnM(Fe)). 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-T-c superconductors.