A large iron isotope effect in SmFeAsO1-xFx and Ba1-xKxFe2As2

The recent discovery of superconductivity in oxypnictides with a critical transition temperature (T-C) higher than the McMillan limit of 39K (the theoretical maximum predicted by Bardeen-Cooper-Schrieffer theory) has generated great excitement(1-5). Theoretical calculations indicate that the electron-phonon interaction is not strong enough to give rise to such high transition temperatures(6), but strong ferromagnetic/antiferromagnetic fluctuations have been proposed to be responsible(7-9). Superconductivity and magnetism in pnictide superconductors, however, show a strong sensitivity to the crystal lattice, suggesting the possibility of unconventional electron-phonon coupling. Here we report the effect of oxygen and iron isotope substitution on T-C and the spin-density wave (SDW) transition temperature (T-SDW) in the SmFeAsO1-xFx and Ba1-xKxFe2As2 systems. The oxygen isotope effect on T-C and T-SDW is very small, while the iron isotope exponent alpha(C) =-dlnT(C)/dlnM is about 0.35 (0.5 corresponds to the full isotope effect). Surprisingly, the iron isotope exchange shows the same effect on T-SDW as T-C. This indicates that electron-phonon interaction plays some role in the superconducting mechanism, but a simple electron-phonon coupling mechanism seems unlikely because a strong magnon-phonon coupling is included.