The currently available results of the isotope effect on the superconducting transition temperature T_c in Fe-based high-temperature superconductors (HTS) are highly controversial. The values of the Fe isotope effect (Fe-IE) exponent \alpha_Fe for various families of Fe-based HTS were found to be as well positive, as negative, or even be exceedingly larger than the BCS value \alpha_BCS=0.5. Here we demonstrate that the Fe isotope substitution causes small structural modifications which, in turn, affect T_c. Upon correcting the isotope effect exponent for these structural effects, an almost unique value of \alpha~0.35-0.4 is observed for at least three different families of Fe-based HTS.
@article{arxiv.1008.4540,
title = {Intrinsic and structural isotope effects in Fe-based superconductors},
author = {R. Khasanov and M. Bendele and A. Bussmann-Holder and H. Keller},
journal= {arXiv preprint arXiv:1008.4540},
year = {2015}
}