Intrinsic and structural isotope effects in iron-based superconductors

The currently available results of the isotope effect on the superconducting transition temperature T(c) in Fe-based high-temperature superconductors (HTSs) are highly controversial. The values of the Fe isotope effect 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 emphasize 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 similar to 0.35-0.4 is observed for at least three different families of Fe-based HTS.