Iron isotope effect on the superconducting transition temperature and the crystal structure of FeSe$_{1-x}$

The Fe isotope effect (Fe-IE) on the transition temperature Tc and the crystal structure was studied in the Fe chalcogenide superconductor FeSe$_{1-x}$ by means of magnetization and neutron powder diffraction (NPD). The substitution of natural Fe (containing $\simeq92$\% of $^{56}$Fe) by its lighter $^{54}$Fe isotope leads to a shift in T$_{\rm c}$ of 0.22(5) K corresponding to an Fe-IE exponent of $\alpha_{\rm Fe}=0.81(15)$. Simultaneously, a small structural change with isotope substitution is observed by NPD. This may help to clarify the currently controversal results of the Fe-IE. Upon correcting the isotope effect exponent for these structural effects, an almost unique value of $\alpha \sim0.35-0.4$ is observed for at least three different families of Fe-based HTS.