Isotope effect on the transition temperature $T_c$ in Fe-based superconductors: the current status

Abstract

The results of the Fe isotope effect (Fe-IE) on the transition temperature $T_c$ obtained up to date in various Fe-based high temperature superconductors are summarized and reanalyzed by following the approach developed in [Phys. Rev. B 82, 212505 (2010)]. It is demonstrated that the very controversial results for Fe-IE on $T_c$ are caused by small structural changes occurring simultaneously with the Fe isotope exchange. The Fe-IE exponent on $T_c$ [$α_{\rm Fe}=-(ΔT_c/T_c)/(ΔM/M)$, $M$ is the isotope mass] needs to be decomposed into two components with the one related to the structural changes ($α_{\rm Fe}^{\rm str}$) and the genuine (intrinsic) one ($α_{\rm Fe}^{\rm int}$). The validity of such decomposition is further confirmed by the fact that $α_{\rm Fe}^{\rm int}$ coincides with the Fe-IE exponent on the characteristic phonon frequencies $α_{\rm Fe}^{\rm ph}$ as is reported in recent EXAFS and Raman experiments.