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

TL;DR

This paper reviews and reanalyzes the Fe isotope effect on the superconducting transition temperature in Fe-based superconductors, emphasizing the importance of separating structural effects from intrinsic isotope effects to understand the true impact on $T_c$.

Contribution

It introduces a method to decompose the isotope effect exponent into structural and intrinsic components, clarifying previous controversial results.

Findings

The intrinsic isotope effect exponent aligns with phonon frequency shifts.

Structural changes during isotope exchange influence $T_c$ measurements.

Decomposition improves understanding of isotope effects in superconductors.

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 $T_c$ [$\alpha_{\rm Fe}=-(\Delta T_c/T_c)/(\Delta M/M)$, $M$ is the isotope mass] needs to be decomposed into two components with the one related to the structural changes ($\alpha_{\rm Fe}^{\rm str}$) and the genuine (intrinsic) one ($\alpha_{\rm Fe}^{\rm int}$). The validity of such decomposition is further confirmed by the fact that $\alpha_{\rm Fe}^{\rm int}$ coincides with the Fe-IE exponent on the characteristic phonon frequencies $\alpha_{\rm Fe}^{\rm ph}$ as is reported in recent EXAFS and Raman experiments.