Comment on "Isotope effect in multi-band and multi-channel attractive systems and inverse isotope effect in iron-based superconductors" by T. Yanagisawa, et al

TL;DR

This paper critiques a theoretical claim that an inverse isotope effect can occur in iron-based superconductors, highlighting misleading assumptions and contrasting experimental results that question the validity of the theory.

Contribution

It provides a critical analysis of the theoretical model claiming inverse isotope effects, emphasizing the importance of accurate assumptions and experimental validation.

Findings

Contradicts the theoretical claim of inverse isotope effect in iron-based superconductors.

Highlights conflicting experimental results on isotope exponents in Ba,KFe2As2.

Questions the assumptions used in the theoretical analysis of isotope effects.

Abstract

In a recent paper Yanagisawa et al. [1] claim from a theoretical analysis of a multi-channel multi-band superconductor model that an inverse isotope exponent on the superconducting transition temperature Tc can be realized in iron-based superconductors. Simultaneously, a subgroup of the authors of Ref. 1 performed the corresponding isotope effect experiment on (Ba, K)Fe2As2 by investigating the iron isotope exchange effect on Tc [2]. In accordance with their theoretical analysis they indeed report an unusually large sign reversed isotope exponent of {\alpha} \simeq -0.18(3) which is in strong contrast to previous experiments on the nominally same system with the same composition in Ba, K content, namely Ba0.6K0.4Fe2As2 [3], where the exponent was determined to be {\alpha} \simeq 0.37(3). This conflict remains unsolved until now with the exception of Ref. 4 where the iron isotope exponent has been determined for FeSe. In accordance with the results of Ref. 3 a large positive isotope exponent has been seen thus questioning the outcome of Ref. 1 and implicitly the findings of Ref. 2. Here, we do not comment on the controversial experimental situation but address the theoretical analysis of Ref. 1, where a variety of misleading assumptions have led to the conclusion that a sign reversed isotope exponent can be realized in a multi-band and multi-channel attractive model for iron based superconductors.