Isotope effect and the role of phonon in the Fe-based superconductors

TL;DR

This paper investigates the isotope effect in Fe-based superconductors, demonstrating that a large isotope coefficient can coexist with high transition temperatures within a two-band model, influenced by asymmetric density of states.

Contribution

It shows that a significant isotope effect does not contradict the $ m extit{s}$-wave pairing state and high Tc, highlighting the role of asymmetric DOS in enhancing the isotope coefficient.

Findings

Large isotope effect is compatible with $ m extit{s}$-wave pairing and high Tc.

Asymmetric DOS enhances the isotope coefficient significantly.

Moderate phonon coupling can produce large isotope effects and high Tc.

Abstract

We studied the isotope effect of phonon in the Fe-based superconductors using a phenomenological two band model for the sign-changing s-wave ($\pm$s-wave) state. Within this mean-field model, we showed that the large isotope effect is not inconsistent with the $\pm$s-wave pairing state and its high transition temperature. In principle, a large phonon isotope coefficient $\alpha$ implies a large phonon coupling constant. However, the asymmetric density of states (DOS) between two bands substantially enhances the value of $\alpha$, so that a moderate value of the phonon coupling constant ($\lambda_{\rm ph} \approx 0.4$) can produce a very large value of $\alpha$ ($\approx 0.4$) as well as a high transition temperature together with an antiferromagnet (AFM)-induced interaction.