Anharmonic softening of Raman active phonons in Iron-Pnictides; estimating the Fe isotope effect due to anharmonic expansion

TL;DR

This study uses Raman spectroscopy and anharmonic phonon modeling to analyze Fe-pnictide superconductors, revealing significant isotope effects and challenging theories that exclude electron-phonon interactions.

Contribution

It provides the first detailed analysis of anharmonic phonon behavior and isotope effects in Fe-pnictides, emphasizing the importance of electron-phonon interactions in superconductivity.

Findings

Phonon energies and lifetimes are well explained by anharmonic phonon interactions.

Fe isotope substitution causes measurable lattice expansion and bond length shifts.

Results suggest a notable Fe isotope effect, challenging purely electronic superconductivity theories.

Abstract

We present Raman measurements on the iron-pnictide superconductors CeFeAsO_{1-x}F_{x} and NdFeAsO{1-x}F_{x}. Modeling the Fe-As plane in terms of harmonic and a cubic anharmonic Fe-As interaction we calculate the temperature dependence of the energy and lifetime of the Raman active Fe B_{1g} mode and fit to the observed energy shift. The shifts and lifetimes are in good agreement with those measured also in other Raman studies which demonstrate that the phonon spectrum is well represented by phonon-phonon interactions without any significant electronic contribution. We also estimate the anharmonic expansion from Fe (56->54) isotope substitution to \Delta a=5.1 10^{-4}\AA and \Delta d_{Fe-As}= 2.510^{-4}\AA and the shift of harmonic zero point fluctuations of bond lengths <\Delta x^2><=3 10^{-5}\AA^2, giving a total relative average decrease of electronic hopping integrals of |\delta t|/t<= 2.0 10^{-4}. The results poses a serious challenge for any theory of superconductivity in the pnictides that does not include electron-phonon interactions to produce a sizable Fe-isotope effect.