Phonon anomalies in pure and underdoped R{1-x}K{x}Fe{2}As{2} (R = Ba, Sr) investigated by Raman light scattering

TL;DR

This study uses Raman light scattering to investigate phonon behavior in iron-based superconductors, revealing effects of magnetic and structural transitions and subtle anomalies at the superconducting transition, with implications for electron-phonon interactions.

Contribution

It provides detailed temperature-dependent phonon data in R{1-x}K{x}Fe{2}As{2} compounds, highlighting magnetic effects and discrepancies with theoretical predictions.

Findings

Phonon linewidths exceed density functional predictions, suggesting persistent local magnetic moments.

Structural transitions cause discontinuous phonon frequency jumps.

Subtle phonon anomalies observed at superconducting transition temperature.

Abstract

We present a detailed temperature dependent Raman light scattering study of optical phonons in Ba{1-x}K{x}Fe{2}As{2} (x ~ 0.28, superconducting Tc ~ 29 K), Sr{1-x}K{x}Fe{2}As{2} (x ~ 0.15, Tc ~ 29 K) and non-superconducting BaFe{2}As{2} single crystals. In all samples we observe a strong continuous narrowing of the Raman-active Fe and As vibrations upon cooling below the spin-density-wave transition Ts. We attribute this effect to the opening of the spin-density-wave gap. The electron-phonon linewidths inferred from these data greatly exceed the predictions of ab-initio density functional calculations without spin polarization, which may imply that local magnetic moments survive well above Ts. A first-order structural transition accompanying the spin-density-wave transition induces discontinuous jumps in the phonon frequencies. These anomalies are increasingly suppressed for higher potassium concentrations. We also observe subtle phonon anomalies at the superconducting transition temperature Tc, with a behavior qualitatively similar to that in the cuprate superconductors.