Superconducting Fluctuations and Anomalous Phonon Renormalization much above superconducting transition temperature in Ca4Al2O5.7Fe2As2

TL;DR

This study reveals that Ca4Al2O5.7Fe2As2 exhibits superconducting fluctuations and phonon anomalies well above its transition temperature, indicating complex interactions between lattice vibrations and superconductivity.

Contribution

It provides evidence of precursor superconducting fluctuations and strong spin-phonon coupling in Ca4Al2O5.7Fe2As2 through Raman spectroscopy analysis.

Findings

Sharp jump in Raman mode frequency at ~60 K

Anomalous softening and linewidth changes below T0

Large blue-shift of phonon mode from 300 K to 60 K

Abstract

Raman studies on Ca4Al2O5.7Fe2As2 superconductor in the temperature range of 5 K to 300 K, covering the superconducting transition temperature Tc ~ 28.3 K, reveal that the Raman mode at ~ 230 cm-1 shows a sharp jump in frequency by ~ 2 % and linewidth increases by ~ 175 % at To ~ 60 K. Below To, anomalous softening of the mode frequency and a large decrease by ~ 10 cm-1 in the linewidth is observed. These precursor effects at T0 (~ 2Tc) are attributed to significant superconducting fluctuations, possibly enhanced due to reduced dimensionality arising from weaked coupling between the well separated (~ 15 {\AA}) Fe-As layers in the unit cell. A large blue-shift of the mode frequency between 300 K to 60 K (~7%) indicates strong spin-phonon coupling in this superconductor.