Formation of partial energy gap below the structural phase transition and the rare-earth element substitution effect on infrared phonons in ReFeAsO (Re=La, Nd, and Sm)

TL;DR

This study investigates how structural phase transitions and rare-earth element substitutions affect the electronic and vibrational properties of ReFeAsO compounds, revealing partial energy gaps, phonon shifts, and electron-phonon interactions.

Contribution

It provides new insights into the effects of rare-earth substitution on infrared phonons and the electronic state near the SDW transition in ReFeAsO compounds.

Findings

Partial energy gaps are present below the structural transition.

Rare-earth substitution shifts phonon frequencies higher.

Evidence of electron-phonon coupling in Fe-pnictides.

Abstract

Single crystals of LaFeAsO, NdFeAsO, and SmFeAsO have been prepared by means of a NaAs flux growth technique and studied by optical spectroscopy measurements. We show that the spectral features corresponding to the partial energy gaps in the spin-density-wave (SDW) state are present below the structural phase transition. This indicates that the electronic state below the structural phase transition is already very close to that in the SDW state. We also show that in-plane infrared phonon modes display systematic shifts towards high frequency upon rare-earth element substitutions for La, suggesting a strong enhancement of the bonding strength. Furthermore, an asymmetric line-shape of the in-plane phonon mode is observed, implying the presence of an electron-phonon coupling effect in Fe-pnictides.