Magnetism dependent phonon anomaly in LaFeAsO observed via inelastic x-ray scattering

TL;DR

This study reveals that magnetism significantly influences phonon behavior in LaFeAsO, with spin-polarized calculations better matching experimental data even above the magnetic transition temperature.

Contribution

The paper demonstrates the importance of magnetic order in phonon dispersion in LaFeAsO, showing magneto-structural effects and the necessity of Fe-spin for accurate modeling.

Findings

Spin-polarized calculations match experimental phonon spectra better than nonmagnetic ones.

Magneto-structural effects similar to AFe_2As_2 are confirmed in LaFeAsO.

Fe-spin presence is crucial for accurate phonon spectrum modeling above T_N.

Abstract

The phonon dispersion was measured at room temperature along (0,0,L) in the tetragonal phase of LaFeAsO using inelastic x-ray scattering. Spin-polarized first-principles calculations imposing various types of antiferromagnetic order are in better agreement with the experimental results than nonmagnetic calculations, although the measurements were made well above the magnetic ordering temperature, T_N. Splitting observed between two A_{1g} phonon modes at 22 and 26 meV is only observed in spin-polarized calculations. Magneto-structural effects similar to those observed in the AFe_2As_2 materials are confirmed present in LaFeAsO. The presence of Fe-spin is necessary to find reasonable agreement of the calculations with the measured spectrum well above T_N. On-site Fe and As force constants show significant softening compared to nonmagnetic calculations, however an investigation of the real-space force constants associates the magnetoelastic coupling with a complex renormalization instead of softening of a specific pairwise force.