Bond-Bending and Bond-Stretching Phonons in Ferromagnetic La_0.7Sr_0.3MnO_3

TL;DR

This study investigates phonon behaviors in La_0.7Sr_0.3MnO_3 using inelastic neutron scattering, revealing how lattice distortions influence phonon interactions and highlighting discrepancies with shell model predictions.

Contribution

It demonstrates the impact of rhombohedral distortion on phonon interactions and identifies limitations of shell model predictions in this material.

Findings

Bond-stretching phonons show anomalous downward dispersion.

Rhombohedral distortion causes strong interaction between bond-stretching and bond-bending modes.

Experimental results differ from shell model predictions due to lattice distortions.

Abstract

Longitudinal optical phonons with oxygen character were measured in La_0.7Sr_0.3MnO_3 by inelastic neutron scattering in the (1 0 0) cubic direction and results were compared with shell model predictions. Measurements were performed in several Brillouin zones, which enabled us to identify the eigenvectors independent of the shell model. All major disagreements between model predictions and experimental results are primarily due to the anomalous downward dispersion of the bond-stretching vibration. The main new result is that the rhombohedral distortion of the cubic lattice makes the bond-stretching vibrations interact strongly with bond-bending modes folded into the cubic Brillouin zone.