# Damping and softening of transverse acoustic phonons in colossal magnetoresistive La$_{0.7}$Ca$_{0.3}$MnO$_3$ and La$_{0.7}$Sr$_{0.3}$MnO$_3$

## TL;DR

This study uses neutron spectroscopy to investigate how transverse acoustic phonons in colossal magnetoresistive manganites soften and dampen near the Curie temperature, revealing differences in quasielastic scattering and magnetic field effects between two compounds.

## Contribution

It provides new insights into phonon behavior and quasielastic scattering in La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3 near Tc, highlighting the impact of magnetic fields on damping.

## Key findings

- Phonons soften and damp significantly near Tc in La0.7Ca0.3MnO3.
- Quasielastic scattering is more intense near Tc in La0.7Ca0.3MnO3.
- Magnetic field partially reverses damping and quasielastic scattering, but not phonon energy.

## Abstract

Neutron spectroscopy is used to probe transverse acoustic phonons near the (2, 2, 0) Bragg position in colossal magnetoresistive La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3. Upon warming to temperatures near Tc = 257 K the phonon peaks in La0.7Ca0.3MnO3 soften and damp significantly with the phonon half width at half maximum approaching 2.5 meV for phonons at a reduced wave vector of q = (0.2, 0.2, 0). Concurrently a quasielastic component develops that dominates the spectrum near the polaron position at high temperatures. This quasielastic scattering is ~5 times more intense near Tc than in La0.7Sr0.3MnO3 despite comparable structural distortions in the two. The damping becomes more significant near the polaron position with a temperature dependence similar to that of polaron structural distortions. An applied magnetic field of 9.5 T only partially reverses the damping and quasielastic component, despite smaller fields being sufficient to drive the colossal magnetoresistive effect. The phonon energy, on the other hand, is unaffected by field. The damping in La0.7Sr0.3MnO3 near Tc at a reduced wave vector of q = (0.25, 0.25, 0) is significantly smaller but displays a similar trend with an applied magnetic field.