Cooperative dynamics in doped manganite films: phonon anomalies in the ferromagnetic state

TL;DR

This study investigates phonon behavior in doped manganite films across magnetic phase transitions, revealing anomalies linked to electron-phonon interactions and differences between two compounds.

Contribution

It provides detailed optical phonon spectra in doped manganite films and uncovers how phonon anomalies relate to electronic correlations and electron-phonon coupling.

Findings

Phonon anomalies occur near the magnetic transition.

Enhanced electron-phonon coupling causes Fano interference effects.

Different phonon behaviors are observed in LCMO and LSMO.

Abstract

We present optical measurements of phononic excitations in La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ (LCMO) and La$_{2/3}$Sr$_{1/3}$MnO$_{3}$ (LSMO) thin films covering the full temperature range from the metallic ferromagnetic to the insulating paramagnetic phase. All eight phonons expected for the R$\bar{3}$c symmetry in LSMO and 17 out of the expected 25 phonons for the Pnma symmetry in LCMO have been determined. Close to the ferromagnetic-to-paramagnetic transition both compounds reveal an anomalous behavior but with different characteristics. Anomalies in the phononic spectra are a manifestation of the coupling of lattice degrees of freedom (DOF) to electronic DOF. Specifically, the low-frequency external group proves to be an indicator for lattice modifications induced by electronic correlations. The enhanced electron-phonon coupling in LCMO is responsible for Fano-like interference effects of distinct phonon modes with electronic continuum excitations: we observe asymmetric phonon line shapes, mode splitting and spectral weight transfer between modes.