Polaronic metal phases in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ uncovered by inelastic neutron and x-ray scattering

TL;DR

This study reveals that La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ exhibits strong electron-phonon coupling and polaronic metallic phases, challenging previous assumptions of weak coupling and showing a correlation between magnetoresistance and polaron lifetime.

Contribution

The paper uncovers the existence of polaronic metal phases in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ through neutron and x-ray scattering, demonstrating strong electron-phonon interactions and their impact on magnetoresistance.

Findings

Strong phonon softening and broadening near T$_C$

Coexistence of metallic resistivity and polarons

Magnetoresistance scales with inverse polaron lifetime

Abstract

Among colossal magnetoresistive manganites the prototypical ferromagnetic manganite La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ has a relatively small magnetoresistance, and has been long assumed to have only weak electron-lattice coupling. Here we report that La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ has strong electron-phonon coupling: Our neutron and x-ray scattering experiments show strong softening and broadening of transverse acoustic phonons on heating through the Curie temperature T$_C$ = 350 K. Simultaneously, we observe two phases where metallic resistivity and polarons coexist. The ferromagnetic polaronic metal phase between 200 K and T$_C$ is characterized by quasielastic scattering from dynamic CE-type polarons with the relatively short lifetime of $\mathbf{\tau}\approx 1\,\rm{ps}$. This scattering is greatly enhanced above T$_C$ in the paramagnetic polaronic metal phase. Our results suggest that the strength of magnetoresistance in manganites scales with the inverse of polaron lifetime, not the strength of electron-phonon coupling.