Polaronic metal phases in La${}_{0.7}$Sr${}_{0.3}$MnO${}_3$ uncovered by inelastic neutron and x-ray scattering

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 $\ddot{\mathbf{I}}„\approx 1\mathrm{ps}$. 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.