Polaronic metal phases in L a 0.7 S r 0.3 Mn O 3 uncovered by inelastic neutron and x-ray scattering

Among colossal magnetoresistive manganites the prototypical ferromagnetic manganite $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ has a relatively small magnetoresistance, and has been long assumed to have only weak electron-lattice coupling. Here we report that $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ 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 ${T}_{C}=350\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Simultaneously, we observe two phases where metallic resistivity and polarons coexist. The ferromagnetic polaronic metal phase between $200\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{C}$ is characterized by quasielastic scattering from dynamic CE-type polarons with the relatively short lifetime of $\ensuremath{\tau}\ensuremath{\approx}1\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$. 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 the polaron lifetime, not the strength of electron-phonon coupling.