Correlated polarons in dissimilar perovskite manganites

We report x-ray scattering studies of broad peaks located at (0.5 0 0)/(0 0.5 0)-type wave vectors in the paramagnetic insulating phases of ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ and ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3.$ We interpret the scattering in terms of correlated polarons and measure isotropic correlation lengths of 1-2 lattice constants in both samples. Remarkably, the size of these correlated polarons remains constant over the entire temperature range investigated. In ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3,$ this range extends up to ∼400 K, at which temperature the peaks are observed to disappear. Based on the wavevector, the correlated polarons are found to be consistent with a CE-type structure. Differences in behavior between the samples arise as they are cooled through their respective transition temperatures and become ferromagnetic metallic $\left({\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3\right)$ or charge and orbitally ordered insulating $\left({\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3\right).$ Since the primary difference between the two samples is the trivalent cation size, these results illustrate the robust nature of the correlated polarons to variations in the relative strength of the electron-phonon coupling, in contrast to the sensitivity of the low-temperature ground state to such variations.