Correlated polarons in dissimilar perovskite manganites

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C. S. Nelson, M. v. Zimmermann, Y. J. Kim, J. P. Hill, D. Gibbs, V. Kiryukhin, T.Y. Koo, S.-W. Cheong,
D. Casa, B. Keimer, Y. Tomioka, Y. Tokura, T. Gog, C. T. Venkataraman

Abstract:
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 La 0.7 Ca 0.3 MnO 3 and Pr 0.7 Ca 0.3 MnO 3 . We interpret the scattering in terms of
correlated polarons and measure isotropic correlation lengths of 1-2 lattice constants in both samples. Remark-ably,
the size of these correlated polarons remains constant over the entire temperature range investigated. In
La 0.7 Ca 0.3 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. Differ-ences
in behavior between the samples arise as they are cooled through their respective transition temperatures
and become ferromagnetic metallic (La 0.7 Ca 0.3 MnO 3 ) or charge and orbitally ordered insulating
(Pr 0.7 Ca 0.3 MnO 3 !. 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.