Evidence for small or intermediate-size polarons in the ferromagnetic state of manganites

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Oxygen-isotope effects on the intrinsic resistivity have been studied in high-quality epitaxial thin films of La₀.₇₅Ca₀.₂₅MnO₃ and Nd₀.₇Sr₀.₃MnO₃. We found that the residual resistivity ρₒ increases by about 15(3)% upon replacing ¹⁶O by ¹⁸O. This provides strong evidence for the presence of small or intermediate-size polarons in the metallic ferromagnetic state. Furthermore, the temperature dependent part of the resistivity at low temperatures consists of an AT⁴.⁵ term contributed from 2-magnon scattering, and a Bωₛ/²(ℏωₛ/2kBT) term which arises from scattering by a soft optical phonon mode. The absolute magnitudes of the coefficient A and the phonon frequency ℏωₛ for both isotope samples are in quantitative agreement with theoretical predictions.