We report a La-139 NMR investigation of low-doped insulating manganite samples (LaMnO3+delta and La1-yCayMnO3+delta) as a function of temperature. A volume fraction with fast nuclear relaxations was revealed by the inhomogeneous loss of the NMR signal over a broad temperature interval. Comparison with muon spin rotation data demonstrates that the wipeout of the La-139 signal is mainly due to slowly fluctuating electric field gradients. This provides strong evidence for the slow diffusion,of lattice excitations, identified with Jahn-Teller small polarons.
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