Ultraslow Polaron Dynamics in Low-Doped Manganites fromL139aNMR-NQR and Muon Spin Rotation

We report a ${}^{139}\mathrm{La}$ NMR investigation of low-doped insulating manganite samples ( ${\mathrm{LaMnO}}_{3+\delta }$ and ${\mathrm{La}}_{1-y}{\mathrm{Ca}}_y{\mathrm{MnO}}_{3+\delta }$) 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 ${}^{139}\mathrm{La}$ 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.