μ⁺ diffusion in cubic f-electron compounds observed by high transverse field μ⁺ SR

Positive muon (μ⁺) diffusion was observed in cubic f-electron compounds PrPb₃ and SmAg₂In by means of the μ⁺ spin rotation (μ⁺SR) method in high transverse field applied along the [001] direction. The applied field splits one crystallographic site into two magnetically inequivalent sites. The μ⁺ diffusion between these magnetically different sites drastically influences the μ⁺SR frequencies, so that two lines are gradually merged into one line at increasing temperature due to the motional narrowing effect. The μ⁺ hopping rate Ω derived from the μ⁺SR line shape follows the Arrhenius law, namely, Ω Ω₀ exp(-U/k_BT) in both cases. The preexponential factor Ω₀ suggests that the dominant mechanism of μ⁺ diffusion is classical over-the-barrier hopping in PrPb ₃ for 140<T<200 K and phonon-assisted quantum tunneling in SmAg₂In for 40<T<70 K.