Quantum Diffusion of Positive Muons in Iron

Muon spin precession in iron has been observed from 300 K down to 3 K. The relaxation rate $\lambda$ shows a pronounced temperature dependence with minima at 15 and 90 K. A fit with an Arrhenius function yields $E_a=0.3\mathrm{}$ meV below 10 K, 14 meV between 35 and 90 K, and 86 meV between 150 and 300 K. The data indicate that coherent tunneling becomes important below 35 K. The temperature region between 35 and 90 K is dominated by incoherent tunneling while above 150 K the over-barrier hopping process sets in.