Quantum Diffusion of Muonium in KCl

The diffusion rate of muonium in an insulator, KCl, has been determined from measurements of muon spin relaxation induced by motion in the presence of nuclear hyperfine interactions. The crossover from stochastic to coherent hopping occurs at about 70 K, as evidenced by a dramatic rise in the hop rate at lower temperatures. The coherent hop rate rises much more rapidly with decreasing temperature than for muon diffusion in metals, as predicted by current theories on the diffusion of light interstitials in the absence of conduction electrons.