Muonium quantum diffusion in cryocrystals

In this paper we review our recent study on muonium (Mu) diffusion in simplest solids — Van der Waals cryocrystals. We give experimental evidence for the quantum nature of Mu diffusion in these matters. The results are compared with the current theory of quantum diffusion in insulators. The first direct observation of theT⁷ power-law for the Mu hop rate (in solid nitrogen) is taken as confirmation of two-phonon scattering mechanism. In solid xenon the one-phonon interaction is shown to be dominant. Muonium diffusion in solid neon is discussed in terms of band-like motion. Finally, we report a dramatic effect of impurities on Mu quantum diffusion.