Muonium quantum diffusion and localization in cryocrystals

The authors review their recent study of atomic muonium (μ+e- or Mu, a light isotope of the hydrogen atom) diffusion in the simplest solids--Van der Walls cryocrystals. They give experimental evidence of the quantum-mechanical nature of the Mu diffusion in these solids. The results are compared with the current theories of quantum diffusion in insulators. The predicted T±7 power-law temperature dependence of the Mu hop rate is observed directly for the first time in solid nitrogen (δ-N2) and is taken as confirmation of a two-phonon scattering mechanism. In solid xenon and krypton, by contrast, the one-phonon interaction is dominant in the whole temperature range under investigation due to the extremely low values of the Debye temperatures in those solids. Particular attention is devoted to processes of inhomogeneous quantum diffusion and Mu localization. It is shown that at low temperatures static crystal disorder results in an inhomogeneity of the Mu quantum diffusion which turns out to be inconsistent with diffusion models using a single correlation time τc. Conventional trapping mechanisms are shown to be ineffective at low temperatures in insulators. Muonium localization effects are studied in detail in solid Kr. In all the cryocrystals studied, muonium atoms turn out to be localized at the lowest temperatures