Muonium quantum diffusion and localization in cryocrystals

We review our recent study of atomic muonium ( ⁺e^– or Mu, a light isotope of the hydrogen atom) diffusion in the simplest solids-van der Waals cryocrystals. We 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. In solid nitrogen bothT ⁷ andT ^–7 temperature dependences of the Mu hop rate are observed directly for the first time, which is taken as a 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 extremely low values of the Debye temperatures. Particular attention is dedicated 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 _c . Conventional trapping mechanisms are shown to be ineffective at low temperatures in insulators. The localization effects in Mu quantum diffusion are studied in detail in solid Kr. In all the cryocrystals studied muonium atom turns out to be localized at low temperatures.