MUONIUM QUANTUM DIFFUSION AND LOCALIZATION IN CRYOCRYSTALS

We review our recent study of atomic muonium (mu(+) e(-) or Mu, a ligh t isotope of the hydrogen atom) diffusion in the simplest solids-van d er Waals cryocrystals. We give experimental evidence of the quantum-me chanical nature of the Mu diffusion in these solids. The results are c ompared with the current theories of quantum diffusion in insulators. In solid nitrogen both T-7 and T-7 temperature dependences of the Mu h op 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 wh ole temperature range under investigation due to extremely low values of the Debye temperatures. Particular attention is dedicated to proces ses of inhomogeneous quantum diffusion and Mu localization. It is show n that at low temperatures static crystal disorder results in an inhom ogeneity of the Mu quantum diffusion which turns out to be inconsisten t with diffusion models using a single correlation time tau(c). Conven tional trapping mechanisms are shown to be ineffective at low temperat ures in insulators. The localization effects in Mu quantum diffusion a re studied in detail in solid Kr. In all the cryocrystals studied muon ium atom turns out to be localized at low temperatures.