QUANTUM DIFFUSION IN CRYOCRYSTALS STUDIED BY MUON SPIN RELAXATION

We review our recent studies of muonium diffusion in cryocrystals. Ato mic muonium (Mu = mu(+)e(-)) may be considered as a light isotope of h ydrogen and so provides the prototypical light interstitial defect in these sytems, themselves the simplest of insulators. Experiments using the muon spin relaxation techniques in, transverse and longitudinal ( zero) magnetic fields reveal tunneling motion at low temperatures, gov erned by various mechanisms of phonon scattering. The results are comp ared with the current theories of quantum diffusion in insulators. A t wo-phonon scattering mechanism is found to dominate at low temperature s in solid nitrogen, methanes and carbon dioxide, whereas a one-phonon interaction provides the main channel at temperatures comparable to t he Debye temperature in solid nitrogen, xenon and krypton. Particular attention is devoted to quantum diffusion processes in the presence of static crystal disorder. At low temperatures, the muonium diffusion c an no longer be described in terms of a single correlation time. The l ocalization and delocalization effects in the Mu diffusion in such inh omogeneous crystals are discussed in detail. Finally, the authors give an analysis of trapping phenomena for muonium in insulators.