Studies of muonium dynamics in insulators and semiconductors at low temperature had led to 2 fundamentally different conclusions. Thus, Mu was found to localize strongly in Van der Waals crystals while, in alkali halides and compound semiconductors, it was believed to undergo band-like propagation with a characteristic bandwidth of about 0.1K. Recent measurements in transverse fields indicated that Mu could be localized at low temperatures in both KCl and GaAs. This apparent discrepancy could markedly change the understanding of muonium quantum dynamics in solids; thus raising the question of whether Mu could ever be truly delocalized at low temperatures or whether its localization was a general phenomenon in solids.

Quantum Diffusion of Muonium Atoms in Solids - Localization vs Band-Like Propagation. V.G.Storchak, D.G.Eshchenko, J.H.Brewer: Physica B, 2006, 374-375, 347-50