Localization of muonium atoms in KCl at low temperature

Muonium (μ+e− or Mu) atoms have long been believed to exhibit band-like propagation at low temperatures in ionic insulators and semiconductors. However, new muon spin relaxation measurements in transverse magnetic field reveal strong localization of Mu atoms at low temperatures in the ionic insulator KCl, similar to that observed in van der Waals cryocrystals. This discrepancy with previous results challenges our understanding of muonium quantum dynamics in solids: can Mu atoms ever truly delocalize at low temperature, or is low-temperature Mu localization a universal phenomenon?