Local tunneling and metastability of muonium in CuCl

Abstract

Measurements of the muon spin relaxation of muonium show that the ${\mathrm{Mu}}^{\mathrm{I}}$ center in CuCl is an example of metastability in which isolated muonium tunnels locally around the tetrahedral interstitial site with four Cu nearest neighbors, with the stable form (${\mathrm{Mu}}^{\mathrm{II}}$) quasistationary at the same interstitial. For T30 K the inverse correlation time associated with the damped tunneling motion increases with decreasing temperature according to a power law ${\mathit{T}}^{\mathrm{\ensuremath{-}}2.7(1)}$, which is close to that observed previously for long-range quantum diffusion of muonium in insulators.