Motion of positively charged muonium in ZnO

We report on a study of the motional characteristics of positively charged muonium defect centers in ZnO as an analog for H+ behavior. Muon spin depolarization measurements at zero applied magnetic field were completed from 20 K to 400 K, with preliminary results to 750 K. Results at the lower temperatures imply that Mu+ occupied two sites, and indicate local motion as thermally assisted tunneling with a characteristic energy of ∼60 meV, as well as a site change transition above 200 K with barrier energy ∼440 meV. Based on theoretical results, we have tentatively assigned these features to tunneling among three equivalent oxygen anti-bonding sites (AB⊥) and a transition to a lower-energy bond-centered site (BC‖) oriented along the c-axis. Preliminary fits suggest that global diffusion of muonium occurs above 400 K, with a diffusion barrier energy of ∼0.7 eV.