Quantum diffusion of muons in solids

Significant advances have recently been made in the long-standing problem of light particle tunneling dynamics in condensed matter. This has been achieved for both charged and neutral particles: experiments using the muon spin relaxation techniques unambiguously show that both muons in metals and muonium atoms (light ‘hydrogen’ isotopes) in insulators reveal tunneling effects at low temperatures. A major step forward has been made in studying the mechanisms of quantum diffusion of these particles under the influence of crystalline disorder. In particular, progress in recent years is connected with the problem of particle localization at low temperatures in disordered media.