Diffusion of light particles in metals

The positively charged light particles as positron, positive muon, proton, deuteron and triton may be considered as isotopes of the element with atomic number one. Thus they offer unique opportunities for studying isotope effects in the regime of 'quantum diffusion'. The basic properties of these particles are reviewed and related to recent experiments on muon depolarization and positron annihilation. The question is whether positrons may show the phenomenon of self-trapping in metals. Recent developments in the quantum theory of diffusion of self-trapped particles are discussed in detail. (30 references)