Diffusion in solid helium (A review)

Abstract

The results of experimental investigations of diffusion processes in solid helium are reviewed. It is shown that 3He impurities in hcp 4He are narrow-band quasiparticles whose motion can be adequately described in the existing theory of quantum diffusion. The observed peculiarities of 3He diffusion in the bcc phase of concentrated 3He–4He solutions, i.e., a noticeable diffusive transport at concentrations higher than critical, the independence of the diffusion coefficient D on the concentration for x<20%3He, an abrupt increase in D for x≃20%, and a dependence of D on the diffusion length in the latter case, have not received a quantitative interpretation. The relation between diffusion phenomena in concentrated solutions and the percolation problem is emphasized. The results of experiments on vacancy diffusion are analyzed, and it is proved that vacancies in 3He–4He are wide-band quasiparticles.