Diffusion in solid helium (Review Article)

Experimental results on diffusion in solid helium are reviewed. It is shown that 3He impurities in hcp 4 He are narrow-band quasi-particles and their motion can be adequately described in the framework of the recent theory of quantum diffusion. Additional peculiarities of 3 He diffusion in a bcc phase of 3He-4 He concentrated solutions are pointed: a marked diffusive transfer at a concentration higher than the critical one: an independence of diffusion coefficient D from concentration down to x < 20% 3 He; a sharp increase of D at x approx = 20% 3 He a dependence of D on diffusion length in the latter case. The above features cannot be now fretted quantitatively. The relation between the diffusion phenomena in the concentrated solutions and the percolation problem is found out. Experimental study into vacancy diffusion is considered which suggests that the vacancies in 3 He and 4 He crystals are wide-band quasi-particles