Theory of quantum diffusion of atoms in crystals

A microscopic theory of the quantum diffusion of light interstitials in a perfect crystal is developed. The basic aspects and features of the quantum diffusion and the role of the polaron effects are revealed. The expressions of the coefficients for under-barrier coherent and noncoherent tunnelling diffusion and for over-barrier diffusion, valid at the appropriate temperatures, are derived in a unified way. The basic role of the intra-well scattering for the coherent diffusion in limiting narrow bands is elucidated. The theory permits one to consider the transition from quantum to classical diffusion. It is found that coherent diffusion predominates at the low temperatures, at least in the perfect crystal. The criteria for the observation of the coherent diffusion mechanism are given.

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