The quantum diffusion of lattice defects was considered on the basis of the non-perturbative description of the quadratic defect-lattice coupling. This allows to take explicitly into account the effect of defect-localized phonon dynamics. This effect was especially important in the case of diffusion of vacancies and self-interstitials while a defect displacement to a nearest site was accompanied by a breaking and reconstitution of bonds of nearest atoms. It was shown that the local softening of the lattice by a vacancy due to the breaking of bonds produces an enhancement of the contribution of the low frequency phonons while the local hardening of the lattice by an interstitial atom leads to the opposite effect. As a consequence, at low temperatures the diffusion coefficient of vacancies was much smaller than that of interstitial atoms; the temperature dependences of these coefficients were also different. The obtained results were compared with experimental data for diffusion of vacancies in solid helium.

Quantum Diffusion - Effect of Defect-Localized Phonon Dynamics. V.Hizhnyakov, G.Benedek: European Physics Journal B, 2005, 43[4], 431-8