Quantum diffusion: effect of defect-localized phonon dynamics

The quantum diffusion of lattice defects is 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 is especially important in the case of diffusion of vacancies and self-interstitials while a defect displacement to a nearest site is accompanied by a breaking and reconstitution of bonds of nearest atoms. It is 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 is much smaller than that of interstitial atoms; the temperature dependences of these coefficients are also different. The obtained results are compared with experimental data for diffusion of vacancies in solid helium