Quantum diffusion: effects of local distortion of phonons

The diffusion of vacancies and self-interstices in crystals is accompanied by a strong change of local springs. We propose a theory, which takes into account this change. It is shown that the local softening of the lattice by a vacancy due to the breaking of bonds produce an enhancement of the contribution of 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 the diffusion of vacancies in helium crystals. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)