A general review was presented of the theoretical description of the low-temperature tunnelling diffusion of light interstitial particles in normally conducting metals, in superconducting metals, and in insulators. The regimes of incoherent diffusion (mean free diffusion path shorter than the distance between interstitial sites) and coherent diffusion (mean free path longer than the distance between interstitial sites) were both considered. In the case of the former regime, it was shown that the behavior of the low-temperature tunnelling rates of light particles was largely determined by so-called energy asymmetry parameters which described the static energy shifts between the ground states of adjacent interstitial sites. These static energy shifts were caused by lattice inhomogeneities. Depending upon the strength of the coupling of the light particles to the electron and phonon systems of the crystal, various regions of behavior could be found. This was demonstrated by means of temperature-energy asymmetry phase diagrams for low-temperature quantum diffusion. It was suggested that the regime of coherent tunnelling diffusion should be further divided into a regime of band diffusion for a localized particle (small polaron) and a regime of Bloch propagation. In the former case, the thermal De Broglie wavelength of the light particles was smaller than the distance between the interstitial sites. In the latter case, it was larger.

T.Regelmann, L.Schimmele: Journal of Alloys and Compounds, 1995, 231, 208-13