Kinetic processes in solid helium with impurities and vacancies (Review Article)
V.A. Maidanov, E.Ya. Rudavskii, and S.S. Sokolov
B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine 47 Pr. Nauky, Kharkiv 61103, Ukraine
Received Jule 7, 2016
A brief review is presented of the kinetic behavior of impurities and vacancies in the solid helium, which according to the prediction of A.F. Andreev and I.M. Lifshitz  are delocalized being a kind of quasi-particles. The focus is on unusual diffusion processes in 3He–4He solid mixtures under their phase separation. The diffusive flow decreases due to the occurrence of mechanical stresses in the crystal at the separation and the effective diffusion coefficient becomes less than that of the coherent quantum diffusion. When the inverse transition from separated mixture to a homogeneous state do occur, abnormally rapid mass transfer is observed, that can be qualitatively explain-ed within the model of three-staged phase dissolution of 3He inclusions. A comparison of experimental data on the kinetics of phase separation with diffusion description of the process is carried out with taking into account differences in diffusion processes inside and outside the nucleus of new phase. A good agreement between the theoretical calculations and experimental results is demonstrated. Using the homogeneous nucleation model the concentration of nucleus is estimated. The first evaluation is made of the mass diffusion coefficient in entire range of solution concentrations. The behavior of delocalized vacancies is investigated in solid solutions of 4He in 3He near the separation temperature. The observed features of the pressure in this system after temperature cycling is explained by the formation of pure 4He vacancy clusters. Although the crystal has no strict periodicity due to the random distribution of 3He and 4He atoms in the lattice sites, a periodic structure is realized within the cluster, and the vacancy becomes delocalized.
PACS: 67.60.–g Mixtures of 3He and 4He; PACS: 67.80.–s Quantum solids.