Vacancies in solid helium

Some theoretical and experimental properties of thermally excited Schottky type vacancies in solid helium are reviewed: the mechanism for creation of a vacancy and the possibility for the latter to tunnel from one lattice site to a neighbouring one are considered. This tunnelling is compared with Pauli exchange interactions in solid 3He. The possibility of vacancy waves is studied and the difference between the cases of 3He and 4He pointed out. X ray scattering in body centered cubic helium three yields the energy of formation of vacancies without ambiguity in principle, although with a so far limited accuracy. Vacancy induced diffusion of magnetization in body centered cubic 3He is studied by the method of moments and existing experimental data are recalled. Spin lattice relaxation is recalled with some detail, both the Zeeman-vacancies relaxation due to the modulation of the dipole-dipole interactions by the motion of the vacancies and the exchange-lattice relaxation through the vacancies. The Zeeman-vacancies relaxation is studied by the method of moments, in parallel with the theory of diffusion. The exchange-lattice relaxation is calculated by the method of strong collisions of Ailion and Slichter. The various published experiments are reviewed. In the case of body centered cubic helium three, it can be concluded to the tunnelling of vacancies by comparison of NMR activation energies with the energies of formation measured by X ray scattering. The case of the 3He impurities in hexagonal close packed 4He is also studied as it also reveals the existence of vacancies in 4He. Finally the possibility of inelastic scattering of either neutrons or light by vacancies is noted. It is not possible to detect directly by such an experiment the vacancy waves