He 4 nanoclusters around vacancies in solid solutions of He4 in He3

Abstract

Precise measurements of the pressure in solid solutions of 4He in 3He are performed with temperature cycling in the phase-separation range. It is found that as a result of such cycling the difference ΔP of the pressures between the minimum (≈100 mK) and maximum (≈200 mK) temperatures decreases by a factor of approximately 3 and then remains unchanged for a long time. The initial values of ΔP and the pressure P0 in a uniform solid solution 4He-3He are restored only after the sample is heated to a temperature substantially above the phase-separation point. The data obtained are explained on the basis of the hypothesis that under these conditions clusters of pure 4He form around quasiequilibrium vacancies. A thermodynamic calculation of such clusters is performed. The computational results made it possible to give a quantitative explanation of the distinctive features observed. Comparing the calculations with experiment made it possible to determine the cluster concentration, 8×10−5, and the cluster radius, which is 3–4 interatomic distances at low temperatures. The characteristic equilibration time under these conditions is determined, and it is suggested that processes occurring at cluster boundaries determine its value.