Nonmonotonic temperature dependence of the mass transfer rate during isotopic phase separation of He-3-He-4 solid mixtures

It is discovered that the characteristic phase separation time constant of solid He-3-He-4 mixtures at low temperatures exhibits a nonmonotonic temper ature dependence with a minimum. It means that the rate of mass transport s lows down at very low temperatures, and the corresponding values of the eff ective diffusion coefficient also depend nonmonotonically on temperature an d concentration and differ significantly from the spin diffusion coefficien t measured earlier in NMR experiments on quantum diffusion. The discovered nonmonotonicity may be associated with the influence of the nonuniform fiel d of elastic stresses in the crystal because of the difference in the molar volumes of the phases. (C) 1999 American Institute of Physics. [S1063-777X (99)00404-1].