Quantum diffusion in solid solutions of helium isotopes with strong localization of 3He atoms

Using NMR, we have measured the diffusion coefficient D of 3He atoms in solid 4He over a wide range of 3He concentration, densities, and temperatures. We have found that in the region of the low-temperature plateau the diffusion coefficient decreases sharply to D≈3 · 10−12 cm−2/sec. The result indicates a strong localization of 3He atoms in the hcp phase of 4He; this localization is lifted by phonon-stimulated quantum diffusion, in total agreement with the predictions of the Kagan theory. Our analysis of the contributions of different mechanisms to 3He diffusion eliminates the contradictions in the treatment of 3He vacancy diffusion. The diffusion of 3He has been investigated near the critical concentration and the parameters of the critical behavior of D have been found. For the first time we have observed diffusion which is essentially independent of the pressure and this is explained in terms of the dipole–dipole diffusion model.

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