Detection and investigation of quantum diffusion in solid solutions of helium isotopes

The phenomenon of quantum diffusion predicted theoretically by Andreev and Lifshitz was detected and investigated experimentally. Measurements of the diffusion coefficient D of He3 impurities in solid He4 by the nuclear magnetic resonance technique established that in the closed-packed hexagonal phase at temperatures below 1°K, D is independent of temperature and its increase is inversely proportional to the He3 concentration. The relationships found indicate a decisive contribution of quantum effects to the diffusion process in weak He3-He4 solutions and correspond to the case when the mobility of the impurity excitations (impuritons) is controlled by their mutual collisions. A comparison of the experimental data with the theory developed permitted estimating the width of the energy band (ΔE) of the impuritons. A continuous transition from vacancion diffusion to impuriton diffusion was recorded and a strong effect of density on the impuriton band width was noted (a 1.5% increase of density leads to an almost twofold decrease of ΔE). Investigations of the diffusion of He3 in the bcc phase of solid helium showed that in solutions containing not more than 2% He3, D decreases exponentially with decrease of temperature in the entire region of existence of the bcc phase, which indicates a vacancion mechanism of diffusion. In more concentrated solutions for which the bcc phase exists down to the lowest investigated temperatures (0.4°K), a complex temperature dependence of the diffusion coefficient was noted. To explain the dependence found, a model is proposed which takes into account the superposition of three mechanisms of diffusion: vacancion and He3-He4 and He3-He3 exchange.

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