Anisotropic reduced diffusion in dilute liquid ³He-⁴He mixture in ordered aerogel

Abstract

© 2021 IOP Publishing Ltd. We report on the first observation of 3He diffusion anisotropy in 3He-4He liquid mixture confined in ordered aerogels at 1.5-4.2 K temperatures. The used aerogels are arrays of long Al2O3 parallel 8 nm strands. The possible origins of diffusion anisotropy are considered and the changes of roton properties introduced by parallel aerogel strands are discussed. Among the responsible mechanisms we account for Knudsen diffusion, potential anisotropy of layer mode excitations or of bulklike excitations, and helium vortices. The observed reduced 3He diffusion in aerogels is discussed and suggested to appear due to helium excitations at strong confinement conditions. These observations pave the way for future experiments to gain insight into the crossover regime expected at lower temperatures (below 1 K) for which roton density is lower and 3He collisions with strands play significant role.