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

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.