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

We report on the first observation of He-3 diffusion anisotropy in He-3-He-4 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 He-3 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 He-3 collisions with strands play significant role.

Automated summary

This study reports the first observation of He-3 diffusion anisotropy in He-3-He-4 liquid mixture confined in ordered aerogels at temperatures between 1.5-4.2 K. Various possible mechanisms for the diffusion anisotropy are considered, including Knudsen diffusion, potential anisotropy of layer mode excitations, and helium vortices. The reduced He-3 diffusion in aerogels is suggested to be due to helium excitations at strong confinement conditions, paving the way for future experiments in the crossover regime at lower temperatures.