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

Area of Science:

• Condensed matter physics
• Quantum fluids
• Materials science

Background:

• Helium-3/Helium-4 mixtures exhibit complex behavior under confinement.
• Aerogels provide unique nanostructured environments for studying quantum phenomena.

Purpose of the Study:

• To investigate helium-3 diffusion anisotropy in liquid helium-3/helium-4 mixtures confined in ordered aerogels.
• To explore the influence of aerogel structure on quantum fluid excitations and diffusion.

Main Methods:

• Utilized ordered aerogels composed of parallel 8 nm Al2O3 strands.
• Conducted experiments at temperatures ranging from 1.5 K to 4.2 K.
• Analyzed helium-3 diffusion using techniques sensitive to anisotropic behavior.

Main Results:

• Observed the first evidence of diffusion anisotropy for helium-3 in helium-3/helium-4 mixtures within aerogels.
• Noted reduced helium-3 diffusion attributed to helium excitations under strong confinement.
• Discussed potential mechanisms including Knudsen diffusion, anisotropic excitations, and helium vortices.

Conclusions:

• The study reveals anisotropic diffusion of helium-3 in confined helium mixtures.
• Aerogel structure significantly impacts quantum fluid properties and diffusion.
• Future research at lower temperatures (<1 K) will explore the crossover regime and the role of helium-3/strand collisions.