In situ monitoring distribution and migration of 3He in liquid-solid 4He mixtures

Helium is ideal for studies of quantum liquids and solids, with its significant advantage of being extremely pure at low temperatures-all except isotopic impurities are frozen out. However, even tiny concentrations of isotopic impurities have significant effects on both dynamic and thermodynamic properties. Difficulties in predicting and measuring impurities' distributions and motions make it hard to systematically study such effects. Here we report a sensitive technique combining dielectric and pressure measurements to resolve concentration changes of isotopic impurities in liquid helium with a resolution of 40 ppm. Using this technique, we observed that He-3 impurities concentrate in liquid phase of solid-liquid coexisting He-4 at low temperatures, and their migration between the liquid and solid phases when temperature is changed. This migration process is much slower than He-3 diffusion in the liquid phase, suggesting the bottleneck is in the solid phase and possibly transmission across the liquid-solid interface.

Automated summary

Helium is an ideal substance for studying quantum liquids and solids due to its purity at low temperatures, but even small concentrations of isotopic impurities can have significant effects. A new sensitive technique combining dielectric and pressure measurements has been developed to resolve concentration changes of isotopic impurities in liquid helium. The migration behavior of He-3 impurities in a coexisting He-4 phase at low temperatures has been observed, highlighting a slower process compared to diffusion in the liquid phase.