In situ monitoring distribution and migration of ^{3}He in liquid-solid ^{4}He 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 ^{3}He impurities concentrate in liquid phase of solid-liquid coexisting ^{4}He at low temperatures, and their migration between the liquid and solid phases when temperature is changed. This migration process is much slower than ^{3}He diffusion in the liquid phase, suggesting the bottleneck is in the solid phase and possibly transmission across the liquid-solid interface.