Mass Diffusivity ofHe3-He4Mixtures near the Superfluid Transition

We report on measurements of relaxation times for the decay of isothermal concentration gradients in ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixtures over the temperature range ${10}^{-5\mathrm{}}\lesssim \left|\epsilon \mathrm{}\right|=|\mathrm{}\frac{T}{T_{\lambda }}-1|\lesssim {10}^{-2\mathrm{}}$. The data yield a mass diffusivity $D$ which diverges when $T_{\lambda }$ is approached from higher temperatures. When fitted with $D\propto {\epsilon }^{-z}$, they give $z=0.40, 0.36, \mathrm{and} 0.33$ for ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ concentrations 0.100, 0.208, and 0.399, respectively. The estimated uncertainty for $z$ is ±0.06. These results are consistent with theoretical predictions, but differ from experimental results by others.