New behavior for the density dependence of vacancy formation and mobility in bccHe3at high molar volumes

Recent careful measurements of the nuclear spin-lattice relaxation times are reported for bcc ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ at low temperatures and for densities close to the minimum of the melting curve. The results indicate an unexpected density dependence for the formation energy $\Phi$ and the mobility $\mu$ of delocalized vacancies in bcc ${}_{}{}^3{}_{}{}^{}\mathrm{He}$. For molar volumes $V_m$ close to the maximum value $V_{\mathrm{mc}}$ for the solid phase (near the melting curve minimum), we find that $\Phi \left(V_m\right)$ decreases continuously towards zero as $V_{\mathrm{mc}}$ is approached and that $\mu$ changes by seven orders of magnitude over the same range of density. Both $\Phi \left(V_m\right)$ and ${log}_{10}\mu \left(V_m\right)$ vary approximately as ${(V_{\mathrm{mc}}-V_m)}^{\frac{1}{2}}$. These results point to new effects as the volume $V_{\mathrm{mc}}$ is approached and can be understood qualitatively in terms of density wave fluctuations associated with delocalized vacancies at the highest molar volumes.