Second-Sound Velocity and Superfluid Density Near the Tricritical Point inHe3-He4Mixtures

Second-sound velocity measurements were made in He II near the tricritical point ($T_t,x_t$). The corresponding superfluid density along the coexistence curve can be described by $\frac{{\rho }_s}{\rho }=2.03\mathrm{}{\epsilon }_t ({\epsilon }_t\equiv 1-\frac{T}{T_t})$. This agrees with recent theories. At constant concentration $x$ and near the superfluid transition temperature $T_{\lambda }$, the results are consistent with $\frac{{\rho }_s}{\rho }\sim k_{\lambda }{{\epsilon }_{\lambda }}^{\frac{2}{3}} ({\epsilon }_{\lambda }\equiv 1-\frac{T}{T_{\lambda }})$ which is expected on the basis of universality arguments. The amplitude $k_{\lambda }$ becomes small for small $x_t-x$, but does not appear to vanish at $x_t$.