Hopf Bifurcation to Convection near the Codimension-Two Point in aHe3-He4Mixture

From measurements of the convective heat transport in a normal ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixture over the range $-0.018\mathrm{}\lesssim \psi \lesssim 0.015$ of the separation ratio $\psi$, we found a time-periodic state for $\psi <{\psi }_{\mathrm{CT}}=-0.0044\mathrm{}$. The dimensionless onset frequency at ${\psi }_{\mathrm{CT}}$ was 1.42, much larger than the value predicted by linear-stability analysis for a spatially uniform, laterally infinite system. For ${\psi }_{\mathrm{CT}}>\psi >{\psi }_c\approx -0.010\mathrm{}$ the bifurcation to oscillations was forward while for $\psi \lesssim {\psi }_c$ it was backward. The results suggests that ${\psi }_c$ is an additional codimension-two point, rather than a Hopf tricritical point.