Nonperiodic time dependence at the onset of convection in a binary liquid mixture

We report measurements of the convective heat transport near the onset of convection in a normal ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixture for values of the separation ratio $\psi$ between -0.044 and -0.010. For $\psi \lesssim -0.018\mathrm{}$, the first bifurcation was to a relatively large-amplitude time-dependent state with characteristic frequencies over an order of magnitude smaller than predicted by linear-stability analysis. The onset of the time dependence was intermittent and not hysteretic. Above onset, the frequency spectrum of the time dependence either contained discrete frequencies or not, depending on $\psi$.