Second sound in metastable and phase-separated ^{3}-^{4}He mixtures

Propagation of second sound has been used to study superfluid ${}_{}{}^3{}_{}{}^{}\mathrm{-}_{}^4{}_{}{}^{}$He mixtures quenched into nonequilibrium states inside the miscibility gap. From the results for the second-sound velocity we conclude that the superfluid density in the metastable state is well described by extrapolation from equilibrium values. The attenuation of the second-sound amplitude displays a sharp increase as nucleation sets in at the cloud point, and during the subsequent coarse-graining process it varies in proportion to the total surface area of the droplets.