Third-sound measurements on ${}_{}{}^3{}_{}{}^{}\mathrm{He}$-${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mixture films at 0.4 K show a stratification which disappears when the ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ thickness, i.e., roughly the interface distance to the wall, is less than 4.5 atomic layers. We propose to take into account for the stratification transition the influence of the steric interaction between the film interface fluctuations, the so-called ripplons, and the substrate. This effect, considered for the first time, allows definition of a crossover length $d^{*}$ whose magnitude agrees with experimental determination.

• Received 1 June 1984

DOI:https://doi.org/10.1103/PhysRevLett.53.2421