Experimental Observation of the Increase of the Two-Dimensional Critical Temperature in Multilayer Adsorption

Measurements of multilayer adsorption of solid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ on graphite have been made for $0.965 \mathrm{K}<~T<~1.24 \mathrm{K}$ at intervals of ∼ 15 mK using fourth sound in He II as a probe. The first observation of the increase of the two-dimensional critical temperature $T_{\mathrm{cj}}$ with increasing layer number $j$ is reported. We extrapolate our values of $T_{\mathrm{cj}}$ for $j\to \infty$ and obtain the bulk-roughening transition temperature, confirming a prediction of current theories. The modeling of the acoustics including effects of the solid-liquid interface is also presented.