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

Abstract

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 \ensuremath{\sim} 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 $j\ensuremath{\rightarrow}\ensuremath{\infty}$ 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.