Liquefaction of second-layerHe4films on graphite

We present the heat capacity of partial second-layer ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films adsorbed on Grafoil below 2 K. The second-layer liquefaction temperature occurs near 0.75 K which is substantially less than the 1.3 K observed for monolayer liquefaction. This rescaling in $T$ is thought to result from a vertical spread in the wave functions and hence a reduced Van der Waals interaction between adsorbed helium atoms. A consistent picture emerges when the total heat-capacity signals are modified by compressible-monolayer and film-heterogeneity considerations. The resulting liquefaction phase diagram clearly indicates a critical point near 0.025/${\mathrm{\AA }}^2$ and some altered higher-coverage behavior.