Low-temperature heat-capacity measurements ofHe4films adsorbed on a disordered substrate

The heat capacity of submonolayer, monolayer, and multilayer ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films adsorbed on a disordered substrate (porous Vycor glass) has been measured in the temperature range 0.08-1.3 K. Two distinct regions as a function of coverage were observed: (i) coverages less than 28.6 μmole ${\mathrm{m}}^{-2\mathrm{}}$ (approximately 1.5 layers) and (ii) coverages greater than 28.6 μmole ${\mathrm{m}}^{-2\mathrm{}}$. Arguments are presented to show that for the low-coverage region the adsorbed helium forms a two-dimensional (2-D) solid with a small number of thermally activated adatoms moving in free-particle-like states on top of this solid layer. Arguments are also presented to show that for the high-coverage region a superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ fraction lies on top of this initial 2-D solid below a transition temperature $T_0$. Measurements of the heat capacity of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ films adsorbed on ${\mathrm{N}}_2$-preplated Vycor are presented to show that a precoating serves only to change the thickness of the 2-D solid layer and does not affect the form of the heat-capacity curves.