Low-temperature heat-capacity measurements of 4He films adsorbed on a disordered substrate

The heat capacity of submonolayer, monolayer, and multilayer 4He films adsorbed on a disordered substrate (porous Vycor glass) has been measured in the temperature range 0.08--1.3 K. The distinct regions as a function of coverage were observed: (i) coverages less than 28.6 μmole m-2 (approximately 1.5 layers) and (ii) coverages greater than 28.6 μmole m-2. 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 4He fraction lies on top of this initial 2-D solid below a transition temperature T0. Measurements of the heat capacity of 4He films adsorbed on N2-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