Specific heat of monolayer films of³He-⁴He mixtures adsorbed on Grafoil between 0.04 and 4.2 K

The specific heat of mixtures of ³He and ⁴He adsorbed on Grafoil has been measured at four partial monolayer coverages corresponding to areal densities of 0.0279, 0.0419, 0.0485, and 0.066 »⁻² for 0.04 < T < 4.2 K. The ³He concentration in the mixtures was x ₃ = 0, 0.050, 0.091, 0.201, 0.485, 0.650, 0.908, and 0.999. A feature in the specific heat not seen in measurements with the pure isotopes is found for the two intermediate coverages at about 0.4 K. The measurements are analyzed at the higher temperatures using a theoretical virial expansion for two-dimensional gaseous mixtures. The entropy of mixing calculated by numerical integration of the specific heat curves shows that at 4 K the two isotopes are mixed. While mixing effects develop at temperatures below 1 K, the evidence for phase separation as in bulk mixtures is inconclusive.