Calorimetric measurements on 3 He-4 He monolayers by Hickernell, McLean, and Vilches indicate that isotopic ordering takes place in both the low-denszty and registered lattice gas regimes. The low-density film heat capacity corresponds to imperfect 2D gas mixtures above T "-1 K. At lower temperatures the 4Heft'action condenses to a 2D liquid, and the concurrent decrease in mixing entropy indicates concurrent isotopic separation. In the registered lattice gas mixtures the mixing entropy changes by a much smaller amount over the experimental region. It is argued that the observed changes are due to shortrange isotopic ordering tending towards segregation. From a detailed comparison with an Ising model, we estimate a transition temperature T c ~ 30 inK.
The recent experiments by Hickernell, McLean, and Vilches ~ on adsorbed monolayer films of 3He-4He mixtures reveal interesting differences between films at the critical density of the order-disorder transition to the registered phase and films of lower density. By comparing the specific heat of a mixture with that of the pure substances at the same coverage, these authors could determine the contribution due to mixing and, by integrating this contribution over the measured temperature range of 40 mK to 4 K, the entropy of mixing. For the lower-density monolayers, this procedure yields the classical value of the mixing entropy. In the registered films, however, only one quarter to one third of the classical mixing entropy is obtained. An additional noteworthy observation is the absence of any feature in the specific heat which might indicate that isotopic ordering is taking place. We believe that these observations can be understood in terms of current models and that the new results can help to further the interpretations in several respects.
The behavior of low-density mixtures can be understood from our knowledge of the behavior of the pure isotopes at low densities. At high