Abstract
Calorimetric measurements on 3He-4He monolayers by Hickernell, McLean, and Vilches indicate that isotopic ordering takes place in both the low-density 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 4He fraction 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 mK.
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References
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Research supported by The National Science Foundation.
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Dash, J.G., Peierls, R.E. & Schick, M. Mixing entropy of 3He-4He monolayers adsorbed on graphite. J Low Temp Phys 23, 491–494 (1976). https://doi.org/10.1007/BF00116937
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DOI: https://doi.org/10.1007/BF00116937