Heat capacity investigation of two-dimensional isotopic fluid helium mixtures

摘要

The properties of a helium fluid mixture, adsorbed as the second atomic layer on the surface of graphite plated by a monolayer of He-4, have been investigated in the temperature range 2-80 mK. Using a small tracer of He-3, we find support for the theoretically predicted phase diagram for the second layer of He-4; a self-bound liquid, followed by a significant coverage range over which the fluid uniformly covers the surface, before solidification. Adding He-3 up to a coverage of 1.0 nm(-2) to the uniform He-4 film, we initially observe an anomalous temperature dependence of the heat capacity, which we attribute to virtually complete phase separation within the layer, giving rise to the formation of two-dimensional He-3 fluid nanoclusters. We also find evidence for a small solubility of He-3 in the He-3 liquid, with a density dependence similar to that predicted. (C) 2003 Elsevier Science B.V. All rights reserved. [References: 6]