Possible Phase Transition at Low mK Temperatures in Liquid Helium Mixture Films Adsorbed on Graphite

Abstract

We report heat capacity and magnetisation measurements of 3He adsorbed on the surface of graphite plated with three atomic layers of 4He. For 3He coverages n3>4 nm−2 the heat capacity corresponds to a 2D Fermi fluid. The inferred hydrodynamic mass of the 3He quasiparticles is 1.38±0.05 m3. The 3He effective mass ratio increases with coverage to 2.4 at n3=4 nm−2, due to Fermi liquid interactions. The heat capacity isotherm exhibits a steplike increase centred on n3=4.5 nm−2, similar to that previously observed on four layers of 4He. This is associated with the population of the first excited Andreev surface bound state. However, in the present case, as n3 is increased through the step a pronounced anomalous feature develops in the temperature dependence of the heat capacity, around 10 mK. Below 5 mK the heat capacity is approximately linear in temperature. With n3=7 nm−2, we find that this behaviour is very sensitive to small changes in the 4He third layer coverage, around the completed third layer. Measurements of the 3He magnetisation,, by continuous wave NMR methods, find a significant increase with decreasing temperature below around 20 mK. Together the data suggest that a phase transition takes place in the film at low mK temperatures.