Low-Temperature Heat Capacity of $^{4}$He Films on Graphite

Abstract

Heat capacities of $^{4}$He films have been measured at rather low temperatures between 2 and 80 mK and at areal densities between 2 and 24 $\hbox {nm}^{-2}$. These areal densities correspond to a monolayer fluid and third-layer fluid. For monolayer films, the results do not contradict previous measurements carried out at high temperatures. On the other hand, at some areal densities, small and broad but definite bumps, whose origin has not yet been understood, have been observed around 15 mK. Between 13 and 24 $\hbox {nm}^{-2}$, the measured heat capacities above 40 mK are proportional to $T^{2}$ and hardly change with areal density. These behaviors suggest that the second atomic layer does not solidify before the third-layer promotion, at least not into a commensurate solid, such as the so-called 4/7 phase.