Fluidity in domain walls in dilute He3−He4 films on graphite: Possible one-dimensional Fermi fluid and Dirac fermions in a helium film

Abstract

The heat capacity of a small number of $^{3}\mathrm{He}$ atoms dissolved in a submonolayer $^{4}\mathrm{He}$ film has been measured. The measured heat capacity is finite and suggests that $^{3}\mathrm{He}$ atoms are mobile in an areal density regime higher than that of the $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ phase, where $^{4}\mathrm{He}$ films are believed to be solid. At higher areal densities, the measured heat capacity is proportional to ${T}^{2}$ and depends on the number of $^{3}\mathrm{He}$ atoms. These behaviors are anomalous to that of a two-dimensional Fermi fluid and cannot be explained by uniform melting of $^{4}\mathrm{He}$ films. One possible explanation for these anomalous behaviors is that helium atoms exhibit fluidity only inside the domain walls of the adsorption structure, and the dissolved $^{3}\mathrm{He}$ atoms gather in them and behave as a one-dimensional Fermi fluid or as Dirac fermions, depending on the structure of the domain walls. The behaviors of the measured heat capacity strongly suggest this possibility.