Effects of Lateral Substrate Fields on Helium Monolayers

Heat capacities of He monolayers on graphite show qualitative differences between ${\mathrm{He}}^3$ and ${\mathrm{He}}^4$, contrary to the usual theories of two-dimensional quantum gases. We attribute the differences to weak lateral fields due to substrate inhomogeneities, and using a two-dimensional ideal-gas model show qualitative agreement between theoretical and observed heat capacities.