Preliminary heat capacity and vapor pressure measurements of 2D ⁴He on ZYX graphite

We report preliminary heat capacity and vapor pressure measurements of the first and second layers of ⁴He adsorbed on ZYX graphite. ZYX is known to have much better crystallinity than Grafoil, the most commonly-used exfoliated graphite substrate, such as a ten-times larger platelet size. This allows us to distinguish different phases in 2D ⁴He much more clearly and may provide qualitatively different insights into this system. We found a significantly asymmetric density-dependence of the heat-capacity peak associated with the \sqrt{3}×\sqrt{3} phase formation comparing with that obtained with Grafoil. The 2nd-layer promotion density is determined as 11.8±0.3 nm^-2 from the heat-capacity measurement of low density samples in the 2nd layer and vapor pressure measurement.