Ordering Transitions in Helium Monolayers

Abstract

${\mathrm{He}}^3$ and ${\mathrm{He}}^4$ monolayers on graphite at coverages $0.45<~x<~0.61$ show strong heat-capacity peaks near 3°K. Temperature dependences and densities indicate that both isotopes undergo second-order transitions from the two-dimensional gas state to a regular lattice in registry with the substrate structure.