Two-dimensional quantum solid phases of adsorbed monolayers

Monolayer films of helium and other gases adsorbed on basal plane surfaces of graphite exhibit phenomena that indicate numerous phases and phase transitions. He3 and He4 at high coverages form two-dimensional quantum solids whose structures are incommensurate with the underlying graphite, contrasting sharply with the (3×3) epitaxial ordered lattice solids occurring at lower coverage. The 2D solid phases melt in a continuous manner, while the lattice solids show a logarithmic second-order heat capacity peak. No monolayer displays a first-order melting transition indicated by a very high and narrow anomaly. This sharp melting point may be related to partial registry between the structure of the monolayer and that of the substrate. In N2 there is an apparent coupling between melting of the 2D type and the disordering of the lattice solid, with promotion of atoms to the second layer. Experimental methods include calorimetry, nuclear magnetic resonance, vapor pressure, and neutron diffraction.