Neutron Scattering Studies of Quantum Films

Abstract

The phase diagrams of a monolayer of adsorbed gases or light molecules on flat substrates look similar to the phase diagrams in 3-dimensions (3-D). This means that the usual coexistence regions, the triple point and the critical point are present as well in 2-dimensions (2-D) as in 3-D. The phase diagrams can be studied by adsorption isotherm or heat capacity measurements, which reveal mainly the coexistence regions or the phase boundaries, respectively. However, the substrate can not always be regarded to be ideally flat. In many cases the adsorbate does see the adsorption sites of the substrate and locks into a commensurate phase (C-phase). In the case of graphite as substrate the R 30° overstructure is seen in many cases. This structure is shown in fig.1. The heavier rare gases and the light molecules (Nr.CD4) exhibit the C-phase only if the lattice parameter of the dense plane in 3-D is close to the nearest neighbor distance in the C-phase. This is the case because the nearest neighbor distance nearly does not change in the densest plane if the adjacent planes are taken off which means the dimensionality changes from 3-D to 2-D.