HEAT-CAPACITY AND NEUTRON-DIFFRACTION STUDIES OF HD MONOLAYERS PHYSISORBED ON GRAPHITE

The phase diagram of HD monolayers physisorbed on graphite is studied in detail by using heat-capacity and neutron-diffraction measurements. The following features are observed: At low coverages the system forms a commensurate (C) (square-root 3 x square-root 3)R30-degrees phase and at coverages near monolayer completion an incommensurate (IC) two-dimensional solid phase. The order-disorder transition of the C phase belongs to the three-state Potts universality class. The C-IC transition occurs via two intermediate phases, i.e., the alpha-phase and a novel phase, which is interpreted as an alpha-IC coexistence region. Though the molecule HD is a strongly incoherent neutron scatterer, for the first time elastic neutron-diffraction spectra of the various phases could be detected. From the analysis of the data it is concluded that the alpha-phase is a density-modulated phase characterized by a striped network of superheavy domain-walls in agreement with theoretical expectations of the C-IC transition. The results are compared to those previously obtained for H-2 and D2 showing the strong influence of quantum effects in these systems.