6533b839fe1ef96bd12a6eac

RESEARCH PRODUCT

Heat-capacity and neutron-diffraction studies of HD monolayers physisorbed on graphite

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Abstract 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) (√3 × √3)R30° 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 α phase and a novel phase, which is interpreted as an α-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 α 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 D 2 showing the strong influence of quantum effects in these systems.