Thermodynamic and Neutron-Diffraction Studies of H2 and D2 Multilayers Physisorbed on Graphite
Due to the strong influence of the quantum zero-point energy the hydrogen isotopes are highly compressible. This leads to strongly compressed monolayers2,3 before further layer condensation occurs. This property makes these systems significant for the exploration of conditions of multilayer growth. It is generally believed,1,4–6 that the incompatibility between the adsorbate and bulk lattice structures gives rise to lateral strains between the overlayers, which may cause reentrant incomplete wetting phenomena7 in the weakly physisorbed quantum systems. Thus the behavior of the first few layers next to the substrate is of crucial importance for the character of the multilayer growth.
The phase diagrams and structures of hydrogen isotope multilayers adsorbed on graphite are unknown so far. Recently, detailed specific-heat studies8–12 mapped out the phase diagrams of the monolayers, and with neutron diffraction2,3,12–17 and LEED17,18 the structures of the observed phases were identified. At low coverages these systems exhibit a commensurate (√3×√3) R30° phase2,3 due to the strong influence of the substrate corrugation potential, and undergo a commensurate-incommensurate (C-IC) transition via a sequence of different domain-wall phases8–18 to an equilaterally spaced triangular compressed incommensurate phase at higher coverages. First exploratory neutron-diffraction measurements of D2 multilayers19 found common oblique bi- and trilayer structures. A recent study of the multilayer growth of H2 adsorbed on MgO20 discovered the occurrence of a conventional van der Waals phase diagram including triple and critical points in the second layer and a succession of layering transitions at higher coverages.
The weakly bound states of hydrogen multilayers on substrates may be close to the limiting case where a quantum system at low temperatures condenses as a liquid rather than as a solid.21 H2 is a Bose particle and is expected to undergo a Bose-Einstein condensation to a superfluid phase provided the solidification can be suppressed to temperatures below 6.6 K22 and probably even lower in a real system.23 For the second layer of H2 on MgO a triple line at 7.2 K was found,20 which is still too high for a superfluid transition. The question remains: Do the molecules in the second layer of H2 on graphite condense into localized states?
KeywordsTriple Line Coexistence Region Total Heat Capacity Triple Point Temperature Graphite Foam
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