Phase diagram for ortho-para-hydrogen monolayers

Abstract

The phase diagram for orientational ordering of hydrogen monolayers on graphite and boron nitride is revised in view of current theory and experimental observations from nuclear magnetic resonance (NMR) studies recently reported for ortho-H2 concentrations 0.35 ≤ c ≤ 0.92 and temperatures 0.14 ≤ T ≤ 1.80 K. The characteristic interaction coupling Γ0 = 0.50 ± 0.03 K and the crystalline field amplitude V0 = 0.70 ± 0.10 K are derived from experimental data, and distinct types of the local orientationally ordered structures are analysed using a proposed model for site-diluted uniaxial quadrupoles on a triangular plane lattice of hexagonal symmetry. The long-range periodic pinwheel structure and the short-range quadrupolar glass (QG) phase are stable above the 2D site-percolation limit, cp = 0.72, and for 0.48 < c < c p, respectively, where quadrupolar-order effects dominate. At very low T , the QG phase shows instability with respect to local dipole-like polarization effects and the ground state changes to a hindered rotor state. Two para-rotational local ordered PRA and PRB structures driven, respectively, by positive and negative crystalline fields are well distinguished, experimentally and theoretically, and are rather different from the unique PR phase suggested earlier by Harris and Berlinsky.