Phase diagram for ortho-para-hydrogen monolayers

Abstract

The phase diagram for orientational ordering of hydrogenmonolayers on graphite and boron nitride is revised in view ofcurrent theory and experimental observations from nuclearmagnetic resonance (NMR) studiesrecently reported for ortho-H2 concentrations0.35 ⩽ 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 amplitudeV0 = 0.70 ± 0.10 K are derived from experimental data, anddistinct types of the local orientationally ordered structuresare analysed using a proposed model for site-diluted uniaxialquadrupoles on a triangular plane lattice of hexagonal symmetry.The long-range periodic pinwheel structure and theshort-range quadrupolar glass (QG) phase are stable above the 2Dsite-percolation limit, cp = 0.72, and for 0.48 < c < cp,respectively, where quadrupolar-order effects dominate. At verylow T, the QG phase shows instability with respect tolocal dipole-like polarization effects and the ground statechanges to a hindered rotor state. Two para-rotationallocal ordered PRA and PRB structures driven, respectively, bypositive and negative crystalline fields are well distinguished,experimentally and theoretically, and are rather different from theunique PR phase suggested earlier by Harris and Berlinsky.