Orientational behavior of quantum rotors physisorbed on boron nitride

We report the results of NMR studies of the two-dimensional (2D) phase diagrams for molecular H-2 films (monolayers and submonolayers) adsorbed on BN for ortho-H-2 concentrations 0.3<X<0.745 and temperatures 0.05<T<4.2 K. A 2D quadrupolar glass state has been identified for submonolayer coverages for 0.48<X<0.69 and 0.2 less than or equal to T less than or equal to 0.6 K. For submonolayer films (75% of full monolayer), an orientationally ordered phase (expected pinwheel structure) was observed for X greater than or equal to 0.69 with a transition to a second distinct ordered phase (expected herringbone structure) for X less than or equal to 0.69.