Orientational ordering in monolayer films of molecular hydrogen on boron nitride

NMR studies of monolayer hydrogen films for both the expected root 3 x root 3 commensurate solid and the incommensurate solid (for full monolayer cove rage) on hexagonal boron nitride (BN) are reported for ortho-H-2 concentrat ions 0.30 less than or equal to X less than or equal to 0.74 and temperatur es 0.08<T<4.2.K. Phase diagrams for these systems are mapped out in terms o f X and T from clear structural changes of the NMR spectra, including the o bservation of a 2D orientational quadrupolar glass ordering for X < X-crit = 0.69, and long range orientational ordering for X > X-crit for commensura te films.