NEW PHASES FOR THE ORIENTATIONAL ORDERING OF QUANTUM ROTORS - H-2 ON HEXAGONAL BN

The phase diagram for the orientational ordering of 2D films of ortho - para H-2 mixtures has been explored for both commensurate and incomm ensurate monolayer coverages on hexagonal BN. NMR techniques were used to study ortho concentrations 0.30 < x < 0.74 and temperatures 0.05 < T < 4.2. Three distinctly different classes of line shapes were obser ved and we interpret these in terms of the different phases predicted for the orientational ordering in 20. At high concentrations a Pake do ublet line shape was observed which is characteristic of the pinwheel (PW) ordering predicted by Harris and Berlinsky. For intermediate conc entrations a distinctive helmet shaped NMR line is observed which is t he signature of a quadrupolar glass state. This line shape was seen on ly for commensurate coverages. At low concentrations (x < 0.45) a new very sharply defined Fake doublet shape is seen which corresponds to a very narrow distribution of order parameters. We interpret this latte r phase as a herringbone (HB) ordering.