Order/disorder Transitions of Ortho-Para Hydrogen Monolayers at Low Temperatures

Experimental studies of the low temperature behavior of mono-layers of ortho-H2 - para-H2 mixtures using low temperature NMR techniques reveal an unusual phase diagram. An extension of the mean field approach is proposed to explain the qualitative features of the observed phase diagram. The competition between the highly frustrated short-range quadrupolar fields and the local crystal fields plays a key role in determining the nature of the ordered structures.