Order/disorder transitions of ortho-para hydrogen monolayers at low temperatures

Experimental studies of the low temperature behavior of mono-layers of ortho-H-2-para-H-2 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.