Orientational order–disorder transitions in solid hydrogen

Abstract

We discuss the orientational ordering in solid ortho–para hydrogen mixtures. Following a brief sketch of the properties of the ortho and para hydrogen modifications and the long-range ordering of an assembly of J = 1 quantum rotors (ortho-H2), we consider the nature of the purely local ordering seen in random dilute ortho–para mixtures. Measurements of the quasi-static glass order parameter and the mean correlation time for the molecular alignments are interpreted in terms of a rapid but continuous freezing of the orientational fluctuations.