Quantum and Classical Orientational Ordering in Solid Hydrogen

Abstract

We present a unified view of orientational ordering in phases I, II, and III of solid hydrogen. Phases II and III are orientationally ordered, but the ordering objects in phase II are angular momenta of rotating molecules, whereas in phase III the molecules order themselves. This concept provides a quantitative explanation for the vibron softening, libron and roton spectra, and the increase of the vibron effective charge in phase III, as well as a framework for understanding the topology of the phase diagram and ortho-para state at high pressure. The effective charge and the infrared and Raman vibron frequency shifts are all linear in the order parameter in phase III.