Order parameter and a critical point on the megabar-pressure hydrogen-A phase line.

Abstract

We present experimental evidence that a critical point, also reported by Hemley and Mao, exists for the hydrogen-A phase line at a pressure of 173(4) GPa and 151(15) K. We give an interpretation of this critical point in terms of its relationship to a low-pressure disordered insulating phase and a possible metallic phase at high pressure. We interpret the temeprature dependence of the vibron frequency of the H-A phase as a measure of one of the two order parameters characterizing this phase.