The status of the low-temperature phase diagram of hydrogen at the turn of the century

Abstract

Hydrogen is the simplest element in nature. This simplicity in the atomic state is often assumed to hold also for its condensed phases. Nevertheless, experiments carried out during the past 15 years of the XXth century have shown that this picture is not necessarily a faithful one. Several different low-temperature solid phases have been identified, in contrast with the simplicity idea. These exhibit outstanding features like pressure-independent phonon bands, large isotope effects, and strong infrared activity. In this paper I will give an overview of the current understanding of the low-temperature region of the phase diagram of hydrogen, as emerges from a fruitful cooperative action between diamond anvil cell experiments and first-principles theoretical calculations.