ABSTRACT
Many developments in high-pressure research have been driven by questions surrounding the nature of the first element in the Periodic Table under extreme conditions. Discovered by Cavendish and reported in 1766 [1], hydrogen is the starting point for our understanding of much of physical science. As such, the element has been a crucial testing ground for theory in atomic, molecular, solid state, and plasma physics. The unique and simple electronic structure of the hydrogen atom gives rise to an elemental dichotomy as a halogen or an alkali metal (Figure 1). Pressure thus serves to illuminate the nature of that chemical duality, as pointed out by Wigner and Huntington in 1935 [2]. Moreover, deeper theoretical inquiry some 40 years ago into the behavior of hydrogen at very high pressures indicated that the element under extreme conditions could exhibit high-temperature superconductivity [3] and a liquid ground state [4]. These propositions in turn led to the prediction of additional novel behavior, including dissipationless properties as a combined superfluid and superconductor: indeed, an altogether new state of matter [5]. Highly compressed hydrogen is also of interest because it is a high energy density material and central to a potential hydrogen-based fuel economy [6]. Finally, hydrogen has long been recognized as the major component of large planets [7]. Approximately 90% of the atoms in the solar system are hydrogen and most of those atoms experience conditions of ultrahigh pressures and temperatures. Indeed, hydrogen is the most abundant element in the visible cosmos, accounting for about 75% of its observable mass.
