The properties of hydrogen at extreme pressures and temperatures are of great interest to condensed matter physics, astrophysics, and planetary science due to the element’s putative simplicity and abundance in the visible universe. Of particular interest is the desire to know pressure-temperature conditions over which hydrogen is converted from solid to liquid and from non-metallic to metallic states including such novel states as a possible superconducting superfluid.  Such knowledge may lead to new physics as well as a better understanding of the interior giant planets.  The new work led by the Geophysical Laboratory's Chang-Sheng Zha reports the first direct measurements of the melting curve for solid hydrogen up to 3 million atmospheres pressure using externally heated diamond anvil and spectroscopic techniques.  Rather than the theoretically predicted temperature-pressure slope, the unexpected discovery is that the melting temperature remained relatively constant above 1.4 million atmospheres.  Furthermore, the spectroscopic measurements suggest that there is a sequence of metallic phases before the predicted transition to the atomic metallic form. 

The new work thus bridges the gap in pressure-temperature regions explored so far in hydrogen by static and dynamic compression techniques, and thus provides important new constraints on theoretical calculations. The experimental techniques employed can be readily extended for a full characterization of the high pressure-temperature phase diagram of hydrogen,  including more complete determination of possible liquid-liquid transitions and the relationship to underlying solid phases. Their work was published in Physical Review Letters.