According to the hypothesis formulated more than 80 years ago by the physicists Wigner and Huntington, hydrogen could exist in a metallic state, i.e. a state of matter in which electrons can circulate freely. Since then, this hypothesis has aroused the interest of theorists and researchers alike, who address several issues: how can hydrogen be converted from the well-known gaseous state, into this metallic state? Which experimental tools can be used to achieve the extreme conditions for this predicted metallization? Because these conditions are extreme: a pressure greater than 4 million atmospheres (reminder: the pressure at sea level is 1 atmosphere, and at the centre of the Earth it is estimated to be around 3.5 million atmospheres). So far, the theoretical and experimental approaches to the identification of this metallic state have revealed the great complexity of this transition, with several intermediate states. Fascinating new properties have been also predicted for this metal, in particular superconductivity at room temperature, but also a very low temperature melting point and a transition into a superfluid liquid state.

Hydrogen, the most abundant element in the universe, consists of only one proton and one electron. So, it is neither rare nor - evidently - complex, and yet it continues to fascinate scientists.

In the course of experimental attempts, new tools have been proposed to push up the achievable pressure limit. However, up to now, the claims of an observation of metallic hydrogen have always been questioned by the scientific community and could never be reproduced.

The technology to obtain and master the very high pressure necessary to observe the metallic state has made recent steps forward and, for the first time, the existence of the transformation of gaseous hydrogen into metal has been unquestionably demonstrated. This major breakthrough was made possible by a French team composed of two researchers from the CEA (The French Alternative Energies and Atomic Energy Commission) and a CNRS researcher working at the French synchrotron radiation centre, SOLEIL. Their results are published in the scientific journal Nature.

The team of researchers on the SMIS beamline, by the experimental set up used to reveal the metallic state of hydrogen. From left to right: Florent Occelli, Paul Loubeyre and Paul Dumas.

"See" hydrogen-metal, thanks to synchrotron infrared radiation

Monitoring and detecting the metallic transformation of hydrogen was the tricky hurdle in this experiment. Considering the very small size of the sample (less than a cube of 5 micrometres size) and the enormous pressures exerted (the new designed cells containing hydrogen can withstand up to 6 million atmospheres), any in-situ probe at the sample level is perturbed and modified.

Only a very specific photon probe, the infrared radiation produced by the SOLEIL Synchrotron, is capable of providing rigorously the required information. Infrared light revealed the successive transformations of hydrogen during the pressure increase: from a gaseous state the hydrogen transforms into an insulating solid, then becomes a semiconductor, like silicon, and finally metallic.

These transformations are revealed by the absorption of an increasing fraction of infrared light by the hydrogen sample as it evolves towards the metallic state. The total absorption of infrared photons is a proof of the disappearance of the energy barrier that was keeping the electrons bound to the molecules from moving freely like in a metal. This total absorption, clearly identified at 4.2 million atmospheres, is the expected "fingerprint" of the metallic state, and it is the main outcome in this study.

Beyond the relevance of the experimental technique employed, the mandatory need for an infrared source that could be focused on a tiny volume was accomplished thanks to the properties of the synchrotron radiation at the SOLEIL Facility.

Why metallic hydrogen?

The ability to transform gaseous hydrogen into metal is a major fundamental discovery that could impact both our technology and our knowledge of the Universe. According to the theorist's calculations, it was predicted that metallic hydrogen might be metastable i.e. it would retain its properties as a metal even without being subjected to the extreme conditions necessary to reach this state (pressure greater than 4 million atmospheres). Because metallic hydrogen might be a room temperature superconductor, this would make it possible to obtain a compound with remarkable properties, a discovery that solid-state physicists’ dream of and whose applications could be multiple! The possibility of stabilizing this metallic hydrogen in hydrides is currently under very active studies.

On the technology side, due to its very high density, we should also mention the importance of metallic hydrogen as a "compact" energy source. Applications could then be conceivable to power electrical devices on Earth, or to produce a fuel ten times more concentrated than those currently used in the conquest of space: enough to shorten the trip to Mars by a few months, for example…

On the other hand, some planets such as Jupiter and Saturn, and many extrasolar planets, are thought to contain large amounts of metallic hydrogen. Developing accurate models of these giant planets to better understand their composition and properties requires a more precise description of the transition between gaseous hydrogen and metallic hydrogen. The demonstration of the existence of this metallic phase and knowledge of its properties should help to model the internal structure of the giant planets.

This first demonstration of the metallic transformation of hydrogen is a trigger and a springboard towards revealing and exploiting all the extraordinary properties of this metal. The quality of the samples makes it possible to envision a whole range of experiments, and for many of them, synchrotron radiation, from infrared to X-rays, will be a crucial tool.

A pooling of skills

Three experts shared their know-how to make this achievement possible. Paul Loubeyre and Florent Occelli, from the CEA, brought their extensive knowledge in the manufacture and optimization of high pressure cells to reach the necessary extreme conditions and are internationally renowned for their research associated with these instruments. Paul Dumas, CNRS Emeritus Research Director of the institute of Chemistry, working at SOLEIL Synchrotron, is acknowledged within the international scientific community for his expertise in the use of synchrotron radiation in the infrared range, radiation which made it possible to demonstrate the metallization of hydrogen.