Experiments studying the properties of dense liquid hydrogen and deuterium in static diamond-anvil cells have achieved one of the most sought-after goals of high pressure science: metallic hdyrogen.
Few materials have ever captured the interest of condensed matter, plasma and astro-physics research more than metallic hydrogen [1]. For as long as quantum mechanics was known, the first element is expected to undergo a phase transition to the metallic state under sufficient compression [2,3]. Almost 90 years passed since these original predictions and yet the existence of that phase transition, let alone its nature and properties, both in the solid and fluid state, remain a mystery. The solid metallic hydrogen state is expected to be a room-temperature superconductor [4]. Whereas, the fluid metallic state constitutes the most the abundant form of condensed matter in our Solar planetary structure. It is also predicted to reveal new ordered quantum states of matter at low temperatures such as metallic superfluidity or superfluid-superconductivity [5,6]. Should any of theses phases prove recoverable to ambient conditions, they would constitute the ultimate hydrogen storage material, a pellet fuel for fusion and the most powerful rocket propellant known to man [7].
In 2016, we have succeeded in realizing the liquid metallic state, the Universe most common metal, in bench-top experiments using static compression and laser-heating.The experiments were time-resolved and the duration of the heating pulse was ~300 ns, which is sufficient to achieve local thermal equilibrium yet short enough to inhibit diffusion. The signature of metallization was the saturation of observed reflectance around values of 50-55%, typical of metallic liquids, and consistent with all previous shockwave experiments. The saturation of high-reflectance as a function of increasing temperatures evinces a state of high-degeneracy and the Drude behavior provides another hallmark of the metallic character. For more information, please see Zaghoo et al., Phys. Rev. B. 93, 155128
In 2017, we have conducted the first experimental determination of the optical conductivity of bulk Liquid metallic state using spectrally resolved reflectance measurements. The main result reported is that the mechanism of metallization is largely dissociative to atomic hydrogen, rather than the previously held experimental model, ionization of molecules. Furthermore, we find that LMH’s electrical conductivity is substantially higher, a factor of 6-8, than the only experimentally reported value in the literature, measured in the DC limit.
These newly reported transport values of LMH yield insights for the fundamental physics of dense hydrogen, as well as the planetary physics of gas giants’ magnetic and thermal history models. LMH’s electrical conductivity is shown to be considerably higher than Mott’s minimum metallic conductivity criterion and increases with density, in contrast to inference by previous shock-wave experiments and standard Jovian interior models. The changes in the transport coefficients are large enough that Jovian planets magnetic dynamo action, zonal flow and perhaps thermal histories models, need to be critically reassessed. Zaghoo & Silvera, PNAS, 2017
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