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Extreme conditions and synchrotron infrared radiation : towards the evidence of metallic hydrogen - Paul Dumas

Synchrotron SOLEIL - Gif Sur Yvette

LPS bât 510, grand amphi

The search of metal hydrogen has a unique place in high pressure physics. Due to increase in electron kinetic energy because of quantum confinement, pressure should turn any insulator into a metal, as observed for molecular oxygen around 100 GPa some 20 years ago. At first, the insulator-metal transition in hydrogen was predicted intertwined with the molecular dissociation. However, it was later suggested that metal hydrogen may exist as a proton paired metal.
However, metal hydrogen is a hydride with remarkable properties. It may exhibit a room temperature superconductivity, a melting transition at very low temperature into an unusual superconducting-superfluid state, a high protonic diffusion and a high energy density storage. Also, metal hydrogen in a dense fluid state is central to planetary interiors. Investigating the quantum many body effects in solid metal hydrogen at low temperature could force physicists to rethink their model of hydrogen at deep planetary conditions where quantum effects prevail even at high temperature due to extreme density.
The change of the direct bandgap of solid hydrogen up to 300 GPa was measured previously by visible absorption mesurements1). By extrapolating to zero the linear decrease of the bandgap with pressure, the transition to metal hydrogen could be predicted to occur around 450 GPa.
Several works have suggested the observation of the metallic state of hydrogen, but each of them has received criticisms and skepticisms. I will quickly recall the most recent ones.
We have chosen a non-intrusive technique ( synchrotron IR spectroscopy) to disclose structural changes and to characterize the electronic properties of hydrogen up to its metal transition... Our approach is based on two experimental developments. First, in order to overcome the 400 GPa limit of conventional DAC, we used the novel T-DAC that enables achieving pressure up to at least 600 GPa. Importantly, it preserves the standard DAC assets in terms of stress distribution, optical access and sample size. Synthetic IIas diamonds were used to provide IR transparency down to 800 cm-1. Second, an infrared horizontal microscope was designed which is coupled to a collimated exit port of a synchrotron-feed FTIR spectrometer at the SMIS beamline at the SOLEIL synchrotron. Such a high brightness broadband IR source is essential for measuring, by transmission through the sample, satisfactory signal to noise ratio spectra on a 5 µm diameter sample. Simultaneous Raman spectroscopy and visual observation could be performed.
In this seminar I will show the recent results we obtained that demonstrate that most probably hydrogen becomes metallic in its molecular structure2) 3
Perspectives and future experiments will be presented regarding the work planned in a near future to reveal its superconductivity properties.

1.Loubeyre, P., Occelli, F. and LeToullec, R. Optical studies of solid hydrogen to 320 GPa and evidence for black hydrogen. Nature 416, 613 – 617 (2002).
2.Loubeyre P., Occelli,F. and Dumas,P. Observation of a first order phase transition to metal hydrogen near 425 GPa arXiv :1906.05634 ( 2019)
3.Loubeyre, P. , Occelli, F. and Dumas, P. Probable transition to metal hydrogen evidenced by synchrotron infrared spectroscopy. Nature ( under revision)


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