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Physics of hydrogen and other light elements under extreme conditions. – HyLightExtreme

Submission summary

In 1935 Wigner and Huntington speculated that metallization in compressed hydrogen should occur at around 25GPa. Present experiments have reached about 350GPa without finding the metallic phase yet. It is now predicted to occur at 400GPa at low temperature. This mismatch is emblematic of the unexpected and rich physics observed in this pressure range. Several crystalline phases have been detected at low temperature but the molecular dissociation has not been observed. The melting line of the molecular crystal has a reentrant behavior with a maximum temperature of 800K around 100GPa and decreasing at higher pressure, suggesting the presence of a new quantum state of matter, a metallic superfluid or a superconducting superfluid, stabilized by nuclear quantum effects against crystallization.
Because of the experimental limitations in reaching higher pressure for hydrogen samples, a new line of thought in searching for hydrogen metallization is to study hydrates compounds comprising H2 molecules hosted in the lattice of heavier elements. Particularly suitable materials appear to be Silane (H4S) and hydrogen sulfide (H2S) which has been very recently found to be a conventional superconductor with a particularly high critical temperature (190K). Very interesting materials are lithium hydrates which are also predicted to favor hydrogen metallization and could be of interest for technological applications
Hydrogen is also the most abundant element in the universe, followed by helium, comprising many astronomical objects, e.g. Jupiter and Saturn in our own solar system and a large number of other objects discovered in the past decade. The basic ingredient of planetary models is the equation of state of hydrogen, helium and their mixtures in a wide range of pressure, temperature and concentrations. Of particular importance is to establish the de-mixing transition of the hydrogen-helium mixtures and its interplay with metallization both in hydrogen and helium. The occurrence and location of phase lines might explain experimental observations about the planets.
First Principles methods have been widely applied to hydrogen, light elements and hydrates under compression. However standard Density Functional Theory (DFT) methods have difficulties to provide accurate predictions of metallization. Conversely, ground state Quantum Monte Carlo methods have proven to provide reliable predictions even at metallization. An additional difficulty for light elements is the proper account of the nuclear quantum effects which are significant at high pressure. Recently we introduced the Coupled Electron-Ion Monte Carlo (CEIMC), based entirely on Quantum Monte Carlo methods, which overcomes both limitations and is particularly suitable for hydrogen and light elements (helium and lithium) under extreme conditions. So far it has been applied to predict a first order liquid-liquid transition in hydrogen and the principal Hugoniot line in deuterium.
Our project is to investigate by CEIMC, metallization in hydrogen and other light elements like helium, lithium and their hydrates compounds, and its interplay with melting and other phase lines. On the methodological side we intend to develop the CEIMC further in the direction of integrating this new method into an open access package which will allow an easier spread of the methodology, and will facilitate its use for systems with heavier elements, where a distinction between core and valence electrons, based on the use of pseudo-potentials, is necessary. For instance the quantitative modeling of water is still very challenging for conventional first-principle methods, while it will be possible to consider electronic correlation, dispersion interactions and quantum nuclei, the three relevant effects, all simultaneously and on the same footing by this new method.
Finally we will develop methods for dynamical properties, such as conductivity and response functions based on Quantum Monte Carlo.

Project coordinator

Monsieur Carlo Pierleoni (Maison de la Simulation)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

Maison de la Simulation

Help of the ANR 599,999 euros
Beginning and duration of the scientific project: September 2016 - 48 Months

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