It is generally assumed1,2,3 that solid hydrogen will transform into a metallic alkali-like crystal at sufficiently high pressure. However, some theoretical models4,5 have also suggested that compressed hydrogen may form an unusual two-component (protons and electrons) metallic fluid at low temperature, or possibly even a zero-temperature liquid ground state. The existence of these new states of matter is conditional on the presence of a maximum in the melting temperature versus pressure curve (the ‘melt line’). Previous measurements6,7,8 of the hydrogen melt line up to pressures of 44 GPa have led to controversial conclusions regarding the existence of this maximum. Here we report ab initio calculations that establish the melt line up to 200 GPa. We predict that subtle changes in the intermolecular interactions lead to a decline of the melt line above 90 GPa. The implication is that as solid molecular hydrogen is compressed, it transforms into a low-temperature quantum fluid before becoming a monatomic crystal. The emerging low-temperature phase diagram of hydrogen and its isotopes bears analogies with the familiar phases of 3He and 4He (the only known zero-temperature liquids), but the long-range Coulomb interactions and the large component mass ratio present in hydrogen would result in dramatically different properties9.
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We thank S. Izvekov for providing the force-matching routines that were used in the analysis shown in the inset of Fig. 4, and F. Gygi for many discussions. This work was performed under the auspices of the US Department of Energy at the University of California/Lawrence Livermore National Laboratory.
The authors declare that they have no competing financial interests.
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The Astrophysical Journal (2019)
Proceedings of the National Academy of Sciences (2019)
Journal of Applied Physics (2019)
The Journal of Physical Chemistry Letters (2019)