Editing Metallic hydrogen (section)

===Liquid metallic hydrogen===
[[Helium-4]] is a [[liquid helium|liquid]] at [[standard pressure|normal pressure]] near [[absolute zero]], a consequence of its high [[zero-point energy]] (ZPE). The ZPE of protons in a dense state is also high,<ref>{{Cite journal |last=Geng |first=Hua Y. |date=2022-11-17 |title=Full Temperature-Dependent Potential and Anharmonicity in Metallic Hydrogen: Colossal NQE and the Consequences |url=https://pubs.acs.org/doi/10.1021/acs.jpcc.2c05027 |journal=The Journal of Physical Chemistry C |language=en |volume=126 |issue=45 |pages=19355–19366 |doi=10.1021/acs.jpcc.2c05027 |issn=1932-7447|arxiv=2211.14474 }}</ref> and a decline in the ordering energy (relative to the ZPE) is expected at high pressures. Arguments have been advanced by [[Neil Ashcroft]] and others that there is a melting point maximum in [[compressed hydrogen]], but also that there might be a range of densities, at pressures around 400 GPa, where hydrogen would be a liquid metal, even at low temperatures.<ref>{{cite journal |last1=Ashcroft |first1=N. W. |year=2000 |title=The hydrogen liquids |journal=[[Journal of Physics: Condensed Matter]] |volume=12 |issue=8A |pages=A129–A137 |bibcode=2000JPCM...12..129A |doi=10.1088/0953-8984/12/8A/314|s2cid=250917368 }}</ref><ref>{{cite journal |last1=Bonev |first1=S. A. |display-authors=etal |year=2004 |title=A quantum fluid of metallic hydrogen suggested by first-principles calculations |journal=[[Nature (journal)|Nature]] |volume=431 |issue=7009 |pages=669–672 |arxiv=cond-mat/0410425 |bibcode=2004Natur.431..669B |doi=10.1038/nature02968 |pmid=15470423|s2cid=4352456 }}</ref>

Geng predicted that the ZPE of protons indeed lowers the melting temperature of hydrogen to a minimum of {{convert|200|to|250|K|C}} at pressures of {{convert|500|-|1500|GPa|atm psi|abbr=on}}.<ref>{{cite journal |last1=Geng |first1=H. Y. |display-authors=etal |year=2015 |title=Lattice stability and high-pressure melting mechanism of dense hydrogen up to 1.5 TPa |journal=[[Physical Review B]] |volume=92 |issue=10 |pages=104103 |doi=10.1103/PhysRevB.92.104103|arxiv=1607.00572 |bibcode=2015PhRvB..92j4103G |s2cid=118358601 }}</ref><ref>{{cite journal |last1=Geng |first1=H. Y. |display-authors=etal |year=2016 |title=Predicted reentrant melting of dense hydrogen at ultra-high pressures |journal=[[Scientific Reports]] |volume=6 |pages=36745 |doi=10.1038/srep36745|pmid=27834405 |pmc=5105149 |arxiv=1611.01418 |bibcode=2016NatSR...636745G }}</ref>

Within this flat region there might be an elemental [[mesophase]] intermediate between the liquid and solid state, which could be [[Metastability|metastably]] stabilized down to low temperature and enter a [[supersolid]] state.<ref>{{cite journal |last1=Geng |first1=H. Y. |display-authors=etal |year=2017 |title=Prediction of a mobile solid state in dense hydrogen under high pressures |journal=[[J. Phys. Chem. Lett.]] |volume=8 |issue=1 |pages=223–228 |doi=10.1021/acs.jpclett.6b02453|pmid=27973848 |arxiv=1702.00211 |s2cid=46843598 }}</ref>