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Metallic hydrogen is a kind of degenerate matter, a phase of hydrogen in which it behaves as an electrical conductor. This phase was predicted theoretically in 1935. It has yet to be unambiguously observed, but some observations consistent with metallic behaviour have been reported such as the possible observation of some new phases of solid hydrogen under static conditions, and, in dense liquid deuterium, electrical insulator-to-conductor transitions associated with an increase in optical reflectivity. At high pressure, on the order of hundreds of gigapascals, hydrogen might exist as a liquid rather than a gas. Liquid metallic hydrogen is thought to be present in large amounts in the gravitationally compressed interiors of Jupiter, Saturn, and in some extrasolar planets.
- 1 Theoretical predictions
- 2 Experimental pursuit
- 3 See also
- 4 References
Metallization of hydrogen under pressure
Though at the top of the alkali metal column in the periodic table, hydrogen is not, under ordinary conditions, an alkali metal. In 1935 physicists Eugene Wigner and Hillard Bell Huntington predicted that under an immense pressure of around GPa ( 25 000 atm or 250500000 psi), 3hydrogen atoms would display metallic properties, losing hold over their electrons. Since then, metallic hydrogen has been described as "the holy grail of high-pressure physics".
The initial prediction about the amount of pressure needed was eventually proven to be too low. Since the first work by Wigner and Huntington, the more modern theoretical calculations were pointing toward higher but nonetheless potentially accessible metallization pressures. Techniques are being developed for creating pressures of up to , higher than the pressure at the 500 GPacenter of the Earth, in hopes of creating metallic hydrogen.
Liquid metallic hydrogen
Helium-4 is a liquid at 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, 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 may be a range of densities (at pressures around ) where hydrogen may be a liquid metal, even at low temperatures. 400 GPa
In 1968, Neil Ashcroft put forward that metallic hydrogen may be a superconductor, up to room temperature (~), far higher than any other known candidate material. This stems from its expected strong 290 Kcoupling between the conduction electrons and the lattice vibrations.
Possibility of novel types of quantum fluid
Presently known "super" states of matter are superconductors, superfluid liquids and gases, and supersolids. Egor Babaev predicted that if hydrogen and deuterium have liquid metallic states, they may have quantum ordered states which cannot be classified as superconducting or superfluid in the usual sense. Instead, they may represent two possible novel types of quantum fluids: "superconducting superfluids" and "metallic superfluids". Such fluids were predicted to have highly unusual reactions to external magnetic fields and rotations, which might provide a means for experimental verification of Babaev's predictions. It has also been suggested that, under the influence of magnetic field, hydrogen may exhibit phase transitions from superconductivity to superfluidity and vice versa.
Lithium doping reduces requisite pressure
In 2009, Zurek et al. predicted that the alloy LiH6 would be a stable metal at only 1⁄4 of the pressure required to metallize hydrogen, and that similar effects should hold for alloys of type LiHn and possibly other alloys of type ?Lin.
Shock-wave compression, 1996
In March 1996, a group of scientists at Lawrence Livermore National Laboratory reported that they had serendipitously produced, for about a microsecond at temperatures of thousands of kelvins, pressures of over a million atmospheres (>100 GPa) and density of approximately 0.6 g/cm3, the first identifiably metallic hydrogen. The team did not expect to produce metallic hydrogen, as it was not using solid hydrogen, thought to be necessary, and was working at temperatures above those specified by metallization theory. Previous studies in which solid hydrogen was compressed inside diamond anvils to pressures of up to 2,500,000 atm (250 GPa), did not confirm detectable metallization. The team had sought simply to measure the less extreme electrical conductivity changes which were expected to occur. The researchers used a 1960s-era light-gas gun, originally employed in guided missile studies, to shoot an impactor plate into a sealed container containing a half-millimeter thick sample of liquid hydrogen. The liquid hydrogen was in contact with wires leading to a device measuring electrical resistance. The scientists found that, as pressure rose to 1,400,000 atm (140 GPa), the electronic energy band gap, a measure of electrical resistance, fell to almost zero. The band-gap of hydrogen in its uncompressed state is about eV, making it an 15 insulator but, as the pressure increases significantly, the band-gap gradually fell to . Because the 0.3 eVthermal energy of the fluid (the temperature became about due to compression of the sample) was above 3000 K, the hydrogen might be considered metallic. 0.3 eV
Other experimental research, 1996 – 2004
Many experiments are continuing in the production of metallic hydrogen in laboratory conditions at static compression and low temperature. Arthur Ruoff and Chandrabhas Narayana from Cornell University in 1998, and later Paul Loubeyre and René LeToullec from Commissariat à l'Énergie Atomique, France in 2002, have shown that at pressures close to those at the center of the Earth (3.2 to 3.4 million atmospheres or 324 to 345 GPa) and temperatures of 100–300 K, hydrogen is still not a true alkali metal, because of the non-zero band gap. The quest to see metallic hydrogen in laboratory at low temperature and static compression continues. Studies are also ongoing on deuterium. Shahriar Badiei and Leif Holmlid from the University of Gothenburg have shown in 2004 that condensed metallic states made of excited hydrogen atoms (Rydberg matter) are effective promoters to metallic hydrogen.
Pulsed laser heating experiment, 2008
The theoretically predicted maximum of the melting curve (the prerequisite for the liquid metallic hydrogen) was discovered by Shanti Deemyad and Isaac F. Silvera by using pulsed laser heating. Hydrogen-rich molecular SiH4 was claimed to be metallized and become superconducting by M.I. Eremets et al.. This claim is disputed, and their results have not been repeated.
Observation of liquid metallic hydrogen, 2011
In 2011 Eremets and Troyan reported observing the liquid metallic state of hydrogen and deuterium at static pressures of 260–300 GPa. This claim was questioned by other researchers in 2012.
Z machine, 2015
Observation of the Wigner-Huntington Transition to Solid Metallic Hydrogen, 2016
On October 5, 2016, Ranga Dias and Isaac F. Silvera of Lyman Laboratory of Physics, Harvard University released the first experimental evidence that solid metallic hydrogen has been synthesised in the laboratory. This manuscript first appeared on the arXiv preprint server.
With increasing pressure we observe changes in the sample, going from transparent, to black, to a reflective metal, the latter studied at a pressure of 495 GPa... the reflectance using a Drude free electron model to determine the plasma frequency of 30.1 eV at T = 5.5 K, with a corresponding electron carrier density of 6.7 x 1023 particles/cm3, consistent with theoretical estimates. The properties are those of a metal. Solid metallic hydrogen has been produced in the laboratory.
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minimum electrical conductivity of a metal at 140 GPa, 0.6 g/cm3, and 3000 K
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0.28–0.36 mol/cm3 and 2200–4400 K
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no evidence for MH
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