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Register a new userA First-order Phase Transition to Metallic Hydrogen
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The insulator-metal transition in hydrogen is one of the most outstanding problems in condensed matter physics. The high-pressure metallic phase is now predicted to be liquid atomic from T=0 K to very high temperatures. We have conducted measurements of optical properties of hot dense hydrogen in the region of 1.1-1.7 Mbar and up to 2200 K. We observe a first-order phase transition accompanied by changes in transmittance and reflectance characteristic of a metal. The phase line of this transition has a negative slope in agreement with theories of the so-called plasma phase transition.
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Hydrogen has been the essential element in the development of atomic and molecular physics1). Moving to the properties of dense hydrogen has appeared a good deal more complex than originally thought by Wigner and Hungtinton in their seminal paper predicting metal hydrogen2): the electrons and the protons are strongly coupled to each other and ultimately must be treated equally3)4). The determination of how and when molecular solid hydrogen will transform into a metal is the stepping stone towards a full understanding of the quantum-many body properties of dense hydrogen. The quest for metal hydrogen has pushed major developments of modern experimental high pressure physics, yet the various claims of its observation over the past 30 years have remained controversial5)6)7). Here we show a first order phase transition near 425 GPa from insulator molecular solid hydrogen to metal hydrogen. Pressure in excess of 400 GPa could be achieved by using the recently developed Toroidal Diamond Anvil Cell (T-DAC)8). The structural and electronic properties of dense solid hydrogen at 80 K have been characterized by synchrotron infrared spectroscopy. The continuous vibron frequency shift and the electronic band gap closure down to 0.5 eV, both linearly evolving with pressure, point to the stability of the insulator C2/c-24 phase up to the metallic transition. Upon pressure release, the metallic state transforms back to the C2/c-24 phase with almost no hysteresis, hence suggesting that the metallization proceeds through a structural transformation within the molecular solid, presumably to the Cmca-12 structure. Our results are in good agreement with the scenario recently disclosed by an advanced calculation able to capture many-body electronic correlations9).
Although hydrogen is the simplest of atoms, it does not form the simplest of solids or liquids. Quantum effects in these phases are considerable (a consequence of the light proton mass) and they have a demonstrable and often puzzling influence on many physical properties, including spatial order. To date, the structure of dense hydrogen remains experimentally elusive. Recent studies of the melting curve of hydrogen indicate that at high (but experimentally accessible) pressures, compressed hydrogen will adopt a liquid state, even at low temperatures. In reaching this phase, hydrogen is also projected to pass through an insulator-to-metal transition. This raises the possibility of new state of matter: a near ground-state liquid metal, and its ordered states in the quantum domain. Ordered quantum fluids are traditionally categorized as superconductors or superfluids; these respective systems feature dissipationless electrical currents or mass flow. Here we report an analysis based on topological arguments of the projected phase of liquid metallic hydrogen, finding that it may represent a new type of ordered quantum fluid. Specifically, we show that liquid metallic hydrogen cannot be categorized exclusively as a superconductor or superfluid. We predict that, in the presence of a magnetic field, liquid metallic hydrogen will exhibit several phase transitions to ordered states, ranging from superconductors to superfluids.
We have studied solid hydrogen under pressure at low temperatures. 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. We have measured the reflectance as a function of wavelength in the visible spectrum finding values as high as 0.90 from the metallic hydrogen. We have fit 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 6.7x1023 particles/cm3, consistent with theoretical estimates. The properties are those of a metal. Solid metallic hydrogen has been produced in the laboratory.
The antiferromagnetic (AFM) to ferromagnetic (FM) first order phase transition of an epitaxial FeRh thin-film has been studied with x-ray magnetic circular dichroism using photoemission electron microscopy. The FM phase is magnetized in-plane due to shape anisotropy, but the magnetocrystalline anisotropy is negligible and there is no preferred in-plane magnetization direction. When heating through the AFM to FM phase transition the nucleation of the FM phase occurs at many independent nucleation sites with random domain orientation. The domains subsequently align to form the final FM domain structure. We observe no pinning of the FM domain structure.
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Alexander F. Goncharov
, Viktor V. Struzhkin (Geophysicaln Laboratory
, Carnegie Institution of Washington
2017
A recent paper of Dias and Silvera (DS) reports on production of metallic hydrogen in a diamond anvil cell at 495 GPa at 5.5 and 83 K. The results are implied to have a great impact on energy and rocketry. Here we argue that the presented (very scarce) results are contradictory with the presented experimental description making their claims unsupported experimentally. Moreover, the proposed implications are highly speculative making this paper very confusing for a broad audience. Elucidating the claims and the related implications is important for building a coherent picture that is currently emerging as the results of theoretical calculations at various levels and experimental investigations employing static and dynamic compression techniques. There is no doubt that hydrogen metallizes at high pressures. But this does not make all claims about reaching this state immediately valid. Scientific community would like to learn at what conditions hydrogen metallizes, what is the nature of the conducting state and its properties (e.g. superconductivity). Here we argue that the presented data do not provide any reliable information on this.
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