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Added by
Alexander Goncharov
Publication date
2017
fields
Physics
and research's language is
English
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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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In a recently published article [1], Ranga P. Dias & Isaac F. Silvera have reported the visual evidence of metallic hydrogen concomitantly with its characterization at a pressure of 495 GPa and low temperatures. We have expressed serious doubts of such a conclusion when interviewed to comment on this publication [2,3]. In the following comment, we would like to detail the reasons, based on experimental evidences obtained by us and by other groups worldwide that sustain our skepticism. We have identified two main flaws in this paper, as discussed in details below: the pressure is largely overestimated; the origin of the sample reflectivity and the analysis of the reflectance can be seriously questioned.
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.
We reported the first observation of metallic hydrogen (MH) in the low temperature limit at a pressure of ~495 GPa in an article published in Science (1). This transition was first predicted by Wigner and Huntington (WE) over 80 years ago (2) at a pressure of ~25 GPa. In recent decades it became clear that the required pressure for metallization was far greater, in the 400-500 GPa range. Until now the observation of the WE transition in diamond anvil cells (DACs) has been prevented by one problem: the diamonds break before a sufficiently high pressure has been achieved. This has driven the high-pressure community to improve DACs and experimental methods to understand and overcome the conditions that limited the performance of diamonds and the pressure. In our experiment, with increasing pressure, we observed a clear transition from a transparent sample of solid molecular hydrogen to an opaque black sample to a shiny reflective sample of MH, as determined by reflectance measurements. There is no doubt that MH was produced at the highest pressures. Yet there have been criticisms concerning the pressure that was achieved, the possibility that the 50 nm alumina layer, deposited on diamonds to inhibit diffusion of hydrogen, might be transformed to a metal and be responsible for the reflectance, and analysis of the reflectance. Here we respond to the criticisms posted on the condensed matter arXiv by Loubeyre, Occelli, and Dumas (LOD)- arXiv:1702.07192, Eremets and Drozdov (ED)- arXiv:1702.05125, and Goncharov and Struzhkin (GS)- arXiv:1702.04246.
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.
We comment on a recent paper published by McWilliams et al claiming that high-pressure/high-temperature hydrogen is a semi-conductor or semi-metal, in conflict with all earlier measurements on this system which show that it is metallic. We point out problems and inconsistencies and propose an alternate interpretation such that their results can be compatible with the observation of metallic hydrogen.
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