Hydrogen has been an essential element in the development of atomic, molecular and condensed matter physics1. It is predicted that hydrogen should have a metal state2; however, understanding the properties of dense hydrogen has been more complex than originally thought, because under extreme conditions the electrons and protons are strongly coupled to each other and ultimately must both be treated as quantum particles3,4. Therefore, how and when molecular solid hydrogen may transform into a metal is an open question. Although the quest for metal hydrogen has pushed major developments in modern experimental high-pressure physics, the various claims of its observation remain unconfirmed5,6,7. Here a discontinuous change of the direct bandgap of hydrogen, from 0.6 electronvolts to below 0.1 electronvolts, is observed near 425 gigapascals. This result is most probably associated with the formation of the metallic state because the nucleus zero-point energy is larger than this lowest bandgap value. Pressures above 400 gigapascals are achieved with the recently developed toroidal diamond anvil cell8, and the structural changes and electronic properties of dense solid hydrogen at 80 kelvin are probed using synchrotron infrared absorption spectroscopy. The continuous downward shifts of the vibron wavenumber and the direct bandgap with increased pressure point to the stability of phase-III hydrogen up to 425 gigapascals. The present data suggest that metallization of hydrogen proceeds within the molecular solid, in good agreement with previous calculations that capture many-body electronic correlations9.
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The data that support the findings of this study are available from the corresponding author upon request.
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We thank O. Marie for focused ion beam machining the toroidal anvils and the gasket holes. We are grateful to the SOLEIL director general, J. Daillant, for giving us regular access to the infrared beamline over the past six years. The inputs of S. Lefrançois in designing and assembling the horizontal infrared microscope and of the optics group at SOLEIL in aligning the Schwarzschild objectives are appreciated. We thank F. Borondics and F. Capitani for their assistance at the SMIS beamline.
The authors declare no competing interests.
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Extended data figures and tables
a, Single-beam spectrum at 123 GPa, used as the reference spectrum for the absorption spectra. The three red stars indicate parasitic effects corresponding to, from right to left, absorption peaks of impurities around 2,800 cm−1, a broad absorption band from the diamond (1,900–2,300 cm−1), and a broad absorption around 1,200 cm−1 from the protected layers of the aluminium mirrors of the beamline. b, Single-beam spectra, after intensity normalization (peak-to-peak value of their respective interferogram before Fourier transform). IR, infrared.
a, b, These spectra show a zeroing at high wavenumbers, progressing towards low wavenumber values with pressure increase (a) and reversibly, to the opposite direction upon pressure release (b).
a, The symbols indicate: red, pressure increase; blue, pressure decrease; and black, previous work performed using standard diamond anvil cells. The filled dots indicate the pressure measured by the diamond pressure scale and the open symbols are the position according to the linear vibron shift as a function of pressure. Inset, infrared spectrum at 406 GPa, in arbitrary absorbance units, with the vibron peak indicated by a red star. The linear fit of the vibron wavenumber with pressure is given by: υvibron [cm−1] = 4,814 − 2.48P [GPa] and that of the phonon wavenumber by: υphonon [cm−1] = 1,163 + 2.69P [GPa]. b, FWHM of the vibron peak versus its wavenumber, υvibron. The increase in the vibron linewidth with pressure remains well below the dissociation limit value, given by FWHM = E/2π, with E = hυvibron the vibron energy and where h is the Planck constant. This limit is estimated by assuming that the lifetime of the vibron state (inferred by assuming it is the only contribution to the line broadening) is equal to the vibration period.
Extended Data Fig. 5 Hydrogen equation of state and evolution of the direct electronic bandgap versus density.
a, Equation of state of solid hydrogen around 80 K. The black dots are neutron diffraction measurements39. The blue dots are our unpublished X-ray diffraction data obtained at the European Synchrotron Radiation Facility; the pressure scale is based on the revised ruby scale40. The red line is the fit of the experimental data by a Vinet form41: P = 3K0(1 − X)X−2exp[3/2(K0′ − 1)(1 − X)], with X = (V/V0)1/3, K0 = 0.191 GPa, K0′ = 7.039 (where K0′ = dK0/dP), and V0 = 23 cm3 mol−1. The present equation of state is in good agreement with that measured previously42. Inset, the Vinet form can be reformulated in terms of expressions analogous to normalized stress, ln[H(X)] = ln[PX2/3(1 − X)], and Eulerian strain, (1 − X). This gives: ln[H(X)] = lnK0 + 3/2(K0′ − 1)(1 − X). The linear fit of the data is shown. b, Evolution of the direct bandgap of solid hydrogen with density, for pressure increase (red), pressure decrease (blue) and from a previous study in the visible range18 (green). The vertical rectangles indicate the maximum 0.14-eV underestimation of the bandgap owing to the limited absorbance value of 2 that could be measured. The density uncertainties (±0.007 H2 mole cm−3) are obtained by propagating the ±10 GPa pressure uncertainties.
a, Scanning electron micrograph of the anvil. b, Profile of the toroidal diamond tip measured by interferometry. c, Scanning electron microscope image of the toroidal anvil recovered after pressure release. The toroidal part of the anvil is intact. Ring cracks are seen on the bevel of the anvil, at a diameter of about 150 μm.
Extended Data Fig. 7 Comparison of the original35 and revised36 Akahama diamond pressure scales for two measurements.
The two scales deviate above 300 GPa. a, Sample pressure–load curve. The force on the piston of the T-DAC is linearly related to the helium gas pressure inflating the membrane, F [kN] = 0.05 × Pm [bar]. The revised scale (orange) gives a convex evolution above 300 GPa that is mechanically incorrect. b, Infrared H2 vibron wavenumber versus pressure. Above 300 GPa, using the original scale (red), the shift is linear, in agreement with the calculated trend in phase III22,25; however, using the revised scale (dots and orange line), a sublinear shift is observed. The error bars in the pressure measurements (±10 GPa) arise from the random uncertainties originating from the positional accuracy of the sample and the stress field at the diamond anvil tip.
Extended Data Fig. 8 Experimental setup on the horizontal infrared microscope at the SMIS beamline of the SOLEIL synchrotron.
a, The L–N2 flow cryostat containing the T-DAC sits between the two Schwarzschild objectives for infrared transmission measurements. In the Raman configuration, one of the Schwarzschild objectives is swapped by the Raman head. Both configurations are reproducibly recovered within about 2 μm, because they are mounted on a precise, long-travel translation stage. b, Raw spectra recorded through two calibrated gasket holes 6 μm and 3 μm in diameter, made by focused ion beam machining in a rhenium gasket, and positioned between the two diamond anvils of the T-DAC. The spectra were recorded with a resolution of 4 cm−1 and after 400 accumulations.
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Loubeyre, P., Occelli, F. & Dumas, P. Synchrotron infrared spectroscopic evidence of the probable transition to metal hydrogen. Nature 577, 631–635 (2020). https://doi.org/10.1038/s41586-019-1927-3