Synchrotron infrared spectroscopic evidence of the probable transition to metal hydrogen

Abstract

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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Fig. 1: A selection of measurements over the investigated pressure range.
Fig. 2: Discontinuous pressure evolution in the infrared absorption and probable signature of metal hydrogen.
Fig. 3: Evolution of the molecular solid hydrogen properties up to the insulator–metal transition: comparison between experiment and calculations.
Fig. 4: Low-temperature phase diagram of solid hydrogen.

Data availability

The data that support the findings of this study are available from the corresponding author upon request.

References

  1. 1.

    Rigden, J. S. Hydrogen, the Essential Element (Harvard Univ. Press, 2002).

  2. 2.

    Wigner, E. & Huntington, H. B. On the possibility of a metallic modification of hydrogen. J. Chem. Phys. 3, 764–770 (1935).

  3. 3.

    Ashcroft, N. Dense hydrogen: the reluctant alkali. Phys. World 8, 43–48 (1995).

  4. 4.

    McMahon, J. M., Morales, M. A., Pierleoni, C. & Ceperley, D. M. The properties of hydrogen and helium under extreme conditions. Rev. Mod. Phys. 84, 1607–1653 (2012).

  5. 5.

    Mao, H. K. & Hemley, R. J. Optical studies of hydrogen above 200 gigapascals: evidence for metallization by band overlap. Science 244, 1462–1465 (1989).

  6. 6.

    Eremets, M. I. & Troyan, I. A. Conductive dense hydrogen. Nat. Mater. 10, 927–931 (2011).

  7. 7.

    Dias, R. P. & Silvera, I. F. Observation of the Wigner–Huntington transition to metallic hydrogen. Science 355, 715–718 (2017).

  8. 8.

    Dewaele, A., Loubeyre, P., Occelli, F., Marie, O. and Mezouar, M. Toroidal diamond anvil cell for detailed measurements under extreme static pressures. Nat. Commun. 9, 2913 (2018).

  9. 9.

    McMinis, J., Clay III, R.C., Lee, D. and Morales, M. A. Molecular to atomic phase transition in hydrogen under high pressure. Phys. Rev. Lett. 114, 105305 (2015).

  10. 10.

    Desgreniers, S., Vohra, Y. K. & Ruoff, A. L. Optical response of very high density solid oxygen to 132 GPa. J. Phys. Chem. 94, 1117–1122 (1990).

  11. 11.

    Johnson, K. A. & Ashcroft, N. W. Structure and bandgap closure in dense hydrogen. Nature 403, 632–635 (2000).

  12. 12.

    Drummond, N.D. et al. Quantum Monte Carlo study of the phase diagram of solid molecular hydrogen at extreme pressures. Nat. Commun. 6, 7794 (2015).

  13. 13.

    Ashcroft, N. W. Metallic hydrogen: a high-temperature superconductor? Phys. Rev. Lett. 21, 1748–1749 (1968).

  14. 14.

    Cudazzo, P. et al. Ab initio description of high temperature superconductivity in dense molecular hydrogen. Phys. Rev. Lett. 100, 257001 (2008).

  15. 15.

    Borinaga, M., Errea, I., Calandra, M., Mauri, F. & Bergara, A. Anharmonic effects in atomic hydrogen: superconductivity and lattice dynamical stability. Phys. Rev. B 93, 174308 (2016).

  16. 16.

    Babaev, E., Sudbo, A. and Ashcroft, N.W. Observability of a new state of matter: a metallic superfluid. Phys. Rev. Lett. 95, 105301 (2005).

  17. 17.

    Geng, H. Y., Wu, Q. & Sun, Y. Prediction of a mobile solid state in dense hydrogen under high pressures. J. Phys. Chem. Lett. 8, 223–228 (2017).

  18. 18.

    Loubeyre, P., Occelli, F. & LeToullec, R. Optical studies of solid hydrogen to 320 GPa and evidence for black hydrogen. Nature 416, 613–617 (2002).

  19. 19.

    Li, B. et al. Diamond anvil cell behavior up to 4 Mbar. Proc. Natl Acad. Sci. USA 115, 1713–1717 (2018).

  20. 20.

    Hanfland, M., Hemley, R. J. & Mao, H. K. Novel infrared vibron absorption of solid hydrogen at megabar pressures. Phys. Rev. Lett. 70, 3760–3763 (1993).

  21. 21.

    Azadi, M., Drummond, N. D. & Foulkes, W. M. C. Nature of the metallization transition in solid hydrogen. Phys. Rev. B 95, 035142 (2017).

  22. 22.

    Pickard, C. J. & Needs, R.J. Structure of phase III of solid hydrogen. Nat. Phys. 3, 473–476 (2007).

  23. 23.

    Eremets, M. I., Drozdov, A. P., Kong, P. P. & Wang, H. Semimetallic molecular hydrogen at pressure above 350 GPa. Nat. Phys. 15, 1246–1249 (2019).

  24. 24.

    Dewaele, A., Loubeyre, P., Dumas, P. and Mezouar, M. Oxygen impurities reduce the metallization pressure of xenon. Phys. Rev. B 86, 014103 (2012).

  25. 25.

    Zhang, C. et al. Finite-temperature infrared and Raman spectra of high-pressure hydrogen from first-principle molecular dynamics. Phys. Rev. B 98, 144301 (2018).

  26. 26.

    Morales, M., Mc Mahon, J. M., Pierleoni, C. & Ceperley, D. M. Towards a predictive first-principles description of solid molecular hydrogen with density functional theory. Phys. Rev. B 87, 184107 (2013).

  27. 27.

    Dalladay-Simpson, P., Howie, R. T. & Gregoryanz, E. Evidence for a new phase of dense hydrogen above 325 gigapascals. Nature 529, 63–67 (2016).

  28. 28.

    Ji, C. et al. Ultrahigh-pressure isostructural electronic transitions in hydrogen. Nature 573, 558–562 (2019).

  29. 29.

    Goncharenko, I. & Loubeyre, P. Neutron and X-ray diffraction study of the broken symmetry phase transition in solid deuterium. Nature 435, 1206–1209 (2005).

  30. 30.

    Geneste, G., Torrent, M., Bottin, F. & Loubeyre, P. Strong isotope effect in phase II of dense solid hydrogen and deuterium. Phys. Rev. Lett. 109, 155303 (2012).

  31. 31.

    Loubeyre, P., Occelli, F. & Dumas, P. Hydrogen phase IV revisited via synchrotron infrared measurements in H2 and D2 up to 290 GPa at 296 K. Phys. Rev. B 87, 134101 (2013).

  32. 32.

    Zhang, X.-W., Wang, E.-G. & Li, X.-Z. Ab initio investigation on the experimental observation of metallic hydrogen. Phys. Rev. B 98, 134110 (2018).

  33. 33.

    Carbotte, J. P., Nicol, E. J. & Timusk, T. Detecting superconductivity in the high pressure hydrides and metallic hydrogen from optical properties. Phys. Rev. Lett. 121, 047002 (2018).

  34. 34.

    Ruoff, A. L., Luo, H. & Vohra, Y. K. The closing diamond anvil optical window in multimegabar research. J. Appl. Phys. 69, 6413–6416 (1991).

  35. 35.

    Akahama, Y. & Kawamura, H. Pressure calibration of diamond anvil Raman gauge to 310 GPa. J. Appl. Phys. 100, 043516 (2006).

  36. 36.

    Akahama, Y. & Kawamura, H. Pressure calibration of diamond anvil Raman gauge to 410 GPa. J. Phys. Conf. Ser. 215, 012195 (2010).

  37. 37.

    Holmes, N. C., Moriarty, J. A., Gathers, G. R. & Nellis, W. J. The equation of state of platinum to 660 GPa. J. Appl. Phys. 66, 2962–2967 (1989).

  38. 38.

    Yokoo, M., Kawai, N., Nakamura, K. & Kondo, K. Ultrahigh-pressure scales for gold and platinum at pressures up to 550 GPa. Phys. Rev. B 80, 104114 (2009).

  39. 39.

    Ishmaev, S. N. et al. Neutron structural investigations of solid parahydrogen at pressures up to 24 kbar. Sov. Phys. JETP 84, 394–403 (1983).

  40. 40.

    Dewaele, A., Torrent, M., Loubeyre, P. & Mezouar, M. Compression curves of transition metals in the Mbar range: experiments and projector augmented-wave calculations. Phys. Rev. B 78, 104102 (2008).

  41. 41.

    Vinet, P., Ferrante, J., Rose, J. H. & Smith, J. R. Compressibility of solids. J. Geophys. Res. 92, 9319–9325 (1987).

  42. 42.

    Akahama, Y. et al. Evidence from X-ray diffraction of orientational ordering in phase III of solid hydrogen at pressures up to 183 GPa. Phys. Rev. B 92, 060101 (2010).

  43. 43.

    Zha, C.-S., Liu, Z. & Hemley, R. J. Synchrotron infrared measurements of dense hydrogen to 360 GPa. Phys. Rev. Lett. 108, 146402 (2012).

Download references

Acknowledgements

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.

Author information

P.L. designed the project. P.L. and F.O. prepared and loaded the T-DAC. P.D., F.O. and P.L. developed the infrared microscope. P.L., F.O. and P.D. conducted the experiment and analysed the data. P.L. and P.D. wrote the manuscript. All authors discussed the results.

Correspondence to Paul Loubeyre.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 Infrared spectra below 350 GPa.

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.

Extended Data Fig. 2 Infrared spectra above 350 GPa with pressure increase and pressure decrease.

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).

Extended Data Fig. 3 Infrared vibron and phonon wavenumbers versus pressure.

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 = vibron 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. 4 Infrared vibron wavenumber versus pressure.

The present data (black dots), and their linear fit (red dashed line) are compared to the linear fits of previous measurements7,43 up to 360 GPa. Pressure uncertainty is ±10 GPa.

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 cm3)  are obtained by propagating the ±10 GPa pressure uncertainties.

Extended Data Fig. 6 Geometry of the toroidal shape.

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.

Extended Data Table 1 Infrared vibron wavenumber, direct bandgap and corresponding pressure

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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

Download citation

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.