Theoretical calculations shine light on the optical properties of metallic hydrogen

Fig. 1: Using two diamonds, scientists squeezed hydrogen to pressures above those in Earth’s core. Photo by Sang-Heon Shim, Arizona State University.

The recent claim by Dias and Silvera of having produced metallic hydrogen at an extremely high pressure of 495 GPa has been a source of intense debate in the scientific community, with pressure determination and the optical approach for characterizing the sample being the main issues put under question. In this work, we suggest optical measurements could be a good alternative for characterizing this material in view of the limitations imposed on conventional techniques as neutron or X-ray scattering.

We have performed optical spectra simulations for metallic hydrogen adopting both atomic and molecular structures. We have combined time-dependent density functional and Migdal-Eliashberg theories incorporate electronic band structure and electron-phonon scattering effects in the optical spectra. As shown in Fig. 2, the atomic hydrogen model structure I4₁/amd shows a sharp decrease of reflectivity for photon energies around 6 eV due to a clear interband plasmon, while for lower energies reflectivity remains high as expected for a metal. On the other hand, the molecular Cmca-4 candidate structure shows a broad region of light absorption at 1-6 eV as the available interband transitions remarkably exceed the ones in the atomic case and therefore deviates from the typical reflectance spectrum of a metal even if it is also a metal. Therefore, our simulations show molecular and atomic hydrogen display very different features in their visible light and ultraviolet reflectance, suggesting that different phases of hydrogen could potentially be distinguished from their optical spectra.

Fig. 2: Calculated reflectivity of atomic I41/amd hydrogen at 500 GPa for different impurity scattering rates and temperatures. A simulation on molecular Cmca-4 is shown for comparison. Experimental values obtained by Dias and Silvera are shown as well. The inset shows a zoom into the infrared region.

In the infrared and low-frequency visible region, where the very strong electron-phonon scattering governs the optical properties, the experiments are better reproduced with the atomic phase. Moreover, according to our calculations, both atomic hydrogen, which is expected to be superconducting even at room temperature, and molecular hydrogen, which is also expected to be a superconductor but at lower temperatures, show a sharp decrease of reflectivity in the infrared region associated to their extraordinary superconducting properties. More precisely, the larger superconducting energy gap of the atomic phase, and the consequent higher critical temperature, manifests in the optical spectra as this sharp decrease of reflectivity appearing at larger energies; in fact, one expects this feature to appear exactly at an energy of twice the gap value (61 meV and 48 meV in the atomic and molecular cases, respectively).

All this deeply encourages further experimental research in order to extend the optical measurements to a wider region of the electromagnetic spectra. Confirming the predicted features would be not only of tremendous interest by itself but also a big step towards characterizing this fascinating material.