High-pressure dielectric measurements of solid hydrogen to 170 GPa

Abstract

AT high pressure (>100 GPa), the valence–conduction band gap in solid hydrogen is predicted to decrease and eventually to close, transforming it from an insulator to a metal¹. Recent experiments at pressures up to ∼300 GPa suggest a gradual closing of the direct gap at very high pressures whereas closure of the indirect gap may occur at lower pressures, perhaps below 200 GPa (refs 2–6). Measurements of the refractive index, n, and its frequency dependence (the dispersion dn/dω) as a function of pressure can provide information on the changes in electronic structure that occur under these conditions^7,8. Here we report refractive-index measurements on solid hydrogen at visible frequencies at pressures up to 170 GPa. Unlike earlier studies⁹, we find no evidence for a divergence (dielectric instability) at 150 GPa, close to the low-temperature phase transition observed previously⁶and suggested as being associated with metallization^4,9. Our results are consistent with closure of the indirect gap. A fit of our data to a dielectric model indicates that the onset of visible absorption owing to direct interband transitions should occur above 200 GPa, consistent with previous direct observations³. Pressure-induced molecular dissociation may occur before closure of the direct gap.