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 unconfirmed5-7. Here a discontinuous change of the direct bandgap of hydrogen, from 0.6electronvolts to below 0.1electronvolts, is observed near 425gigapascals. This result is most probably associated with the formation of the metallic state because the nucleus zero-point energyis larger than this lowest bandgap value. Pressures above 400gigapascals are achieved with the recently developed toroidal diamond anvil cell8, and the structural changes and electronic properties of dense solid hydrogen at 80kelvin 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 425gigapascals. 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.