Molecular semimetallic hydrogen*

Establishing the metallic hydrogen has been a goal of intensive theoretical and experimental work since 1935 when Wigner and Hungtinton predicted the dissociation of insulating molecular hydrogen into atomic metallic hydrogen at pressures above 25 GPa. The recent calculations give a value of ~450 GPa. Following the alternative scenario, metallization can be achieved prior the dissociation, in molecular state, through gradual overlapping of the electronic bands.
Our experiments have already shown that above ~360 GPa reflection significantly increases; the electrical conductivity increases with decreasing of temperature (metallic behavior) and then slowly decreases at temperatures <~ 100 K (the activation energy is few meVs). This peculiar temperature dependence is characteristic for poor metals or semimetals. With pressure, the overall electrical conductivity measured up to 440 GPa, strongly increases approaching values of poor metals. The Raman spectroscopy measurements evidence that hydrogen preserves its molecular phase III (likely C2/c or Pbcn structure) at least up to 440 GPa. At higher pressures up to 480 GPa, the Raman signal gradually disappears indicating further transformation either into a good molecular metal or in atomic state. We interpret our results as transformation of molecular hydrogen into a semimetal. This conclusion agrees with the recent calculations showing that the metallization happens through an indirect band gap closure in molecular hydrogen at pressure ~350 GPa.