Title: Molecular semimetallic hydrogen,
Abstract: Establishing metallic hydrogen is a goal of intensive theoretical and
experimental work since 1935 when Wigner and Hungtinton [1] predicted that
insulating molecular hydrogen will dissociate at high pressures and transform
to a metal. This metal is predicted to be a superconductor with very high
critical temperature [2]. In another scenario, the metallization can be
realized through overlapping of electronic bands in molecular hydrogen in the
similar 400 - 500 GPa pressure range [3-5]. The calculations are not accurate
enough to predict which option will be realized. Our data are consistent with
transforms of hydrogen to semimetal by closing the indirect band gap in the
molecular phase III at pressure ~ 360 GPa. Above this pressure, the metallic
behaviour in the electrical conductivity appears, the reflection significantly
increases. With pressure, the electrical conductivity strongly increases as
measured up to 440 GPa. The Raman measurements evidence that hydrogen is in the
molecular phase III at pressures at least up to 440 GPa. At higher pressures
measured up to 480 GPa, the Raman signal gradually disappears indicating
further transformation to a good molecular metal or to an atomic state.