  In previous work,1 we showed that hydrogen metallizes in phase III at
temperatures below ~200 K and at pressures near ~350 GPa. Here, we perform a
detailed study of electrical conductivity R(T) in phase III over a pressure
range of 200-400 GPa and a temperature range of 80-300 K, and we show that
hydrogen transforms from a semiconducting to a metallic state already at ~315
GPa. This transformation is also supported by Raman spectroscopy: the Raman
signal intensity decreases with pressure in accordance with the appearance and
increase of electrical conductivity. Moreover, the Raman and electrical
measurements yield the same boundary between the hydrogen phases III and V.
