We found from the measurements of electrical resistance that hydrogen is in a conductive state in the domain of 350-450 GPa and temperatures below 200 K in accordance with the previous measurement at 360 GPa . For these measurements, four electrodes (tantalum covered with gold) were sputtered directly at the surface of a diamond anvil. We did not use any protection layer against hydrogen (such as Al2O3) because the measurements were done at low temperatures T<~200 K. We proved that we measure electrical resistance of the sample, not a gasket or diamond. We obtained the same results in the runs with very different insulating materials: CaSO4, NaCl, CaF2 mixed with epoxy, and MgO mixed with epoxy. We exclude a conductivity from diamond because the same results were obtained in Ref. , Fig. 9a, where electrodes were sputtered on the opposite anvils.
The temperature dependence of resistance of hydrogen is peculiar: metallic in the 100-200 K temperature range, and increases at lower temperatures. However, the activation energy derived from this increase is only of order of meV -too small for an intrinsic semiconductor. In addition, the activation energy does not change much (1- 4 meV) within the wide pressure range 350-450 GPa. More likely, hydrogen is a metal, namely poor metal or semimetal that can have a temperature dependence of conductivity with a rise at low temperatures. The measured resistivity is high but strongly decreases with pressure, and an extrapolation shows that the values of resistivity which is characteristic for good metals will be reached at P~500 GPa. Puzzlingly, the conductive samples also reveal Raman spectra characteristic for the phase III. The Raman intensity strongly decreases with pressure and cannot be tracked above P~430 GPa up to the highest reached and well documented pressure of 475 GPa. The combination of electrical conductivity with the Raman activity can be explained by closing of the indirect band gap of the molecular phase III and transformation of hydrogen to a semimetal at pressure above ~350 GPa. Above this pressure, the hydrogen samples reflect well.
1. M. I. Eremets, I.A. Troyan, and A.P. Drozdov, Low temperature phase diagram of hydrogen at pressures up to 380 GPa. A possible metallic phase at 360 GPa and 200 K. arXiv:1601.04479, 2016.