Comment on A First-order Phase Transition to Metallic Hydrogen
  A recent paper by Zaghoo et al. presents optical data at high-pressure and high-temperature and interprets the data as evidence for a first-order phase transition to metallic hydrogen during heating. Here we argue that the presented data are contradictory with these claims. Elucidating this issue is important for building a coherent picture that is emerging as the results of theoretical calculations of various levels and experimental investigations employing static and dynamic compression techniques. In this context, the use of adequate probes of the electronic and chemical state is crucial. Optical probes that do not address the energy dependent conductivity while making multiple references to the Drude model are highly speculative. Indeed, the available dynamic and static compression data and theoretical modeling show that study of energy dependent conductivity is important for understanding the nature of hot dense hydrogen. Moreover, the most recent investigations suggest that the critical point, which demarcates the regimes of crossover between insulating and plasma state at low densities to the first order liquid-liquid transition, is above 200 GPa. Below we concentrate on inconsistencies in the interpretation of data in Zaghoo et al, which call for careful examination of their claims and further detailed investigations. We analyze their optical data and use finite element calculations and argue that the high-temperature state studied in Zaghoo et al. is not metallic and that the data cannot discriminate between a first-order phase transition and a continuous phase transition, or even a bandgap drop within one phase 
Alexander F. Goncharov and Zachary M. Geballe
cond-mat.mtrl-sci