Metallization of fluid hydrogen

The electrical resistivity of liquid hydrogen has been measured at the high dynamic pressures, densities and temperatures that can be achieved with a reverberating shock wave. The resulting data are most naturally interpreted in terms of a continuous transition from a semiconducting to a metallic largely diatomic fluid, the latter at 140 GPa (nine-fold compression) and 3000 K. While the fluid at these conditions resembles common liquid metals by the scale of its resistivity of 500 mu Omega cm, it differs by retaining a strong pairing character, and the precise mechanism by which a metallic state might be attained is still a matter of debate. Some evident possibilities include: (i) physics of a largely one-body character, such as a band-overlap transition; (ii) physics of a strong-coupling or many-body character, such as a Mott-Hubbard transition; and (iii) processes in which structural changes are paramount.