Electrical Conductivity and Metallization of Fluid Hydrogen in the Pressure Range 90-180 GPa (0. 9-1. 8 Mbar)

Abstract

Conductivity measurements indicate that hydrogen becomes a metallic fluid at 140 GPa, ninefold initial liquid density, and ∼3000 K. Metallization occurs when the electronic energy gap E_g is reduced by pressure to k_BT . High pressures and temperatures were obtained by a shock wave reverberating in hydrogen between stiff Al₂O₃ anvils. Resistivity decreases four orders of magnitude from 93 to 140 GPa and is constant from 140 to 180 GPa. About ∼5% of the H₂ molecules are dissociated at metallization . The free-electron Fermi energy is ∼12 ev. The measured conductivities of hydrogen are essentially the same as those of fluid Cs and Rb at 2000 K undergoing the same transition. The transition to the metal occurs at a Mott-scaled density of D_m^1/3a*=0. 30, where D_m is the metallization density and a* is the Bohr radius of the molecule. The measured metallic conductivity is in good agreement with (1) the calculated minimum conductivity of a metal, (2) the conductivity of hydrogen calculated with the strong-scattering free-electron model, (3) a preliminary calculation by Louis and Ashcroft using the weak-scattering Ziman model for a molecular liquid metal, and (4) tight-binding molecular dynamics calculations by Lenosky et al.