Minimum metallic conductivity of fluid hydrogen at 140 GPa (1.4 Mbar)

Electrical conductivity measurements indicate that fluid hydrogen achieves the minimum conductivity of a metal at 140 GPa, ninefold initial liquid-H-2 density, and 2600 K. Metallization density is defined to be that at which the electronic mobility gap E-g is reduced by pressure to E(g)similar to k( B)T, at which point E-g is filled in by fluid disorder to produce a metalli c density of states with a Fermi surface and the minimum conductivity of a metal. High pressures and temperatures were obtained with a two-stage gun, which accelerates an impactor up to 7 km/sec. A strong shock wave is genera ted on impact with a holder containing liquid hydrogen at 20 K. The impact shock is split into a shock wave reverberating in hydrogen between two stif f Al2O3 anvils. This compression heats hydrogen quasi-isentropically to abo ut twice its melting temperature and lasts similar to 100 ns, sufficiently long to achieve equilibrium and sufficiently short to preclude loss of hydr ogen by diffusion and chemical reactions. The measured conductivity increas es four orders of magnitude in the range 93 to 140 GPa and is constant at 2 000 (Ohm cm)(-1) from 140 to 180 GPa. This conductivity is that of fluid Cs and Rb undergoing the same transition at 2000 K. This measured value is wi thin a factor of 5 or less of hydrogen conductivities calculated with (i) m inimum conductivity of a metal, (ii) Ziman model of a liquid metal, and (ii i) tight-binding molecular dynamics. At metallization this fluid is similar to 90 at. % H-2 and 10 at. % H with a Fermi energy of similar to 12 eV. Fl uid hydrogen at finite temperature undergoes a Mott transition at Dm(1/3)a* =0.30, where D-m is the metallization density and nh is the Bohr radius of the molecule. Metallization occurs at a lower pressure in the fluid than pr edicted for the solid probably because crystalline and orientational phase transitions in the ordered solid do not occur in the fluid and because of m any-body and structural effects. Tight-binding molecular dynamics calculati ons by Lenosky et al. suggest that fluid metallic hydrogen is a novel state of condensed matter. Protons are paired transiently and exchange on a time scale of a few molecular vibrational periods, similar to 10(-14) sec. Also, the kinetic, vibrational, and rotational energies of the dynamically paire d protons are comparable. [S0163-1829(99)02805-2].