Response to Comment on "Insulator-metal transition in dense fluid deuterium".

In their comment, Desjarlais et al claim that a small temperature drop occurs after isentropic compression of fluid deuterium through the first-order insulator-metal transition. We show that their calculations do not correspond to the experimental thermodynamic path, and that thermodynamic integrations with parameters from first-principles calculations produce results in agreement with our original estimate of the temperature drop.

Insulator-metal transition in dense fluid deuterium.

Dense fluid metallic hydrogen occupies the interiors of Jupiter, Saturn, and many extrasolar planets, where pressures reach millions of atmospheres. Planetary structure models must describe accurately the transition from the outer molecular envelopes to the interior metallic regions. We report optical measurements of dynamically compressed fluid deuterium to 600 gigapascals (GPa) that reveal an increasing refractive index, the onset of absorption of visible light near 150 GPa, and a transition to metal-like reflectivity (exceeding 30%) near 200 GPa, all at temperatures below 2000 kelvin. Our measurements and analysis address existing discrepancies between static and dynamic experiments for the insulator-metal transition in dense fluid hydrogen isotopes. They also provide new benchmarks for the theoretical calculations used to construct planetary models.

Achieving high-density states through shock-wave loading of precompressed samples.

Materials can be experimentally characterized to terapascal pressures by sending a laser-induced shock wave through a sample that is precompressed inside a diamond-anvil cell. This combination of static and dynamic compression methods has been experimentally demonstrated and ultimately provides access to the 10- to 100-TPa (0.1-1 Gbar) pressure range that is relevant to planetary science, testing first-principles theories of condensed matter, and experimentally studying a new regime of chemical bonding.

Laser-shock compression of diamond and evidence of a negative-slope melting curve.

Diamond is the only known high-pressure structure of carbon. In spite of its fundamental and planetary importance, the stability domain of this strong covalent material is largely unknown. After decades of experimental efforts, evidence was obtained that the diamond-liquid melting line has a positive slope above the graphite-diamond-liquid triple point. At higher pressure, theoretical studies have suggested that the melting curve of diamond should have a maximum, owing to a loss of stability of the sp3 hybridization in the fluid phase. Accurate Hugoniot data of diamond exist up to 590 GPa (ref. 6). Higher-pressure measurements along the diamond Hugoniot have recently been achieved by laser shocks, showing that diamond probably melts to a conducting fluid. We report here laser-shock Hugoniot data across the melting transition. The shocked diamond crystal begins to melt around 750 GPa. Furthermore, a negative volume discontinuity at melting is observed. This requires a negative melting slope and thus supports the existence of a maximum on the diamond melting curve. These melting data allow us to test various calculations of the phase diagram of carbon at very high pressure. Finally, the stability domain of the diamond crystal is now constrained in a relevant region for Uranus-like planetary interiors.