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We use pulsed-laser heating of hydrogen at static pressures in the megabar pressure region to search for the plasma phase transition to liquid atomic metallic hydrogen. We heat our samples substantially above the melting line and observe a plateau in a temperature vs. laser power curve that otherwise increases with power. This anomaly in the heating curve appears correlated with theoretical predictions for the plasma phase transition. I n 1935, Wigner and Huntington predicted that solid molecular hydrogen would become atomic and metallic when compressed to a pressure of 25 GPa in the ground state of the solid (1). Subsequent theoretical work showed that there could be two pathways to the metallic state: isothermal compression to the solid atomic metallic phase, also predicted to be a high temperature superconductor by Ashcroft (2), and the plasma phase transition (PPT), first discussed by Landau and Zeldovich (3). For hydrogen, the PPT is predicted to be a first-order phase transition (involving molecular dissociation) to the atomic metallic liquid phase (for a recent short review, see ref. 4). In this contribution, we shall discuss the phase diagram of hydrogen and our experiments designed to observe the PPT at static pressure (P) and high temperature (T).A mixed experimental/theoretical phase diagram of hydrogen is shown in Fig. 1. At the present time, hydrogen has been pressurized to almost 400 GPa in diamond anvil cells (DACs) at room temperature and lower. Some years ago, new high-pressure, lowertemperature phases were discovered, and their phase lines were determined. The lowest pressure phase has a hexagonal closepacked structure with all molecules in the spherically symmetric rotational ground state J = 0. With increased pressure, new phase lines were observed: the broken symmetry phase (5), and the A phases (6-8) for pure parahydrogen, also referred to as I, II, and III for mixtures of ortho-and para-hydrogen. These are insulating phases in which the spherically symmetric single-molecule states (at low pressure) become admixed with higher rotational states and thus have nonspherically symmetric distributions; the phase transitions involve orientational order of the molecules along crystalline directions. At still higher pressures (270 GPa), there was a recent claim of metallization of hydrogen (9) that has been refuted (10) or not supported by subsequent experiments (11,12). However, there is recent evidence of a phase IV in the room temperature region above 200 GPa (12) that may be a semimetal (13).Several years ago, Bonev et al. (14) carried out a theoretical study of the melting line as a function of pressure and predicted a maximum in this line. These calculations were only valid for high temperatures; it has been speculated that the melting line can be extended to higher pressures so that hydrogen would be liquid atomic metallic at multimegabar pressures in the ground state in the limit that temperature goes to zero K (Fig. 1, dotted line). A calculation by Attaccalite and Sorella (15) supports th...

Liquid metallic hydrogen (LMH) is the most abundant form of condensed matter in our solar planetary structure. The electronic and thermal transport properties of this metallic fluid are of fundamental interest to understanding hydrogen's mechanism of conduction, atomic or pairing structure, as well as the key input for the magnetic dynamo action and thermal models of gas giants. Here, we report spectrally resolved measurements of the optical reflectance of LMH in the pressure region of 1.4-1.7 Mbar. We analyze the data, as well as previously reported measurements, using the free-electron model. Fitting the energy dependence of the reflectance data yields a dissociation fraction of 65 ± 15%, supporting theoretical models that LMH is an atomic metallic liquid. We determine the optical conductivity of LMH and find metallic hydrogen's static electrical conductivity to be 11,000-15,000 S/cm, substantially higher than the only earlier reported experimental values. The higher electrical conductivity implies that the Jovian and Saturnian dynamo are likely to operate out to shallower depths than previously assumed, while the inferred thermal conductivity should provide a crucial experimental constraint to heat transport models.

Liquid atomic metallic hydrogen is the simplest, lightest, and most abundant of all liquid metals 1,2 . The role of nucleon motions or ion dynamics has been somewhat ignored in relation to the dissociative insulator-metal transition. Almost all previous experimental high-pressure studies have treated the fluid isotopes, hydrogen and deuterium, with no distinction 3-8 .Studying both hydrogen and deuterium at the same density, most crucially at the phase transition line, can experimentally reveal the importance of ion dynamics. We use static compression to study the optical properties of dense deuterium in the pressure region of 1.2-1.7 Mbar and measured temperatures up to ~3000 K. We observe an abrupt increase in reflectance, consistent with dissociation-induced metallization, at the transition. Here we show that at the same pressure (density) for the two isotopes, the phase line of this transition reveals a prominent isotopic shift, ~700 K. This shift is lower than the isotopic difference in the free-molecule dissociation energies 9 , but it is still large considering the high density of the liquid and the complex many-body effects. Our work reveals the importance of quantum nuclear effects in describing the metallization transition and conduction properties in dense hydrogen systems at conditions of giant planetary interiors, and provides an invaluable benchmark for ab-initio calculations.As the lightest atoms, hydrogen and its isotopes exhibit the largest mass ratios of the elements, giving rise to large differences in their properties. The binding energies of the homonuclear freemolecules H 2 and D 2 differ by ~900 K (4.477eV for H 2 and 4.556 eV for D 2 ) 9 . The isotopic shift in the binding energy is related to the different zero-point energies (ZPE) arising from the fundamental vibrational mode of the molecules. At low temperatures (T), where both hydrogen and deuterium form quantum solids due to their ZPE, the large isotopic effects are manifested in phonon, vibrational, and rotational excitations, as well as differences in their equations of state and melting temperatures 10 . In 1935, Wigner and Huntington first discussed the role of density in destabilizing

Using conceptually and procedurally consistent density functional theory (DFT) calculations with an advanced meta-generalized gradient approximation (GGA) exchange-correlation functional in ab initio Born-Oppenheimer molecular dynamics (BOMD) simulations, we determine the insulator-metal transition (IMT) of warm dense fluid hydrogen from 50 to 300 GPa to be in better agreement with experiment than previous DFT predictions. The inclusion of nuclear quantum effects via path-integral molecular dynamics (PIMD) sharpens the transition and lowers its temperature relative to the BOMD results. A rapid decrease in the molecular character of the system, as observed via the ionic pair correlation function, coincides with an abrupt conductivity increase, confirming a metallic transition due to molecular hydrogen dissociation that is coincident with abrupt band-gap closure. Comparison of the PIMD and BOMD results clearly demonstrates an isotope effect on the IMT. Exploitation of differing methodologies for using the orbital-dependent and deorbitalized versions of the meta-GGA enables us to quantify exchange-correlation approximation effects. Distinct from stochastic simulations, these results do not depend upon any clever but uncontrolled combination of ground-state and finite-T methodologies and should provide a reliable benchmark for further studies.