A platform for planar dynamic compression of crystalline hydrogen toward the terapascal regime.

We describe a method for laser-driven planar compression of crystalline hydrogen that starts with a sample of solid para-hydrogen (even-valued rotational quantum number j) having an entropy of 0.06 kB/molecule at 10 K and 2 atm, with Boltzmann constant kB. Starting with this low-entropy state, the sample is compressed using a small initial shock (<0.2 GPa), followed by a pressure ramp that approaches isentropic loading as the sample is taken to hundreds of GPa. Planar loading allows for quantitative compression measurements; the objective of our low-entropy compression is to keep the sample cold enough to characterize crystalline hydrogen toward the terapascal range.

Imaging the Meissner effect in hydride superconductors using quantum sensors.

By directly altering microscopic interactions, pressure provides a powerful tuning knob for the exploration of condensed phases and geophysical phenomena1. The megabar regime represents an interesting frontier, in which recent discoveries include high-temperature superconductors, as well as structural and valence phase transitions2-6. However, at such high pressures, many conventional measurement techniques fail. Here we demonstrate the ability to perform local magnetometry inside a diamond anvil cell with sub-micron spatial resolution at megabar pressures. Our approach uses a shallow layer of nitrogen-vacancy colour centres implanted directly within the anvil7-9; crucially, we choose a crystal cut compatible with the intrinsic symmetries of the nitrogen-vacancy centre to enable functionality at megabar pressures. We apply our technique to characterize a recently discovered hydride superconductor, CeH9 (ref. 10). By performing simultaneous magnetometry and electrical transport measurements, we observe the dual signatures of superconductivity: diamagnetism characteristic of the Meissner effect and a sharp drop of the resistance to near zero. By locally mapping both the diamagnetic response and flux trapping, we directly image the geometry of superconducting regions, showing marked inhomogeneities at the micron scale. Our work brings quantum sensing to the megabar frontier and enables the closed-loop optimization of superhydride materials synthesis.

Evidence of hydrogen-helium immiscibility at Jupiter-interior conditions.

The phase behaviour of warm dense hydrogen-helium (H-He) mixtures affects our understanding of the evolution of Jupiter and Saturn and their interior structures1,2. For example, precipitation of He from a H-He atmosphere at about 1-10 megabar and a few thousand kelvin has been invoked to explain both the excess luminosity of Saturn1,3, and the depletion of He and neon (Ne) in Jupiter's atmosphere as observed by the Galileo probe4,5. But despite its importance, H-He phase behaviour under relevant planetary conditions remains poorly constrained because it is challenging to determine computationally and because the extremes of temperature and pressure are difficult to reach experimentally. Here we report that appropriate temperatures and pressures can be reached through laser-driven shock compression of H2-He samples that have been pre-compressed in diamond-anvil cells. This allows us to probe the properties of H-He mixtures under Jovian interior conditions, revealing a region of immiscibility along the Hugoniot. A clear discontinuous change in sample reflectivity indicates that this region ends above 150 gigapascals at 10,200 kelvin and that a more subtle reflectivity change occurs above 93 gigapascals at 4,700 kelvin. Considering pressure-temperature profiles for Jupiter, these experimental immiscibility constraints for a near-protosolar mixture suggest that H-He phase separation affects a large fraction-we estimate about 15 per cent of the radius-of Jupiter's interior. This finding provides microphysical support for Jupiter models that invoke a layered interior to explain Juno and Galileo spacecraft observations1,4,6-8.

Equation of State of CO_{2} Shock Compressed to 1 TPa.

Equation-of-state (pressure, density, temperature, internal energy) and reflectivity measurements on shock-compressed CO_{2} at and above the insulating-to-conducting transition reveal new insight into the chemistry of simple molecular systems in the warm-dense-matter regime. CO_{2} samples were precompressed in diamond-anvil cells to tune the initial densities from 1.35 g/cm^{3} (liquid) to 1.74 g/cm^{3} (solid) at room temperature and were then shock compressed up to 1 TPa and 93 000 K. Variation in initial density was leveraged to infer thermodynamic derivatives including specific heat and Gruneisen coefficient, exposing a complex bonded and moderately ionized state at the most extreme conditions studied.

Imaging stress and magnetism at high pressures using a nanoscale quantum sensor.

Pressure alters the physical, chemical, and electronic properties of matter. The diamond anvil cell enables tabletop experiments to investigate a diverse landscape of high-pressure phenomena. Here, we introduce and use a nanoscale sensing platform that integrates nitrogen-vacancy (NV) color centers directly into the culet of diamond anvils. We demonstrate the versatility of this platform by performing diffraction-limited imaging of both stress fields and magnetism as a function of pressure and temperature. We quantify all normal and shear stress components and demonstrate vector magnetic field imaging, enabling measurement of the pressure-driven [Formula: see text] phase transition in iron and the complex pressure-temperature phase diagram of gadolinium. A complementary NV-sensing modality using noise spectroscopy enables the characterization of phase transitions even in the absence of static magnetic signatures.

Erratum: Evidence for a Phase Transition in Silicate Melt at Extreme Pressure and Temperature Conditions [Phys. Rev. Lett. 108, 065701 (2012)].

This corrects the article DOI: 10.1103/PhysRevLett.108.065701.

Development of a broadband reflectivity diagnostic for laser driven shock compression experiments.

A normal-incidence visible and near-infrared shock wave optical reflectivity diagnostic was constructed to investigate changes in the optical properties of materials under dynamic laser compression. Documenting wavelength- and time-dependent changes in the optical properties of laser-shock compressed samples has been difficult, primarily due to the small sample sizes and short time scales involved, but we succeeded in doing so by broadening a series of time delayed 800-nm pulses from an ultrafast Ti:sapphire laser to generate high-intensity broadband light at nanosecond time scales. This diagnostic was demonstrated over the wavelength range 450-1150 nm with up to 16 time displaced spectra during a single shock experiment. Simultaneous off-normal incidence velocity interferometry (velocity interferometer system for any reflector) characterized the sample under laser-compression and also provided an independent reflectivity measurement at 532 nm wavelength. The shock-driven semiconductor-to-metallic transition in germanium was documented by the way of reflectivity measurements with 0.5 ns time resolution and a wavelength resolution of 10 nm.

Planetary science. Shock compression of stishovite and melting of silica at planetary interior conditions.

Deep inside planets, extreme density, pressure, and temperature strongly modify the properties of the constituent materials. In particular, how much heat solids can sustain before melting under pressure is key to determining a planet's internal structure and evolution. We report laser-driven shock experiments on fused silica, α-quartz, and stishovite yielding equation-of-state and electronic conductivity data at unprecedented conditions and showing that the melting temperature of SiO2 rises to 8300 K at a pressure of 500 gigapascals, comparable to the core-mantle boundary conditions for a 5-Earth mass super-Earth. We show that mantle silicates and core metal have comparable melting temperatures above 500 to 700 gigapascals, which could favor long-lived magma oceans for large terrestrial planets with implications for planetary magnetic-field generation in silicate magma layers deep inside such planets.

Ramp compression of diamond to five terapascals.

The recent discovery of more than a thousand planets outside our Solar System, together with the significant push to achieve inertially confined fusion in the laboratory, has prompted a renewed interest in how dense matter behaves at millions to billions of atmospheres of pressure. The theoretical description of such electron-degenerate matter has matured since the early quantum statistical model of Thomas and Fermi, and now suggests that new complexities can emerge at pressures where core electrons (not only valence electrons) influence the structure and bonding of matter. Recent developments in shock-free dynamic (ramp) compression now allow laboratory access to this dense matter regime. Here we describe ramp-compression measurements for diamond, achieving 3.7-fold compression at a peak pressure of 5 terapascals (equivalent to 50 million atmospheres). These equation-of-state data can now be compared to first-principles density functional calculations and theories long used to describe matter present in the interiors of giant planets, in stars, and in inertial-confinement fusion experiments. Our data also provide new constraints on mass-radius relationships for carbon-rich planets.

Clapeyron slope reversal in the melting curve of AuGa2 at 5.5 GPa.

We use x-ray diffraction in a resistively heated diamond anvil cell to extend the melting curve of AuGa2 beyond its minimum at 5.5 GPa and 720 K, and to constrain the high-temperature phase boundaries between cubic (fluorite structure), orthorhombic (cottunite structure) and monoclinic phases. We document a large change in Clapeyron slope that coincides with the transitions from cubic to lower symmetry phases, showing that a structural transition is the direct cause of the change in slope. In addition, moderate (~30 K) to large (90 K) hysteresis is detected between melting and freezing, from which we infer that at high pressures, AuGa2 crystals can remain in a metastable state at more than 5% above the thermodynamic melting temperature.

Evidence for a phase transition in silicate melt at extreme pressure and temperature conditions.

Laser-driven shock compression experiments reveal the presence of a phase transition in MgSiO(3) over the pressure-temperature range 300-400 GPa and 10 000-16 000 K, with a positive Clapeyron slope and a volume change of ∼6.3 (±2.0) percent. The observations are most readily interpreted as an abrupt liquid-liquid transition in a silicate composition representative of terrestrial planetary mantles, implying potentially significant consequences for the thermal-chemical evolution of extrasolar planetary interiors. In addition, the present results extend the Hugoniot equation of state of MgSiO(3) single crystal and glass to 950 GPa.

Insulator-to-conducting transition in dense fluid helium.

By combining diamond-anvil-cell and laser-driven shock wave techniques, we produced dense He samples up to 1.5 g/cm(3) at temperatures reaching 60 kK. Optical measurements of reflectivity and temperature show that electronic conduction in He at these conditions is temperature-activated (semiconducting). A fit to the data suggests that the mobility gap closes with increasing density, and that hot dense He becomes metallic above approximately 1.9 g/cm(3). These data provide a benchmark to test models that describe He ionization at conditions found in astrophysical objects, such as cold white dwarf atmospheres.

Hugoniot data for helium in the ionization regime.

Hugoniot data were obtained for fluid He in the 100 GPa pressure range by shock compression of samples statically precompressed in diamond-anvil cells. The initial (precompressed) He density (rho_(1)) for each experiment was tuned to a value between rho_(0L) or =3rho_{0L} (i.e., rho/rho_(0L)> or =12). Data show an increase in compressibility at the onset of ionization, similar to theoretical predictions.

The chemistry of cyanuric acid (H3C3N3O3) under high pressure and high temperature.

We have studied cyanuric acid (H(3)C(3)N(3)O(3)) at static pressures up to 8.1 GPa and simultaneous temperatures up to 750 K, using primarily infrared absorption spectroscopy and visual observation. The corresponding phase diagram compares favorably with theoretical predictions of metastable organic materials. Two reactions were observed and characterized; both are irreversible. Below 2 GPa, melting is accompanied by a decomposition reaction, and upon cooling, cyanuric acid is not recovered. Above 2 GPa, heating results in a solid product recoverable at ambient conditions. Corresponding infrared spectra suggest that pressure leads to the formation of heterocycles of increasing complexity and biological potential, with the composition determined by the pressure of formation. Cyanuric acid is of interest at these conditions because it and its monomer, isocyanic acid, are "prebiotic" compounds found in stellar dust clouds, meteorites, and other remnants of the early Earth.

Origin of the Verwey transition in magnetite.

Comprehensive x-ray powder diffraction studies were carried out in magnetite in the 80-150 K and 0-12 GPa ranges with a membrane-driven diamond anvil cell and helium as a pressure medium. Careful data analyses have shown that a reversible, cubic to a distorted-cubic, structural transition takes place with increasing pressure, within the (P,T) regime below the Verwey temperature TV(P). The experimental documentation that TV(P)=Tdist(P) implies that the pressure-temperature-driven metal-insulator Verwey transition is caused by a gap opening in the electronic band structure due to the crystal-structural transformation to a lower-symmetry phase. The distorted-cubic insulating phase comprises a relatively small pressure-temperature range of the stability field of the cubic metallic phase that extends to 25 GPa.

Iron spin transition in Earth's mantle.

High-pressure Mössbauer spectroscopy on several compositions across the (Mg,Fe)O magnesiowüstite solid solution confirms that ferrous iron (Fe(2+)) undergoes a high-spin to low-spin transition at pressures and for compositions relevant to the bulk of the Earth's mantle. High-resolution x-ray diffraction measurements document a volume change of 4-5% across the pressure-induced spin transition, which is thus expected to cause seismological anomalies in the lower mantle. The spin transition can lead to dissociation of Fe-bearing phases such as magnesiowüstite, and it reveals an unexpected richness in mineral properties and phase equilibria for the Earth's deep interior.

A new paradigm to extend diffraction measurements beyond the megabar regime.

The possibility of using X-ray diffraction to precisely monitor crystal structure at the extremes of pressure and temperature produced by shock-wave loading is explored. A summary of the advantages of using various X-ray sources for this work and an outline of the necessary experimental layout is given.

Melting curve and high-pressure chemistry of formic acid to 8 GPa and 600 K.

We have determined the melting temperature of formic acid (HCOOH) as a function of pressure to 8.5 GPa using infrared absorption spectroscopy, Raman spectroscopy and visual observation of samples in a resistively heated diamond-anvil cell. The experimentally determined incongruent melting curve compares favorably with a two-phase thermodynamic model. Decomposition reactions were observed above the melting temperature up to a pressure of 6.5 GPa, with principal products being CO2, H2O, and CO. At pressures above 6.5 GPa, decomposition led to reaction products that could be quenched as solids to zero pressure, and infrared and Raman spectra indicate that pressure leads to the presence of sp3 carbon-carbon bonding in these reaction products.

Pressure induced self-oxidation of Fe(OH)2.

Mössbauer spectroscopy, x-ray diffraction, and electrical resistance [R(P,T)] studies in Fe(OH)(2) to 40 GPa revealed an unforeseen process by which a gradual Fe2+ oxidation takes place, starting at approximately 8 GPa reaching 70% Fe3+ abundance at 40 GPa. The nonreversible process Fe2+-->Fe3++e(-) occurs with no structural transition. The "ejected" electrons form a deep band within the high-pressure electronic manifold becoming weakly localized at P>50 GPa. This process is attributed to an effective ionization potential created by the pressure induced orientationally deformed (OH) dipoles and the unusual small binding energy of the valence electron in Fe2+(OH)(2).

Sediments at the top of Earth's core.

Unusual physical properties at the core-mantle boundary have been inferred from seismic and geodetic observations in recent years. We show how both types of observations can be explained by a layer of silicate sediments, which accumulate at the top of the core as Earth cools. Compaction of the sediments expels most of the liquid iron but leaves behind a small amount of core material, which is entrained in mantle convection and may account for the isotopic signatures of core material in some hot spot plumes. Extraction of light elements from the liquid core also enhances the vigor of convection in the core and may increase the power available to the geodynamo.