The insulator-metal transition in hydrogen is one of the most outstanding problems in condensed-matter physics. The high-pressure metallic phase is now predicted to be liquid atomic from the low-temperature limit with the system in the ground state to very high temperatures. We have conducted measurements of optical properties of hot dense hydrogen in the region of 1.1\char21{}1.7 Mbars and up to 2200 K. We present evidence supportive of a first-order phase transition accompanied by changes in transmittance and reflectance, characteristic of a metal. The phase line of this transition has a negative slope in agreement with theories of the so-called plasma phase transition.

Evidence of a first-order phase transition to metallic hydrogen

The phase diagram of high-pressure hydrogen is of great interest for fundamental research, planetary physics, and energy applications. A first-order phase transition in the fluid phase between a molecular insulating fluid and a monoatomic metallic fluid has been predicted. The existence and precise location of the transition line is relevant for planetary models. Recent experiments reported contrasting results about the location of the transition. Theoretical results based on density functional theory are also very scattered. We report highly accurate coupled electron–ion Monte Carlo calculations of this transition, finding results that lie between the two experimental predictions, close to that measured in diamond anvil cell experiments but at 25–30 GPa higher pressure. The transition along an isotherm is signaled by a discontinuity in the specific volume, a sudden dissociation of the molecules, a jump in electrical conductivity, and loss of electron localization.

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Liquid-liquid phase transition in hydrogen by coupled electron-ion Monte Carlo simulations

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.

https://science.sciencemag.org/content/sci/361/6403/677.full.pdf

Insulator-metal transition in dense fluid deuterium

We use finite-temperature density functional theory coupled to classical molecular dynamics simulation to calculate the miscibility gap of hydrogen-helium mixtures. The van der Waals density functional (vdW-DF) theory is used, which leads to lower demixing temperatures compared to computations using the Perdew-Burke-Ernzerhof functional. Our calculations suggest that current Jupiter models are most likely too hot to allow demixing in the interior. A Jupiter isentrope based on our vdW-DF data is presented. Our demixing phase diagram still predicts phase separation in Saturn, but in a significantly reduced fraction of its volume.

Ab Initio Calculation of the Miscibility Diagram for Hydrogen-Helium Mixtures.

Superconductivity at 253 K in lanthanum–yttrium ternary hydrides

New developments in high-pressure X-ray diffraction beamline for diamond anvil cell at SPring-8

We investigated the phase transformation of hot dense fluid hydrogen using static high-pressure laser-heating experiments in a laser-heated diamond anvil cell. The results show anomalies in the heating efficiency that are likely to be attributed to the phase transition from a diatomic to monoatomic fluid hydrogen (plasma phase transition) in the pressure range between 82 and 106 GPa. This study imposes tighter constraints on the location of the hydrogen plasma phase transition boundary and suggests higher critical point than that predicted by the theoretical calculations.

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Phase boundary of hot dense fluid hydrogen

Two-dimensional (2D) triangular lattice antiferromagnets (2D-TLA) often manifest intriguing physical and technological properties, due to the strong interplay between lattice geometry and electronic properties. The recently synthesized 2-dimensional transition metal dichalcogenide LiCrTe[Formula: see text], being a 2D-TLA, enriched the range of materials which can present such properties. In this work, muon spin rotation ([Formula: see text]SR) and neutron powder diffraction (NPD) have been utilized to reveal the true magnetic nature and ground state of LiCrTe[Formula: see text]. From high-resolution NPD the magnetic spin order at base-temperature is not, as previously suggested, helical, but rather collinear antiferromagnetic (AFM) with ferromagnetic (FM) spin coupling within the ab-plane and AFM coupling along the c-axis. The value if the ordered magnetic Cr moment is established as [Formula: see text]. From detailed [Formula: see text]SR measurements we observe an AFM ordering temperature [Formula: see text] K. This value is remarkably higher than the one previously reported by magnetic bulk measurements. From [Formula: see text]SR we are able to extract the magnetic order parameter, whose critical exponent allows us to categorize LiCrTe[Formula: see text] in the 3D Heisenberg AFM universality class. Finally, by combining our magnetic studies with high-resolution synchrotron X-ray diffraction (XRD), we find a clear coupling between the nuclear and magnetic spin lattices. This suggests the possibility for a strong magnon-phonon coupling, similar to what has been previously observed in the closely related compound LiCrO[Formula: see text]. 

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Nuclear and magnetic spin structure of the antiferromagnetic triangular lattice compound LiCrTe2 investigated by [Formula: see text]SR, neutron and X-ray diffraction.

X-ray diffraction measurements of period 4 elements, including Zn, Ge, As, and Se, were extended up to pressures of 251, 249, 250, and 317 GPa, respectively, and the structural phase transitions were investigated. The hcp phase of Zn was stable up to 251 GPa, while a decrease in the optical reflectance of the sample above 50 GPa suggests a change in the electronic structure, such as a semimetallic transition. Ge transformed from the Cmca phase to the hcp phase with a volume reduction (−ΔV) of approximately 1% at 180 GPa. In the stable region of the Cmca phase, the hcp phase, which has a larger atomic volume than the Cmca phase, coexisted with the Cmca phase, and both atomic volumes reversed at approximately 180 GPa. For As, the monoclinic host–guest phase transformed to the bcc phase with −ΔV = 2.6% at 126 GPa. The β-Po-type phase of Se transformed to the bcc phase with −ΔV = 1.9% at 140 GPa, and the bcc phase was stable up to 317 GPa although a bcc–fcc phase transition was theoretically predicted. The equations of state of the monatomic metallic high-pressure phases were determined. The atomic volume of the monatomic metallic phases for period 4 elements increased with increasing atomic number, and the systematicity reported for period 5 elements was observed. Moreover, in the monatomic metallic phases of typical group 12–17 elements for periods 4 and 5, the atomic volume is ordered with increasing atomic number above 200 GPa.

Volume compression of period 4 elements: Zn, Ge, As, and Se above 200 GPa: Ordering of atomic volume by atomic number

We report that complete suppression of a phonon-driven structural phase transition causes partial breakdown of a three-dimensional translation symmetry in a well-defined sublattice. This state is revealed for a dielectric compound, ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}$, that comprises an ${\mathrm{AlO}}_{4}$ network incorporated into a hexagonal Ba(Sr) sublattice. Pair distribution function analyses and inelastic neutron scattering experiments provide clear-cut evidence of the ${\mathrm{AlO}}_{4}$ network forming a continuum of Al-O short-range correlations similar to glasses, whereas the Ba(Sr) sublattice preserves the original translational symmetry. This glassy network significantly dampens the phonon spectrum and transforms it into the broad one resembling those typically observed in glass materials. 

Sho Kawaguchi

Papers

New developments in high-pressure X-ray diffraction beamline for diamond anvil cell at SPring-8

Phase boundary of hot dense fluid hydrogen

Nuclear and magnetic spin structure of the antiferromagnetic triangular lattice compound LiCrTe2 investigated by [Formula: see text]SR, neutron and X-ray diffraction.

Volume compression of period 4 elements: Zn, Ge, As, and Se above 200 GPa: Ordering of atomic volume by atomic number

Partial breakdown of translation symmetry at a structural quantum critical point associated with a ferroelectric soft mode