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There is great current interest in multicomponent superhydrides due to their unique quantum properties under pressure. A remarkable example is the ternary superhydride Li_{2}MgH_{16} computationally identified to have an unprecedented high superconducting critical temperature T_{c} of ∼470 K at 250 GPa. However, the very high synthesis pressures required remains a significant hurdle for detailed study and potential applications. In this Letter, we evaluate the feasibility of synthesizing ternary Li-Mg superhydrides by the recently proposed pressure-potential (P^{2}) method that uniquely combines electrochemistry and applied pressure to control synthesis and stability. The results indicate that it is possible to synthesize Li-Mg superhydrides at modest pressures by applying suitable electrode potentials. Using pressure alone, no Li-Mg ternary hydrides are predicted to be thermodynamically stable, but in the presence of electrode potentials, both Li_{2}MgH_{16} and Li_{4}MgH_{24} can be stabilized at modest pressures. Three polymorphs are predicted as ground states of Li_{2}MgH_{16} below 300 GPa, with transitions at 33 and 160 GPa. The highest pressure phase is superconducting, while the two at lower pressures are not. Our findings point out the potentially important role of the P^{2} method in controlling phase stability of complex multicomponent superhydrides.

The discovery of near room temperature superconductivity in clathrate hydrides has ignited the search for both higher temperature superconductors and deeper understanding of the underlying physical phenomena. In a conventional electron-phonon mediated picture for the superconductivity for these materials, the high critical temperatures predicted and observed can be ascribed to the low mass of the protons, but this also poses nontrivial questions associated with how the proton dynamics affect the superconductivity. Using clathrate superhydride ${\mathrm{Li}}_{2}{\mathrm{MgH}}_{16}$ as an example, we show through ab initio path integral simulations that proton diffusion in this system is remarkably high, with a diffusion coefficient, for example, reaching $6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\text{ }\text{ }{\mathrm{cm}}^{2}/\mathrm{s}$ at 300 K and 250 GPa. The diffusion is achieved primarily through proton transfer among interstitial voids within the otherwise rigid ${\mathrm{Li}}_{2}\mathrm{Mg}$ sublattice at these conditions. The findings indicate the coexistence of proton quantum diffusion together with hydrogen-induced superconductivity, with implications for other very-high-temperature superconducting hydrides.

ANITA's fourth long-duration balloon flight in late 2016 detected 29 cosmic-ray (CR)-like events on a background of $0.37^{+0.27}_{-0.17}$ anthropogenic events. CRs are mainly seen in reflection off the Antarctic ice sheets, creating a characteristic phase-inverted waveform polarity. However, four of the below-horizon CR-like events show anomalous non-inverted polarity, a $p = 5.3 \times 10^{-4}$ chance if due to background. All anomalous events are from locations near the horizon; ANITA-IV observed no steeply-upcoming anomalous events similar to the two such events seen in prior flights., Comment: 6 pages, 4 figures, to appear in Physical Review Letters. Supplemental material (reference 17) available from corresponding author

Unifying principles that underlie recently discovered transitions between metallic and insulating states in elemental solids under pressure are developed. Using group theory arguments and first-principles calculations, we show that the electronic properties of the phases involved in these transitions are controlled by symmetry principles. The valence bands in these systems are described by simple and composite band representations constructed from localized Wannier functions centered on points unoccupied by atoms, and which are not necessarily all symmetrical. The character of the Wannier functions is closely related to the degree of $s\text{\ensuremath{-}}p(\text{\ensuremath{-}}d)$ hybridization and reflects multicenter chemical bonding in these insulating states. The conditions under which an insulating state is allowed for structures having an integer number of atoms per primitive unit cell as well as reentrant (i.e., metal-insulator-metal) transition sequences are detailed, resulting in predictions of behavior such as phases having band-contact lines. The general principles developed are tested and applied to the alkali and alkaline earth metals, including elements where high-pressure insulating phases have been reported (e.g., Li, Na, and Ca).

The negatively charged nitrogen-vacancy (NV-) center in diamond has realized new frontiers in quantum technology. Here, the optical and spin resonances of the NV- center are observed under hydrostatic pressures up to 60 GPa. Our results motivate powerful new techniques to measure pressure and image high-pressure magnetic and electric phenomena. Additionally, molecular orbital analysis and semiclassical calculations provide insight into the effects of compression on the electronic orbitals of the NV- center.

The electrical transport properties of solid xenon were directly measured at pressures up to 155 GPa and temperatures from 300 K to 27 mK. The temperature dependence of resistance changed from semiconducting to metallic at pressures between 121 and 138 GPa, revealing direct proof of metallization of a rare-gas solid by electrical transport measurements. Anomalies in the conductivity are observed at low temperatures in the vicinity of the transition such that purely metallic behavior is observed only at 155 GPa over the entire temperature range.

We report evidence for a preserved magnetic state in FeO up to 143 GPa at room temperature using high-resolution x-ray emission spectroscopy. This observation is based on the spectral line shape of the Fe Kb emission line. Up to the highest pressure, FeO remains a magnetic insulator. Combining our results with previous Mossbauer data, we present a new magnetic phase diagram of FeO. Features like a closed-loop P-T antiferromagnetic domain confirm that high-pressure investigations can reveal new physical properties and unexpected phenomena.

High-pressure Raman measurements of ${\mathrm{H}}_{2}\mathrm{O}$ ice using synthetic diamond anvils reveal major changes associated with the transition to the nonmolecular, symmetric hydrogen-bonded state. At 60 GPa the strongly pressure-dependent O-H symmetric stretching mode disappears, and the translational modes exhibit frequency and damping anomalies. With further increase in pressure, a single peak appears and becomes the dominant feature in the spectrum in the megabar range. The band is assigned to the predicted Raman-active O-O mode of the nonmolecular phase, consistent with the formation of cuprite-type ice X with static, symmetric hydrogen bonds.

Fundamental crystal chemistry and first-principles total-energy calculations are used to examine stable and metastable high-pressure silica structures. We find that a large class of energetically competitive phases can be generated from hcp arrays of oxygen with silicon occupying one-half of the octahedral sites. Calculations for specific structures provide an explanation for a number of recent high-pressure results for crystalline silica and allow us to understand the nature of the short- and intermediate-range order in the high-pressure amorphous state.

Raman and infrared spectroscopy of para-H{sub 2} to pressures in excess of 200GPa and to 8K using new ultrapure synthetic diamond anvils reveals numerous new vibrational excitations in the three high-pressure phases. Highly resolved Raman-active librons indicate differences in orientational ordering between phasesII and III, including evidence for changes within phaseII. The librons in phaseIII are strongly pressure dependent and reflect a substantial increase in ordering with pressure. Multiple vibrons in all three phases (I, II, and III) are observed. The results place new bounds on predicted crystal structures and dynamics of the dense molecular solid. {copyright} {ital 1997} {ital The American Physical Society}

We have measured superconducting T{sub c} using a highly sensitive magnetic susceptibility technique to megabar pressures ({gt}100 GPa). We observed anomalies in T{sub c}(P) for Nb at 5{endash}6GPa and 60{endash}70GPa, at which pressure T{sub c} increases by 0.7K and decreases by about 1K, respectively. In contrast, T{sub c} in Ta remains nearly constant up to 45GPa. We suggest that the anomalies in Nb arise from stress-sensitive electronic topological transitions. Between 70 and 132GPa, T{sub c} for Nb drops continuously to 4.7K, which is related to the decrease in density of states at the Fermi level with increasing pressure. {copyright} {ital 1997} {ital The American Physical Society}

Synchrotron infrared reflectance and absorbance spectroscopy reveals a cascading sequence of pressure-induced Fermi resonances in compressed H{sub 2}O and D{sub 2}O ices up to and through the transition to the symmetric hydrogen-bonded phase. These resonances arise from the extremely large pressure shift of the O-H(D) stretching mode associated with the transition, which causes multiple avoided crossings between this and other infrared-active vibrational states. The highest frequency LO mode has a minimum at the transition, and the LO-TO splitting increases dramatically in the high-pressure phase. Fano-like features are present in absorbance even in the absence of mode coupling. {copyright} {ital 1997} {ital The American Physical Society}

We present a unified view of orientational ordering in phases I, II, and III of solid hydrogen. Phases II and III are orientationally ordered, while the ordering objects in phase II are angular momenta of rotating molecules, and in phase III the molecules themselves. This concept provides quantitative explanation of the vibron softening, libron and roton spectra, and increase of the IR vibron oscillator strength in phase III. The temperature dependence of the effective charge parallels the frequency shifts of the IR and Raman vibrons. All three quantities are linear in the order parameter., Replaced with the final text, accepted for publication in PRL. 1 Fig. added. Misc. text revisions

Phase IV of dense solid hydrogen has been identified by its infrared spectrum using high-pressure synchrotron radiation techniques. The spectrum exhibits a sharp vibron band at higher frequency and lower intensity than that for phase III, indicating the stability of molecular H(2) with decreased intermolecular interactions and charge transfer between molecules. A low-frequency vibron having a strong negative pressure shift indicative of strongly interacting molecules is also observed. The character of the spectrum is consistent with an anisotropic, mixed layer structure related to those recently predicted theoretically. Phase IV was found to be stable from 220 GPa (300 K) to at least 340 GPa (near 200 K), with the I-III-IV triple point located. Infrared transmission observed to the lowest photon energies measured places constraints on the electronic properties of the phase.

The negatively charged nitrogen-vacancy (NV-) center in diamond has realized new frontiers in quantum technology. Here, the optical and spin resonances of the NV- center are observed under hydrostatic pressures up to 60 GPa. Our results motivate powerful new techniques to measure pressure and image high-pressure magnetic and electric phenomena. Additionally, molecular orbital analysis and semiclassical calculations provide insight into the effects of compression on the electronic orbitals of the NV- center.

New synchrotron infrared absorption and reflectivity techniques have been developed to test predicted band overlap metallization in ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ above 150 GPa at various temperatures. Measurements on both isotopes to maximum pressures of 216 GPa show no evidence for Drude excitations in the midinfrared to near-infrared spectrum. The most definitive are measurements on ${\mathrm{D}}_{2}$, which remains transparent down to 0.15 eV at 170 GPa and 83 K. The results put new bounds on the optical conductivity and possible band-gap closure at the 150-GPa transition. In addition to strong vibrational transitions, new infrared absorption features are observed.

An extensive and detailed study of deuterium to 200 GPa and 20--300 K reveals a number of new features of its phase diagram. A well-defined Raman vibron discontinuity is observed across the I-III phase line above the I-II-III triple point at 167 GPa and 130 K. Evidence for additional invariant points is found, including one at 182( $\ifmmode\pm\else\textpm\fi{}$3) GPa and 235( $\ifmmode\pm\else\textpm\fi{}$15) K where the discontinuity disappears. Measurements of the phase III vibron reveal a new functional dependence of the order parameter with temperature and a previously undisclosed parallel to classical orientational phase transitions.

We present experimental results which demonstrate that the effective third-order susceptibility of a composite optical material can exceed those of the materials from which it is constructed. In particular, we have formed a composite of alternating, sub-wavelength-thick layers of titanium dioxide and the conjugated polymer poly( $p$-phenylene-benzobisthiazole), and find that its nonlinear susceptibility exceeds that of its more nonlinear constituent by 35%. The enhancement of the nonlinear susceptibility, which under more ideal but still realistic conditions can be as large as a factor of 10, can be understood as a consequence of local field corrections.

The equation of state, elasticity, and shear strength of magnesium oxide were examined to 227 GPa using synchrotron x-ray diffraction in a diamond anvil cell. Static compression, ultrasonic elasticity, and shock data for MgO from ambient pressure to above 200 GPa can all be described by a single Birch-Murnaghan equation of state when the static shear strength, which is determined to be at least 11 GPa at 227 GPa, is taken into account. Our results show that there are significant changes in the degree and character of the elastic anisotropy of MgO at high pressure.

We demonstrate an all-organic, one-dimensional photonic crystal tuned to the regime of strong exciton-photon coupling. The structure consists of a high index of refraction light-absorbing active layer periodically distributed in a low index of refraction nonabsorbing background medium. The strongly coupled state in this structure is shared between multiple active layers separated by distances longer than the wavelength of light in the structure. In order to investigate the potential for long-range energy transfer, photon-mediated exciton hybridization is demonstrated between multiple absorbing species spatially separated in the photonic crystal.

Hydrogen sulfide (H(2)S) and hydrogen (H(2)) crystallize into a 'guest-host' structure at 3.5 GPa and, at the initial formation pressure, the rotationally disordered component molecules exhibit weak van der Waals-type interactions. With increasing pressure, hydrogen bonding develops and strengthens between neighboring H(2)S molecules, reflected in a pronounced drop in S-H vibrational stretching frequency and also observed in first-principles calculations. At 17 GPa, an ordering process occurs where H(2)S molecules orient themselves to maximize hydrogen bonding and H(2) molecules simultaneously occupy a chemically distinct lattice site. Intermolecular forces in the H(2)S+H(2) system may be tuned with pressure from the weak hydrogen-bonding limit to the ordered hydrogen-bonding regime, resulting in a novel clathrate structure stabilized by cooperative interactions.

We report the experimental demonstration of storing optical information as a mechanical excitation in a silica optomechanical resonator. We use writing and readout laser pulses tuned to one mechanical frequency below an optical cavity resonance to control the coupling between the mechanical displacement and the optical field at the cavity resonance. The writing pulse maps a signal pulse at the cavity resonance to a mechanical excitation. The readout pulse later converts the mechanical excitation back to an optical pulse. The storage lifetime is determined by the relatively long damping time of the mechanical excitation.

Hydrogen sulfide (H(2)S) and hydrogen (H(2)) crystallize into a 'guest-host' structure at 3.5 GPa and, at the initial formation pressure, the rotationally disordered component molecules exhibit weak van der Waals-type interactions. With increasing pressure, hydrogen bonding develops and strengthens between neighboring H(2)S molecules, reflected in a pronounced drop in S-H vibrational stretching frequency and also observed in first-principles calculations. At 17 GPa, an ordering process occurs where H(2)S molecules orient themselves to maximize hydrogen bonding and H(2) molecules simultaneously occupy a chemically distinct lattice site. Intermolecular forces in the H(2)S+H(2) system may be tuned with pressure from the weak hydrogen-bonding limit to the ordered hydrogen-bonding regime, resulting in a novel clathrate structure stabilized by cooperative interactions.

High-pressure optical and x-ray studies of ${\mathrm{H}}_{2}$-${\mathrm{H}}_{2}$O mixtures have revealed the formation of the first hydrogen clathrate hydrates. A rhombohedral hydrate with a ${\mathrm{H}}_{2}$O sublattice similar to ice II and a ${\mathrm{H}}_{2}$:${\mathrm{H}}_{2}$O ratio of 1:6 is stable between 0.75 and 3.1 GPa (295 K). Above 2.3 GPa, a novel hydrate forms with the ${\mathrm{H}}_{2}$O molecules in a cubic diamond structure and with a very high ${\mathrm{H}}_{2}$:${\mathrm{H}}_{2}$O stoichiometry of 1:1. The ${\mathrm{H}}_{2}$ molecules occupy voids in the ${\mathrm{H}}_{2}$O framework, thus improving the packing efficiency and stabilizing this hydrate to very high pressures of at least 30 GPa.

Recent experimental and theoretical studies report that quenched pressure-amorphized \ensuremath{\alpha}-quartz is elastically anisotropic and displays a reversal of anisotropy with respect to the original crystallographic orientation. We demonstrate that samples recovered from static compression experiments of \ensuremath{\alpha}-quartz exhibit an unusual pressure-induced microstructure, which arises from a new phase transformation in \ensuremath{\alpha}-quartz at 21 GPa. Upon decompression, the high-pressure phase reverts to a quartzlike structure in a twinned state, accounting for the previously reported elastic behavior.

We have observed a two-vibron bound-to-unbound transition in solid D2 by Raman scattering at a pressure of 34(2) GPa. We investigated the transition by increasing the vibron bandwidth, through the application of pressure, until it dominated the intramolecular anharmonicity. We present an analysis of a simple Hamiltonian that gives the experimental bivibron binding energy and the critical bandwidth-to-anharmonicity ratio. Our results indicate that while the vibron bandwidth increases markedly with pressure, the anharmonicity remains constant.

Y2O3 with particle sizes ranging from 5 nm to 1 μm were studied at high pressure using x-ray diffraction and Raman spectroscopy techniques. Nanometer-sized Y2O3 particles are shown to be more stable than their bulk counterparts, and a grain size-dependent crystalline-amorphous transition was discovered in these materials. High-energy atomic pair distribution function measurements reveal that the amorphization is associated with the breakdown of the long-rang order of the YO6 octahedra, while the nearest-neighbor edge-shared octahedral linkages are preserved.

Using inelastic x-ray scattering techniques, we have succeeded in probing the high-pressure electronic structure of helium at 300 K. Helium has the widest known valence-conduction band gap of all materials a property whose high-pressure response has been inaccessible to direct measurements. We observed a rich electron excitation spectrum, including a cutoff edge above 23 eV, a sharp exciton peak showing linear volume dependence, and a series of excitations and continuum at 26 to 45 eV. We determined the electronic dispersion along the Γ-M direction over two Brillouin zones, and provided a quantitative picture of the helium exciton beyond the simplified Wannier-Frenkel description.

Y2O3 with particle sizes ranging from 5 nm to 1 μm were studied at high pressure using x-ray diffraction and Raman spectroscopy techniques. Nanometer-sized Y2O3 particles are shown to be more stable than their bulk counterparts, and a grain size-dependent crystalline-amorphous transition was discovered in these materials. High-energy atomic pair distribution function measurements reveal that the amorphization is associated with the breakdown of the long-rang order of the YO6 octahedra, while the nearest-neighbor edge-shared octahedral linkages are preserved.

We report novel molecular compound formation from silane-hydrogen mixtures with intermolecular interactions unprecedented for hydrogen-rich solids. A complex ${\mathrm{H}}_{2}$ vibron spectrum with anticorrelated pressure-frequency dependencies and a striking H-D exchange below 10 GPa reveal strong and unusual attractive interactions between ${\mathrm{SiH}}_{4}$ and ${\mathrm{H}}_{2}$ and molecular bond destabilization at remarkably low pressure. The unique features of the observed ${\mathrm{SiH}}_{4}({\mathrm{H}}_{2}{)}_{2}$ compound suggest a new range of accessible pressure-driven intermolecular interactions for hydrogen-bearing simple molecular systems and a new approach to perturb the hydrogen covalent bond.

Spectroscopic data for the high-pressure phases of hydrogen together with the topology of the phase diagram provide new insight on the behavior of the material at megabar pressure. Structural mechanisms are proposed for the transitions to phases II and III. These mechanisms include a partially ordered structure (possibly incommensurate) and an ordered isotranslational structure for the two phases, respectively. The analysis supports the existence of an additional phase, isostructural to phase III, with boundary that meets to form a second triple point with phases I and III.

The study of Taylor-Couette flow has been useful in understanding classical fluid flows and non-linear phenomena because of its simplicity and its versatility. The Taylor-Couette system promises to be equally useful in understanding the behavior of liquid helium II, a non-classical fluid.

Raman spectra of solid and fluid nitrogen to pressures up to 120 GPa and temperatures up to 2500 K reveal that the melting line exhibits a maximum near 70 GPa, followed by a triple point near 87 GPa, after which the melting temperature rises again. Fluid nitrogen remains molecular over the entire pressure range studied, and there is no sign of a fluid-fluid transition. Solid phases obtained on quenching from the melt above 48 GPa are identical to the recently discovered $\ensuremath{\iota}$ and ${\ensuremath{\zeta}}^{\ensuremath{'}}$ phases. We find that kinetics plays a major role in the experimentally observed phase changes and account for the metastability of various crystalline molecular phases and the existence of an amorphous single bonded $\ensuremath{\eta}\ensuremath{-}\mathrm{N}$.

The electronic and lattice dynamical properties of compressed solid ${\mathrm{SiH}}_{4}$ have been calculated in the pressure range up to 300 GPa with density functional theory. We find two energetically preferred insulating phases with $P{2}_{1}/c$ and $Fdd2$ symmetries at low pressures. We demonstrate that the $Cmca$ structure having a layered network is the most likely candidate of the metallic phase of ${\mathrm{SiH}}_{4}$ over a wide pressure range above 60 GPa. The superconducting transition temperature in this layered metallic phase is found to be in the range of 20--75 K.

Two new transition metal nitrides, IrN2 and OsN2, were synthesized at high pressures and temperatures using laser-heated diamond-anvil cell techniques. Synchrotron x-ray diffraction was used to determine the structures of novel nitrides and the equations of states of both the parent metals as well as the newly synthesized materials. The compounds have bulk moduli comparable with those of the traditional superhard materials. For IrN2, the measured bulk modulus [K0 = 428(12) GPa] is second only to that of diamond (K0 = 440 GPa). Ab initio calculations indicate that both compounds have a metal:nitrogen stoichiometry of 1:2 and that nitrogen intercalates in the lattice of the parent metal in the form of single-bonded N-N units.

Two new transition metal nitrides, IrN2 and OsN2, were synthesized at high pressures and temperatures using laser-heated diamond-anvil cell techniques. Synchrotron x-ray diffraction was used to determine the structures of novel nitrides and the equations of states of both the parent metals as well as the newly synthesized materials. The compounds have bulk moduli comparable with those of the traditional superhard materials. For IrN2, the measured bulk modulus [K0 = 428(12) GPa] is second only to that of diamond (K0 = 440 GPa). Ab initio calculations indicate that both compounds have a metal:nitrogen stoichiometry of 1:2 and that nitrogen intercalates in the lattice of the parent metal in the form of single-bonded N-N units.

High-pressure high-temperature synchrotron diffraction measurements reveal a maximum on the melting curve of Na in the bcc phase at approximately 31 GPa and 1000 K and a steep decrease in melting temperature in its fcc phase. The results extend the melting curve by an order of magnitude up to 130 GPa. Above 103 GPa, Na crystallizes in a sequence of phases with complex structures with unusually low melting temperatures, reaching 300 K at 118 GPa, and an increased melting temperature is observed with further increases in pressure.

We demonstrate strong exciton-photon coupling in an optical microcavity containing a thermally evaporated polycrystalline organic thin film. Microcavity polaritons result from coupling between the 0-0 excitonic transition of 3,4,7,8 napthalenetetracarboxylic dianhydride and a cavity photon. For thicker films, the 0-1 transition also couples to the cavity mode, as vibronic relaxation is overcome by the short Rabi period for strong coupling. To our knowledge, this is the first report of strong coupling between a cavity photon and multiple vibronic transitions in a single material, made possible by the pronounced vibronic absorption features characteristic of crystalline organic materials.