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.

Possible Quantum Paramagnetism in Compressed Sr_{2}IrO_{4}.

The effect of compression on the magnetic ground state of Sr_{2}IrO_{4} is studied with x-ray resonant techniques in the diamond anvil cell. The weak interlayer exchange coupling between square-planar 2D IrO_{2} layers is readily modified upon compression, with a crossover between magnetic structures around 7 GPa mimicking the effect of an applied magnetic field at ambient pressure. Higher pressures drive an order-disorder magnetic phase transition with no magnetic order detected above 17-20 GPa. The persistence of strong exchange interactions between J_{eff}=1/2 magnetic moments within the insulating IrO_{2} layers up to at least 35 GPa points to a highly frustrated magnetic state in compressed Sr_{2}IrO_{4}, opening the door for realization of novel quantum paramagnetic phases driven by extended 5d orbitals with entangled spin and orbital degrees of freedom.

Anomalous optical and electronic properties of dense sodium.

Synchrotron infrared spectroscopy on sodium shows a transition from a high reflectivity, nearly free-electron metal to a low-reflectivity, poor metal in an orthorhombic phase at 118 GPa. Optical spectra calculated within density functional theory (DFT) agree with the experimental measurements and predict a gap opening in the orthorhombic phase at compression beyond its stability field, a state that would be experimentally attainable by appropriate choice of pressure-temperature path. We show that a transition to an incommensurate phase at 125 GPa results in a partial recovery of good metallic character up to 180 GPa, demonstrating the strong relationship between structure and electronic properties in sodium.

Persistence of Jahn-Teller distortion up to the insulator to metal transition in LaMnO3.

High pressure, low temperature Raman measurements performed on LaMnO3 up to 34 GPa provide the first experimental evidence for the persistence of the Jahn-Teller distortion over the entire stability range of the insulating phase. This result resolves the ongoing debate about the nature of the pressure driven insulator to metal transition (IMT), demonstrating that LaMnO3 is not a classical Mott insulator. The formation of domains of distorted and regular octahedra, observed from 3 to 34 GPa, sheds new light on the mechanism behind the IMT suggesting that LaMnO3 becomes metallic when the fraction of undistorted octahedra domains increases beyond a critical threshold.

Superconductivity in boron.

Metals formed from light elements are predicted to exhibit intriguing states of electronic order. Of these materials, those containing boron are of considerable current interest because of their relatively high superconducting temperatures. We have investigated elemental boron to very high pressure using diamond anvil cell electrical conductivity techniques. We find that boron transforms from a nonmetal to a superconductor at about 160 gigapascals (GPa). The critical temperature of the transition increases from 6 kelvin (K) at 175 GPa to 11.2 K at 250 GPa, giving a positive pressure derivative of 0.05 K/GPa. Although the observed metallization pressure is compatible with the predictions of first-principles calculations, superconductivity in boron remains to be explored theoretically. The present results constitute a record pressure for both electrical conductivity studies and investigations of superconductivity in dense matter.

Phonon density of states of iron up to 153 gigapascals.

We report phonon densities of states (DOS) of iron measured by nuclear resonant inelastic x-ray scattering to 153 gigapascals and calculated from ab initio theory. Qualitatively, they are in agreement, but the theory predicts density at higher energies. From the DOS, we derive elastic and thermodynamic parameters of iron, including shear modulus, compressional and shear velocities, heat capacity, entropy, kinetic energy, zero-point energy, and Debye temperature. In comparison to the compressional and shear velocities from the preliminary reference Earth model (PREM) seismic model, our results suggest that Earth's inner core has a mean atomic number equal to or higher than pure iron, which is consistent with an iron-nickel alloy.

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.

Metal fluoride nanotubes featuring square-planar building blocks in a high-pressure polymorph of AgF2.

At a pressure of ca. 15 GPa, AgF2 transforms to an unprecedented orthorhombic polymorph featuring an array of tubular subunits which are built of corner sharing [AgF4] squares. This seems to be the first type of a metal fluoride nanowire and also the only one showing rigid square planar rather than common hexagonal or octahedral moieties.

High-pressure resistivity technique for quasi-hydrostatic compression experiments.

Diamond anvil cell techniques are now well established and powerful methods for measuring materials properties to very high pressure. However, high pressure resistivity measurements are challenging because the electrical contacts attached to the sample have to survive to extreme stress conditions. Until recently, experiments in a diamond anvil cell were mostly limited to non-hydrostatic or quasi-hydrostatic pressure media other than inert gases. We present here a solution to the problem by using focused ion beam ultrathin lithography for a diamond anvil cell loaded with inert gas (Ne) and show typical resistivity data. These ultrathin leads are deposited on the culet of the diamond and are attaching the sample to the anvil mechanically, therefore allowing for measurements in hydrostatic or nearly hydrostatic conditions of pressure using noble gases like Ne or He as pressure transmitting media.

Miniature diamond anvil cell for broad range of high pressure measurements.

A miniature versatile nonmagnetic diamond anvil cell for diverse physical property measurement under cryogenic environments and high magnetic fields at high pressure has been developed. Several such cells have been manufactured and tested in the Physical Properties Measurement System (PPMS) by Quantum Design at high pressures and low temperatures. The cells have good pressure stability during temperature scans down to helium temperatures and back to room temperature. The cells have been tested in strong magnetic fields and demonstrated excellent nonmagnetic properties. The wide-angle side openings give the possibility to use this cell as a "panoramic cell" in synchrotron experiments requiring large angle off-axis access. The possible experiments, which may use this cell, include spectroscopic experiments (optical, synchrotron Mossbauer, Raman, x-ray emission, etc.), different types of x-ray diffraction experiments, transport measurements (resistivity, magnetoresistivity, thermoelectromotive force, etc.), measurements of susceptibility, and many other conventional and synchrotron experiments at very low temperatures and in strong magnetic fields.

Uncovering a pressure-tuned electronic transition in Bi(1.98)Sr(2.06)Y(0.68)Cu(2)O(8+delta) using Raman scattering and x-ray diffraction.

We report pressure-tuned Raman and x-ray diffraction data of Bi(1.98.)Sr(2.06)Y(0.68)Cu(2)O(8+delta) revealing a critical pressure at 21 GPa with anomalies in electronic Raman background, electron-phonon coupling lambda, spectral weight transfer, density dependent behavior of phonons and magnons, and a compressibility change in the c axis. For the first time in a cuprate, mobile charge carriers, lattice, and magnetism all show anomalies at a distinct critical pressure in the same experimental setting. Furthermore, the spectral changes suggest that the critical pressure at 21 GPa is related to the critical point at optimal doping.

Ordering of hydrogen bonds in high-pressure low-temperature H2O.

The near K-edge structure of oxygen in liquid water and ices III, II, and IX at 0.25 GPa and several low temperatures down to 4 K has been studied using inelastic x-ray scattering at 9884.7 eV with a total energy resolution of 305 and 175 meV. A marked decrease of the preedge intensity from the liquid phase and ice III to ices II and IX is attributed to ordering of the hydrogen bonds in the proton-ordered lattice of the latter phases. Density functional theory calculations including the influence of the Madelung potential of the ice IX crystal correctly account for the remaining preedge feature. Furthermore, we obtain spectroscopic evidence suggesting a possible new phase of ice at temperatures between 4 and 50 K.

Nuclear inelastic x-ray scattering of FeO to 48 GPa.

The partial density of vibrational states has been measured for Fe in compressed FeO (wüstite) using nuclear resonant inelastic x-ray scattering. Substantial changes have been observed in the overall shape of the density of states close to the magnetic transition around 20 GPa from the paramagnetic (low pressure) to the antiferromagnetic (high pressure) state. The results indicate that strong magnetoelastic coupling in FeO is the driving force behind the changes in the phonon spectrum of FeO. The paper presents the first observation of changes in the density of terahertz acoustic phonon states under magnetic transition at high pressure.