Search

Experiments in laser-heated diamond anvil cells (LH DACs) are conducted to assess phase diagrams of planetary materials at high pressure-temperature (P-T) conditions; thus, reliable determination of temperature in LH DAC experiments is essential. Radiometric temperature determination in LH DACs relies on the assumption of sample's wavelength-independent optical properties (graybody assumption), which is not justified for major lower mantle materials. The result is that experimental phase diagrams contain systematic unconstrained errors. Here we estimate the systematic error in radiometric temperature of nongray polycrystalline bridgmanite (Bgm; Mg0.96Fe2+0.036Fe3+0.014Si0.99O3) in a LH DAC by modeling emission and absorption of thermal radiation in a sample with experimentally-constrained optical properties. A comparison to experimental data validates the models and reveals that thermal spectra measured in LH DAC experiments record the interaction of radiation with the hot nongray sample. The graybody assumption in the experiments on translucent Bgm (light extinction coefficient, k < ∼250 cm-1 at 500–900 nm) yields temperatures ∼5% higher than the maximum temperature in the sample heated to ∼1900 K. In contrast, the graybody temperature of dark Bgm (k > ∼1500 cm−1), such as that produced upon melt quenching in LH DACs, underestimates the maximum temperature by ∼10%. Our experimental results pose quantitative constraints on the effect of nongray optical properties on the uncertainty of radiometric temperature determination in Bgm in the LH DACs. Evaluating nongray temperature in the future would enable a revision of the Bgm to post-perovskite phase transition and the high-pressure melting curve of Bgm.

Significance Earth’s liquid outer core is mainly composed of iron alloyed with ∼8 to 10% of light elements (e.g., silicon). Convection of the liquid core generates Earth’s magnetic field, which is controlled by the thermal conductivity of the core. In this study, we investigated the resistivity and thermal conductivity of iron-silicon alloys as a candidate composition in Earth’s core via high-pressure and -temperature experiments and numerical calculations. We found a near temperature independence of the resistivity in iron-silicon alloys at Earth core’s pressure and thus a high thermal conductivity. This work indicates that if silicon is the sole major light element in Earth’s core it could depress thermal convection and promote a thermally stratified layer at the topmost outer core., Light elements in Earth’s core play a key role in driving convection and influencing geodynamics, both of which are crucial to the geodynamo. However, the thermal transport properties of iron alloys at high-pressure and -temperature conditions remain uncertain. Here we investigate the transport properties of solid hexagonal close-packed and liquid Fe-Si alloys with 4.3 and 9.0 wt % Si at high pressure and temperature using laser-heated diamond anvil cell experiments and first-principles molecular dynamics and dynamical mean field theory calculations. In contrast to the case of Fe, Si impurity scattering gradually dominates the total scattering in Fe-Si alloys with increasing Si concentration, leading to temperature independence of the resistivity and less electron–electron contribution to the conductivity in Fe-9Si. Our results show a thermal conductivity of ∼100 to 110 W⋅m−1⋅K−1 for liquid Fe-9Si near the topmost outer core. If Earth’s core consists of a large amount of silicon (e.g., > 4.3 wt %) with such a high thermal conductivity, a subadiabatic heat flow across the core–mantle boundary is likely, leaving a 400- to 500-km-deep thermally stratified layer below the core–mantle boundary, and challenges proposed thermal convection in Fe-Si liquid outer core.

The melting curve of vanadium at high pressure and temperature (P-T) is of great interest to our understanding of $d$-orbital transition metals with simple crystal structures at extreme P-T conditions. Here we have investigated the melting curve and crystal structures of polycrystalline vanadium at high P-T using synchrotron x-ray diffraction (XRD) in laser-heated diamond anvil cells (LH DACs) up to \ensuremath{\sim}100 GPa and \ensuremath{\sim}4400 K, a two-stage light-gas gun with in situ shock temperature measurements up to \ensuremath{\sim}256 GPa and \ensuremath{\sim}6200 K, and ab initio molecular dynamics (AIMD) with density functional theory computations up to \ensuremath{\sim}200 GPa. The occurrence of the diffuse scattering signals in high P-T XRD patterns is used as the primary criterion to determine the melting curve of body-centered cubic (bcc) vanadium up to \ensuremath{\sim}100 GPa in LH DACs. Analysis of thermal radiation spectra of shocked vanadium using a quasispectral pyrometer constrains the melting curve up to \ensuremath{\sim}246 GPa and \ensuremath{\sim}5830 K, which is consistent with our static results using the Simon equation. The present static and dynamic experiments on the melting curve of vanadium are consistent with our AIMD simulations with the two-phase melting modeling, and are overall consistent with other theoretical simulations using the Z method. The results reconcile the recently reported theoretical discrepancy, and refute a higher melting curve report given by self-consistent ab initio lattice dynamics calculations. The consistencies among our studies indicate that one does not have to invoke superheating as a hypothesis to describe the solid-liquid equilibrium boundary of vanadium as an explanation for static vs dynamic experimental results. Our static and dynamic results with in situ diagnostics of melting and two-phase AIMD simulation have implications for studying melting curves of other $d$-orbital transition metals and their alloys at extreme P-T conditions.

${\mathrm{WN}}_{6}$ phase discovered at 126--165 GPa after heating of W in nitrogen. XRD refinements reveal a unit cell in space group $R\overline{3}m$ which is consistent with the ${\mathrm{WN}}_{6}$ structure with armchairlike hexazine (${\mathrm{N}}_{6}$) rings, while strong ${A}_{1g}$ Raman mode confirms its $\mathrm{N}─\mathrm{N}$ single bonds. Density functional theory (DFT) calculations reveal balanced contributions of attractive interactions between W and covalent ${\mathrm{N}}_{6}$ rings, and repulsions between ${\mathrm{N}}_{6}$ rings that make ${\mathrm{WN}}_{6}$ ultrastiff and tough. The ${\mathrm{WN}}_{6}$ phase displays long bond lengths in the nearest N-N and pressure-enhanced electronic band gap, which pave the way for finding novel nitrides.

The amount of heat transport from the core, which constrains the dynamics and thermal evolution of the region, depends on the transport properties of iron. Ohta et al.(2016) and Konopkova et al.(2016) measured electrical resistivity and thermal conductivity of iron, respectively, in laser-heated diamond anvil cells (DACs) at relevant Earth's core pressure-temperature (P-T) conditions, and obtained dramatically contradictory results. Here we measure the electrical resistivity of hcp-iron up to ~170 GPa and ~3,000 K using a four-probe van der Pauw method coupled with homogeneous flat-top laser-heating in a DAC. We also compute its electrical and thermal conductivity by first-principles methods including electron-phonon and electron-electron scattering. We find that the measured resistivity of hcp-iron increases almost linearly with increasing temperature, and is consistent with current first-principles computations. The proportionality coefficient between resistivity and thermal conductivity (the Lorenz number) in hcp-iron differs from the ideal value (2.44*10^-8 W Omega K^-2), so a non-ideal Lorenz number of ~(2.0-2.1)*10^-8 W Omega K^-2 is used to convert the experimental resistivity to the thermal conductivity of hcp-Fe at high P-T. The results constrain the resistivity and thermal conductivity of hcp-iron to ~80(5) u Omega cm and ~100(10) W/mK, respectively, at conditions near core-mantle boundary. Our results indicate an adiabatic heat flow of ~10(1) TW through the core-mantle boundary for a liquid Fe alloy outer core, supporting a present-day geodynamo driven by thermal convection through the core's secular cooling and by compositional convection through the latent heat and gravitational energy during the inner core's solidification., Comment: 39 pages, 14 figures

Elastic moduli (${C}_{ij}$) of single-crystal stishovite and post-stishovite are determined using Brillouin light scattering, impulsive stimulated light scattering, and x-ray diffraction up to 70 GPa. The ${C}_{12}$ of stishovite converges with the ${C}_{11}$ at $\ensuremath{\sim}55\text{ }\text{ }\mathrm{GPa}$, where the transverse wave ${V}_{S1}$ propagating along [110] also vanishes. Landau modeling of the ${C}_{ij}$, ${B}_{1g}$ optic mode, and lattice parameters reveals a pseudoproper type ferroelastic post-stishovite transition. The transition would cause peculiar anomalies in ${V}_{S}$ and Poisson's ratio in silica-bearing subducting slabs in the mid-lower mantle.

$\mathrm{LiOs}{\mathrm{O}}_{3}$ synthesized under high pressure in recent years is a rare metal since it undergoes a nonpolar to polar phase transition at ${T}_{s}=140\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Forming a polar axis through a phase transition in a metal seems against common sense. It is also not clear whether the transition to a polar phase in the oxide fits the mechanism predicted by Anderson and Blount in 1965. As monitored by an anomaly of resistivity in $\mathrm{LiOs}{\mathrm{O}}_{3}$ at ${T}_{s}$ reported recently, ${T}_{s}$ increases under pressure. The structural study under high pressure could give us a useful clue for understanding how dipoles form in this metallic oxide. Here, we report the identification of a polar phase of $\mathrm{LiOs}{\mathrm{O}}_{3}$ at room temperature under high pressure by using in situ probes of Raman and synchrotron x-ray diffraction. In the Raman study, the pressure-induced modes and their responses to polarized light, the linewidth change, the peak profile change, and mode softening have been directly compared with the corresponding changes of $\mathrm{LiOs}{\mathrm{O}}_{3}$ on cooling through ${T}_{s}$ at ambient pressure. Whereas a complete set of Raman modes from the $R3c$ phase can be found at $P\ensuremath{\ge}15.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$, a Raman mode of the $R3c$ phase appears in the $R\overline{3}c$ phase at 4.11 GPa. A significant drop in the linewidth occurs at 12.6 GPa that coincides with the critical pressure for the phase transition to the polar phase detected by x-ray diffraction. Fitting the peak profile of a Raman mode to the Fano formula also indicates a clear change of electron-phonon coupling at 16 GPa. In contrast to a sharp structural transition to the polar phase at ${T}_{s}$ under ambient pressure, our results reveal all the structural ingredients to facilitate the polar phase over a broad range of pressure. A bond valence sum analysis has been introduced to reveal the local structural instability under pressure. The transition to the polar phase in metallic $\mathrm{LiOs}{\mathrm{O}}_{3}$ is solely caused by optimizing the local structure in order to make the bond valence sum close to the formal valence of the Li ion.

Crystal structure prediction (CSP) methods recently proposed a series of new rare-earth (RE) hydrides at high pressures with novel crystal structures, unusual stoichiometries, and intriguing features such as high

The recent observation of unusually high thermal conductivity exceeding 1000 W m-1 K-1 in single-crystal boron arsenide (BAs) has led to interest in the potential application of this semiconductor for thermal management. Although both the electron/hole high mobilities have been calculated for BAs, there is a lack of experimental investigation of its electronic properties. Here, a photoluminescence (PL) measurement of single-crystal BAs at different temperatures and pressures is reported. The measurements reveal an indirect bandgap and two donor-acceptor pair (DAP) recombination transitions. Based on first-principles calculations and time-of-flight secondary-ion mass spectrometry results, the two DAP transitions are confirmed to originate from Si and C impurities occupying shallow energy levels in the bandgap. High-pressure PL spectra show that the donor level with respect to the conduction band minimum shrinks with increasing pressure, which affects the release of free carriers from defect states. These findings suggest the possibility of strain engineering of the transport properties of BAs for application in electronic devices.

Background and purpose: Rapid identification of microbes in the bloodstream is crucial in managing septicemia because of its high disease severity, and direct identification from positive blood culture bottles through matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) can shorten the turnaround time. Therefore, we developed a simple method for rapid microbiological identification from positive blood cultures by using MALDI-TOF MS. Methods: We modified previously developed methods to propose a faster, simpler and more economical method, which includes centrifugation and hemolysis. Specifically, our method comprises two-stage centrifugation with gravitational acceleration (g) at 600g and 3000g, followed by the addition of a lysis buffer and another 3000g centrifugation. Results: In total, 324 monomicrobial bacterial cultures were identified. The success rate of species identification was 81.8%, which is comparable with other complex methods. The identification success rate was the highest for Gram-negative aerobes (85%), followed by Gram-positive aerobes (78.2%) and anaerobes (67%). The proposed method requires less than 10 min, costs less than US$0.2 per usage, and facilitates batch processing. Conclusion: We conclude that this method is feasible for clinical use in microbiology laboratories, and can serve as a reference for treatments or further complementary diagnostic testing. Keywords: Blood culture, Direct identification, MALDI-TOF MS

Two distinct stacking orders in ReS

The synchrotron radiation technique of nuclear resonant inelastic X-ray scattering (NRIXS), also known as nuclear resonance vibrational spectroscopy or nuclear inelastic scattering, provides a wealth of information on the vibrational properties of solids. It has found applications in studies of lattice dynamics and elasticity, superconductivity, heme biochemistry, seismology, isotope geochemistry and many other fields. It involves probing the vibrational modes of solids by using the nuclear resonance of Mössbauer isotopes such as 57Fe, 83Kr, 119Sn, 151Eu and 161Dy. After data reduction, it provides the partial phonon density of states of the Mössbauer isotope that is investigated, as well as many other derived quantities such as the mean force constant of the chemical bonds and the Debye velocity. The data reduction is, however, not straightforward and involves removal of the elastic peak, normalization and Fourier–Log transformation. Furthermore, some of the quantities derived are highly sensitive to details in the baseline correction. A software package and several novel procedures to streamline and hopefully improve the reduction of the NRIXS data generated at sector 3ID of the Advanced Photon Source have been developed. The graphical user interface software is named SciPhon and runs as a Mathematica package. It is easily portable to other platforms and can be easily adapted for reducing data generated at other beamlines. Several tests and comparisons are presented that demonstrate the usefulness of this software, whose results have already been used in several publications. Here, the SciPhon software is used to reduce Kr, Sn, Eu and Dy NRIXS data, and potential implications for interpreting natural isotopic variations in those systems are discussed.

The electron spin states of iron in minerals of the Earth’s mantle at high pressures mostly determine the physicochemical properties of deep layers of the Earth and are of great interest not only for geophysics but also for fundamental physics of strongly correlated electron systems. In this work, using Raman and synchrotron Mossbauer nuclear forward scattering (NFS) spectroscopies, iron-containing magnesium–silicate perovskite (Mg, Fe)SiO3 (10% Fe) has been studied in the cryogenic temperature range of 35–300 K and at high pressures up to 48 GPa, which are created in diamond anvil cells. The analysis of NFS spectra has indicated that iron ions are in a nonmagnetic (para- or diamagnetic) state in the entire region of temperatures and pressures and the electronic properties can be controlled by means of the quadrupole splitting parameter. It has been found that an increase in the pressure and a decrease in the temperature are accompanied by a significant increase in the parameter Δ from 2 mm/s to ~4 mm/s, which indicates that the electronic state of Fe2+ ions changes. The maximum Δ value has been observed at P > 20 GPa, but the pressure behavior of a transition strongly depends on the temperature. Possible mechanisms of the transition have been discussed.

Vertical heterostructures (HSs) constructed with two-dimensional (2D) materials is expected to generate fascinating properties due to interlayer coupling between neighboring layers. However, interlayer coupling can be easily obscured by cross-contamination during transfer processes, rendering their experimental demonstration challenging. Here, we explore the coupling-assisted renormalization of excitons and vibrations in a mechanically fabricated MoSe2–WSe2 HS through high-pressure photoluminescence, Raman spectra, and density functional theory calculations. Accompanied by the interlayer coupling enhancement, the excitonic and vibrational renormalizations involving dimensionality and composition variations were achieved. A cycle of 2D–3D–2D excitonic evolution was disclosed and pressure-induced emergence of X– exciton of MoSe2 in HS was found reflecting the band structure transition in the MoSe2–WSe2 HS. The Raman spectra reveals that the coupled A2″ vibrations of WSe2 and MoSe2 in HS was stiffened and ou...

Thermal conductivity of the lowermost mantle governs the heat flow out of the core energizing planetary-scale geological processes. Yet, there are no direct experimental measurements of thermal conductivity at relevant pressure–temperature conditions of Earth's core–mantle boundary. Here we determine the radiative conductivity of post-perovskite at near core–mantle boundary conditions by optical absorption measurements in a laser-heated diamond anvil cell. Our results show that the radiative conductivity of Mg0.9Fe0.1SiO3 post-perovskite (∼1.1 W/m/K) is almost two times smaller than that of bridgmanite (∼2.0 W/m/K) at the base of the mantle. By combining this result with the present-day core–mantle heat flow and available estimations on the lattice thermal conductivity we conclude that post-perovskite is at least as abundant as bridgmanite in the lowermost mantle which has profound implications for the dynamics of the deep Earth.

A carrier doping by a hydrogen substitution in LaFeAsO1−xHx is known to cause two superconducting (SC) domes with the magnetic order at both end sides of the doping. In contrast, SmFeAsO1−xHx has a similar phase diagram but shows single SC dome. Here, we investigated the electronic and crystal structures for iron oxynitride LnFeAsO1−xHx (Ln = La, Sm) with the range of x = 0-0.5 by using x-ray absorption spectroscopy, x-ray emission spectroscopy, and x-ray diffraction. For both compounds, we observed that the pre-edge peaks of x-ray absorption spectra near the Fe-K edge were reduced in intensity on doping. The character arises from the weaker As–Fe hybridization with the longer As–Fe distance in the higher doped region. We can reproduce the spectra near the Fe-K edge according to the Anderson impurity model with realistic valence structures using the local-density approximation (LDA) plus dynamical mean-field theory (DMFT). For Ln = Sm, the integrated-absolute difference (IAD) analysis from x-ray Fe-Kβ emission spectra increases significantly. This is attributed to the enhancement of magnetic moment of Fe 3d electrons stemming from the localized picture in the higher doped region. A theoretical simulation implementing the self-consistent vertex-correction method reveals that the single dome superconducting phase for Ln = Sm arises from a better nesting condition in comparison with Ln = La.

Tailoring the excitonic properties in two-dimensional monolayer transition metal dichalcogenides (TMDs) through strain engineering is an effective means to explore their potential applications in optoelectronics and nanoelectronics. Here we report pressure-tuned photon emission of trions and excitons in monolayer MoSe2 via a diamond anvil cell (DAC) through photoluminescence measurements and theoretical calculations. Under quasi-hydrostatic compressive strain, our results show neutral (X0) and charged (X–) exciton emission of monolayer MoSe2 can be effectively tuned by alcohol mixture vs inert argon pressure transmitting media (PTM). During this process, X– emission undergoes a continuous blue shift until reaching saturation, while X0 emission turns up splitting. The pressure-dependent charging effect observed in alcohol mixture PTM results in the increase of the X– exciton component and facilitates the pressure-tuned emission of X– excitons. This substantial tunability of X– and X0 excitons in MoSe2 can ...

Complex seismic, thermal, and chemical features have been reported in Earth's lowermost mantle. In particular, possible iron enrichments in the large low shear-wave velocity provinces (LLSVPs) could influence thermal transport properties of the constituting minerals in this region, altering the lower mantle dynamics and heat flux across core-mantle boundary (CMB). Thermal conductivity of bridgmanite is expected to partially control the thermal evolution and dynamics of Earth's lower mantle. Importantly, the pressure-induced lattice distortion and iron spin and valence states in bridgmanite could affect its lattice thermal conductivity, but these effects remain largely unknown. Here we precisely measured the lattice thermal conductivity of Fe-bearing bridgmanite to 120 gigapascals using optical pump-probe spectroscopy. The conductivity of Fe-bearing bridgmanite increases monotonically with pressure, but drops significantly around 45 gigapascals due to pressure-induced lattice distortion on iron sites. Our findings indicate that lattice thermal conductivity at lowermost mantle conditions is twice smaller than previously thought. The decrease in the thermal conductivity of bridgmanite in mid-lower mantle and below would promote mantle flow against a potential viscosity barrier, facilitating slabs crossing over the 1000-km depth. Modeling of our results applied to LLSVPs shows that variations in iron and bridgmanite fractions induce a significant thermal conductivity decrease, which would enhance internal convective flow. Our CMB heat flux modeling indicates that, while heat flux variations are dominated by thermal effects, variations in thermal conductivity also play a significant role. The CMB heat flux map we obtained is substantially different from those assumed so far, which may influence our understanding of the geodynamo.

In two dimensional (2D) transition metal dichalcogenides, defect-related processes can significantly affect carrier dynamics and transport properties. Using femtosecond degenerate pump-probe spectroscopy, exciton capture, and release by mid-gap defects have been observed in chemical vapor deposition (CVD) grown monolayer MoSe2. The observed defect state filling shows a clear saturation at high exciton densities, from which the defect density is estimated to be around 0.5 × 1012/cm2. The exciton capture time extracted from experimental data is around ~ 1 ps, while the average fast and slow release times are 52 and 700 ps, respectively. The process of defect trapping excitons is found to exist uniquely in CVD grown samples, regardless of substrate and sample thickness. X-ray photoelectron spectroscopy measurements on CVD and exfoliated samples suggest that the oxygen-associated impurities could be responsible for the exciton trapping. Our results bring new insights to understand the role of defects in capturing and releasing excitons in 2D materials, and demonstrate an approach to estimate the defect density nondestructively, both of which will facilitate the design and application of optoelectronics devices based on CVD grown 2D transition metal dichalcogenides. The temporal dynamics of photo-generated electrons and holes in MoSe2 trapped by defects are revealed. While transitional metal dichalcogenides have significant potential for optoelectronic applications, samples tend to contain defects such as vacancies and impurities, most of which affect carrier mobility by inducing mid-gap states, i.e. within the bandgap. Now a team led by Yaguo Wang from the University of Texas elucidates the role of defects in samples grown by chemical vapor deposition. Femtosecond pump probe spectroscopy reveals that such defects are prone to capture (within few picoseconds) and then release (at slightly longer timescales of hundreds of picoseconds) electrons and holes. Such dynamics are intrinsic to samples grown with this particular method and possibly linked to the oxygen-associated impurities introduced during growth. This knowledge is relevant to engineering the properties of 2D materials for optoelectronics applications.

The concentrations of Fe, Al, and Ni and their distributions were determined for all known natural assemblages of ferropericlase (fPer) and bridgmanite (Bridg), coexisting as inclusions in deep-mantle diamonds from Brazil, Canada, Guinea, and South Australia. Based upon these data, it is likely that some areas within the deep lower mantle are iron-rich and differ markedly from a pyrolitic composition. In the lowermost lower mantle, Bridg is Al-rich and fPer is Ni-poor, witnessing the presence of a free metallic phase in the mineral-forming environment. The iron partitioning in the Bridg + fPer association [ K D Bridg-fPer = ( [ Fe / Mg ] Bridg ) / ( [ Fe / Mg ] fPer ) at in juvenile diamond inclusions is as low as 0.1–0.2. During ascent of the diamonds with their inclusions to the surface, the K D Bridg-fPer eventually increases to values of 0.4–0.5 and even as high as 0.7. The details of the element partitioning between natural Bridg and fPer in the lower mantle are as follows: iron in Bridg is ferrous Fe 2+ in the A site, substituting for Mg 2+ ; almost all iron in fPer is ferrous Fe 2+ ; the share of ferric Fe 3+ iron in fPer is Fe 3+ /ΣFe = 8–12 at%; iron concentrations in both Bridg and fPer increase with depth (pressure), reflecting the increasing Fe content in the lower part of the lower mantle, different from that of a pyrolitic model. Al in Bridg is mainly in the cation site B and partly in the cation site A, in both cases substituting for Si, Mg, and Fe with vacancy formation; and in the case of Al positioning into both B and A sites, a charge-balanced reaction occurs. The natural samples show very diverse K D Bridg-fPer values and elemental distribution that cannot be simply explained by our current understanding on alumina dissolution in Bridg and the spin transition of Fe 2+ in fPer. These major differences between experimental results and observations in natural samples demonstrate the complex, inhomogeneous iron speciation and chemistry in the lower mantle.

In this study, we performed synchrotron X-ray diffraction (XRD) and Mossbauer spectroscopy (SMS) measurements on two single-crystal bridgmanite samples [ Mg 0.94 Fe 0.04 2 + Fe 0.02 3 + Al 0.01 Si 0.99 O 3 ( Bm 6 ) and Mg 0.89 Fe 0.024 2 + Fe 0.096 3 + Al 0.11 Si 0.89 O 3 ( Al-Bm 11 ) ] to investigate the combined effect of Fe and Al on the hyperfine parameters, lattice parameters, and equation of state (EoS) of bridgmanite up to 130 GPa. Our SMS results show that Fe 2+ and Fe 3+ in Bm6 and Al-Bm11 are predominantly located in the large pseudo-dodecahedral sites (A-site) at lower-mantle pressures. The observed drastic increase in the hyperfine quadrupole splitting (QS) between 13 and 32 GPa can be associated with an enhanced local distortion of the A-site Fe 2+ in Bm6. In contrast to Bm6, the enhanced lattice distortion and the presence of extremely high QS values of Fe 2+ are not observed in Al-Bm11 at high pressures. Our results here support the notion that the occurrence of the extremely high QS component of approximately 4 mm/s in bridgmanite is due to the lattice distortion in the high-spin (HS) A-site Fe 2+ , instead of the occurrence of the intermediate-spin state. Both A-site Fe 2+ and Fe 3+ in Bm6 and Al-Bm11 remain in the HS state at lower-mantle pressures. Together with XRD results, we present the first experimental evidence that the enhanced lattice distortion of A-site Fe 2+ does not cause any detectable variation in the EoS parameters, but is associated with anomalous variations in the bond length, tilting angle, and shear strain in the octahedra of Bm6. Analysis of the obtained EoS parameters of bridgmanite at lower-mantle pressures indicates that the substitution of Fe in bridgmanite will cause an enhanced density and a reduced bulk sound velocity ( V Φ ), whereas the Al and Fe substitution has a reduced effect on density and a negligible effect on V Φ . These experimental results provide new insight into the correlation between lattice, hyperfine, and EoS parameters of bridgmanite in the Earth’s lower mantle.

Deciphering the origin of seismic heterogeneity has been one of the major challenges in understanding the geochemistry and geodynamics of the deep mantle. Fully anisotropic elastic properties of constituent minerals at relevant pressure-temperature conditions of the lower mantle can be used to calculate seismic heterogeneity parameters in order to better understand chemically- and thermally-induced seismic heterogeneities. In this study, the single-crystal elastic properties of ferropericlase (Mg0.94Fe0.06)O were measured using Brillouin spectroscopy and X-ray diffraction at conditions up to 50 GPa and 900 K. The velocity-density results were modeled using third-order finite-strain theory and thermoelastic equations along a representative geotherm to investigate high pressure-temperature and compositional effects on the seismic heterogeneity parameters. Our results demonstrate that from 660 to 2000 km, compressional wave anisotropy of ferropericlase increased from 4% to 9.7% while shear wave anisotropy increased from 9% to as high as 22.5%. The thermally-induced lateral heterogeneity ratio (RS/P = ∂lnVS/∂lnVP) of ferropericlase was calculated to be 1.48 at ambient pressure but decreased to 1.43 at 40 GPa along a representative geotherm. The RS/P of a simplified pyrolite model consisting of 80% bridgmanite and 20% ferropericlase was approximately 1.5, consistent with seismic models at depths from 670 to 1500 km, but showed an increased mismatch at lower mantle depths below ~1500 km. This discrepancy below mid-lower mantle could be due to either a contribution from chemically-induced heterogeneity or the effects of the Fe spin transition in the deeper parts of the Earth's lower mantle.

Hydrogen-rich superhydrides are believed to be very promising high-Tc superconductors. Recent experiments discovered superhydrides at very high pressures, e.g. FeH5 at 130 GPa and LaH10 at 170 GPa. With the motivation of discovering new hydrogen-rich high-Tc superconductors at lowest possible pressure, here we report the prediction and experimental synthesis of cerium superhydride CeH9 at 80–100 GPa in the laser-heated diamond anvil cell coupled with synchrotron X-ray diffraction. Ab initio calculations were carried out to evaluate the detailed chemistry of the Ce-H system and to understand the structure, stability and superconductivity of CeH9. CeH9 crystallizes in a P63/mmc clathrate structure with a very dense 3-dimensional atomic hydrogen sublattice at 100 GPa. These findings shed a significant light on the search for superhydrides in close similarity with atomic hydrogen within a feasible pressure range. Discovery of superhydride CeH9 provides a practical platform to further investigate and understand conventional superconductivity in hydrogen rich superhydrides., Hydrogen-rich superhydrides are promising high-temperature superconductors which have been observed only at pressures above 170 GPa. Here the authors show that CeH9 can be synthesized at 80-100 GPa with laser heating, and is characterized by a clathrate structure with a dense 3-dimensional atomic hydrogen sublattice.

Interest in studying power flow dynamics has grown in recent years, with new power flow diagnostics being developed at Sandia National Laboratories for the $Z$ Pulsed Power Facility. Presently, the only power flow loads that have been studied are cylindrical static or imploding loads that are driven by synchronous short pulse (100 ns rise time). Presented is a design that utilizes the dynamic materials properties program's stripline geometry in a high voltage pulsed shaped (asymmetric asynchronous) driving mode. This design has exhibited repeatable current loss with a large time-varying inductance that is well matched to the machine at pulse initialization but which triples to high inductance in 800 ns. Evidence is presented that plasma not captured in the magnetohydrodynamic approximation and ill represented by any of our existing predictive pulsed power codes is adversely affecting load current delivery. The authors believe this design could be of great interest to the experimental and modeling communities for studying power flow dynamics.

The Kohn anomaly and topological change of the Fermi surface in $d$-block metals can occur under high pressure with affiliated significant changes in elastic, mechanical, and transport properties. However, our understanding of their origin and associated physical phenomena remains limited both experimentally and theoretically. Here we study the pressure effect on the Kohn anomaly, electronic topological transition (ETT), and the associated anomalies in physical properties of body-centered cubic (bcc) single-crystal tantalum (Ta). The phonon dispersions of Ta crystal were directly measured up to $\ensuremath{\sim}47$ GPa using high-energy resolution inelastic x-ray scattering in a diamond anvil cell with hydrostatic helium medium. A Kohn anomaly in Ta was observed and became significantly stronger at 47.0 GPa at the reduced wave vector of $\ensuremath{\sim}0.7$ in the longitudinal acoustic mode along the [\ensuremath{\xi},0,0] direction. Our theoretical and experimental results indicate that the electron-phonon coupling and Fermi surface nesting mainly contribute to the Kohn anomaly, and the latter plays a dominant role at high pressures of 17--47 GPa. First-principles calculations further reveal an ETT with a topology change of the Fermi surface to occur at $\ensuremath{\sim}100$ GPa in Ta, which causes a softening in the elastic constants (${C}_{11}$ and ${C}_{44}$) and mechanical properties (shear, Young's, and bulk moduli). Our study shows that the $d$-orbital electrons in Ta play a key role in the stability of its electronic topological structure, where electron doping in Ta could significantly depress its ETT and elastic anomaly at high pressures. It is conceivable that our observed Kohn anomaly and ETT in a representative bcc Ta are much more prevalent in $d$-block transition metals under compression than previously thought.

A pressing challenge in engineering devices with topological insulators (TIs) is that electron transport is dominated by the bulk conductance, and so dissipationless surface states account for only a small fraction of the conductance. Enhancing the surface-to-volume ratio is a common method to enhance the relative contribution of such states. In thin films with reduced thickness, the confinement results in symmetry-breaking and is critical for the experimental observation of topologically protected surface states. We employ micro-Raman and tip-enhanced Raman spectroscopy to examine three different mechanisms of symmetry breaking in Bi2Te3 TI thin films: surface plasmon generation, charge transfer, and application of a periodic strain potential. These mechanisms are facilitated by semiconducting and insulating substrates that modify the electronic and mechanical conditions at the sample surface and alter the long-range interactions between Bi2Te3 and the substrate. We confirm the symmetry breaking in Bi2Te3 via the emergence of the Raman-forbidden $${{\boldsymbol{A}}}_{{\bf{1}}{\boldsymbol{u}}}^{{\bf{2}}}$$ A 1 u 2 mode. Our results suggest that topological surface states can exist at the Bi2Te3/substrate interface, which is in a good agreement with previous theoretical results predicting the tunability of the vertical location of helical surface states in TI/substrate heterostructures.

Because of their weak interlayer bonding, van der Waals (vdW) solids are very sensitive to external stimuli such as strain. Experimental studies of strain tuning of thermal properties in vdW solids have not yet been reported. Under $\ensuremath{\sim}9%$ cross-plane compressive strain created by hydrostatic pressure in a diamond anvil cell, we observed an increase of cross-plane thermal conductivity in bulk ${\mathrm{MoS}}_{2}$ from 3.5 to about $25\text{ }\text{ }\mathrm{W}\text{ }{\mathrm{m}}^{\ensuremath{-}1}\text{ }{\mathrm{K}}^{\ensuremath{-}1}$, measured with a picosecond transient thermoreflectance technique. First-principles calculations and coherent phonon spectroscopy experiments reveal that this drastic change arises from the strain-enhanced interlayer interaction, heavily modified phonon dispersions, and decrease in phonon lifetimes due to the unbundling effect along the cross-plane direction. The contribution from the change of electronic thermal conductivity is negligible. Our results suggest possible parallel tuning of structural, thermal, and electrical properties of vdW solids with strain in multiphysics devices.

The heat flux across the core-mantle boundary (Q(CMB)) is the key parameter to understand the Earth's thermal history and evolution. Mineralogical constraints of the Q(CMB) require deciphering contributions of the lattice and radiative components to the thermal conductivity at high pressure and temperature in lower mantle phases with depth-dependent composition. Here we determine the radiative conductivity (k(rad)) of a realistic lower mantle (pyrolite) in situusing an ultra-bright light probe and fast time-resolved spectroscopic techniques in laser-heated diamond anvil cells. We find that the mantle opacity increases critically upon heating to similar to 3000 K at 40-135 GPa, resulting in an unexpectedly low radiative conductivity decreasing with depth from similar to 0.8 W/m/K at 1000 km to similar to 0.35 W/m/K at the CMB, the latter being similar to 30 times smaller than the estimated lattice thermal conductivity at such conditions. Thus, radiative heat transport is blocked due to an increased optical absorption in the hot lower mantle resulting in a moderate CMB heat flow of similar to 8.5 TW,on the lower end of previous Q(CMB) estimates based on the mantle and core dynamics. This moderate rate of core cooling implies an inner core age of about 1 Gy and is compatible with both thermally- and compositionally-driven ancient geodynamo. (C) 2020 Elsevier B.V. All rights reserved.

We have developed a transient thermoreflectance technique using picosecond pulsed and cw laser to study thermal conductivity and interface conductance in both thin-films and bulk materials. A real time-resolved system observes a thermal transport along the cross-plane direction of the sample during a single pulse excitation. The suggested TTR technique can measure thermal conductivity in up to a few hundred nm of thin films with a reasonable uncertainty by carefully selecting metal transducer thickness. In this paper, we examine thermal conductivity in several substrates including Si, GaAs, Sapphire, and Glass after depositing Au thin film as metal transducer and compare with reported values to validate our technique. For further study on our method, MoS2 thin-films with different thicknesses are prepared via exfoliating, and their thermal conductivity are measured as average value of 3.4 W/mK. Compared to TDTR technique, TTR is a simpler and inexpensive method to study thermophysical properties and can also measure in-plane thermal property using a grating imaging technique. TTR can be one of the available options for observing thermal transport phenomena in both horizontal and vertical directions with a simple and inexpensive preparation., Comment: 18 pages

Seismic observations show a reduced compressional-wave gradient at the base of the outer core relative to the preliminary reference Earth model and seismic wave asymmetry between the east-west hemispheres at the top of the inner core. Here, we propose a model for the inner core boundary (ICB), where a slurry layer forms through fractional crystallization of an Fe alloy at the base of the outer core (F layer) above a compacting cumulate pile at the top of the inner core (F' layer). Using recent mineral physics data, we show that fractional crystallization of an Fe alloy (e.g., Fe-Si-O) with light element partitioning can explain the observed reduced velocity gradient in the F layer, in cases with a solid fraction of ~15(5)% in liquid with a compositional gradient due to preferential light element partitioning into liquid. The compacting cumulate pile in the F' layer may exhibit lateral variations in thickness between the east-west hemispheres due to lateral variations of large-scale heat flow in the outer core, which may explain the east-west asymmetry observed in the seismic velocity. Our interpretations suggest that the inner core with solid Fe alloy has a high shear viscosity of ~10^23 Pa s., Comment: v2, 39 pages with 10 figures

We examine electronic and crystal structures of iron-based superconductors LnFeAsO1−x H x (Ln = La, Sm) under pressure by means of x-ray absorption spectroscopy (XAS), x-ray emission spectroscopy (XES), and x-ray diffraction. In LaFeAsO the pre-edge peak on high-resolution XAS at the Fe-K absorption edge gains in intensity on the application of pressure up to 5.7 GPa and it saturates in the higher pressure region. We found integrated-absolute difference values on XES for Ln = La, corresponding to a spin state, decline on the application of pressure, and then it is minimized when the T c approaches the maximum at around 5 GPa. In contrast, such the optimum value was not detected for Ln = Sm. We reveal that the superconductivity is closely related to the lower spin state for Ln = La unlike Sm case. We observed that As height from the Fe basal plane and As–Fe–As angle on the FeAs4 tetrahedron for Ln = La deviate from the optimum values of the regular tetrahedron in superconducting (SC) phase, which has been widely accepted structural guide to SC thus far. In contrast, the structural parameters were held near the optimum values up to ∼15 GPa for Ln = Sm.

Earth’s lowermost mantle displays complex geological phenomena that likely result from its heterogeneous physical interaction with the core. Geophysical models of core-mantle interaction rely on the thermal and electrical conductivities of appropriate geomaterials which, however, have never been probed at representative pressure and temperature (P-T) conditions. Here we report on the opacity of single crystalline bridgmanite and ferropericlase and link it to their radiative and electrical conductivities. Our results show that light absorption in the visible spectral range is enhanced upon heating in both minerals but the rate of change in opacity with temperature is a factor of six higher in ferropericlase. As a result, bridgmanite in the lowermost mantle is moderately transparent while ferropericlase is highly opaque. Our measurements support previous indirect estimates of low (< 1 W/m/K) and largely temperature-independent radiative conductivity in the lowermost mantle. This implies that the radiative mechanism has not contributed significantly to cooling the Earth’s core throughout the geologic time. Opaque ferropericlase is electrically conducting and mediates strong core-mantle electromagnetic coupling, explaining the intradecadal oscillations in the length of day, low secular geomagnetic variations in Central Pacific, and the preferred paths of geomagnetic pole reversals.

Thermal and compositional convection in Earth's core are thought to be the main power sources driving geodynamo. The viability and strength of thermally and compositionally-driven convection over Earth's history depend on the adiabatic heat flow across the core-mantle boundary (CMB) which is governed by the thermal conductivity of a constituent Fe-Ni-light element alloy at the pressure-temperature (P-T) conditions relevant to the core. Silicon is often proposed to be an abundant light element alloyed with Fe along with ∼5 wt% Ni, but the thermal transport properties of Fe-Ni-Si alloys at high P-T remain largely uncertain. Here we measured the electrical resistivities of Fe-10wt%Ni and Fe-1.8wt%Si alloys up to ∼142 GPa and ∼3400 K using four-probe van der Pauw method in laser-heated diamond anvil cell experiments. Our results show that the resistivities of hcp-Fe-1.8Si and Fe-10Ni display quasi-linear temperature dependence from ∼1500 to 3400 K at each given high pressure. Addition of ∼2 wt% Si in hcp-Fe significantly increases its resistivity by ∼25% at ∼138 GPa and 4000 K, but Fe-10wt%Ni has similar resistivity to pure hcp-Fe at near CMB P-T conditions. Using our measured values of electrical resistivities, we model thermal conductivities via the Wiedemann-Franz law, giving a nominal thermal conductivity of ∼50 W m−1 K−1 for liquid Fe-5Ni-8Si alloy at the topmost outer core, implying an adiabatic (conductive) core heat flow of ∼8.0 TW. The outer core has a much lower thermal conductivity than the inner core due to light-element differentiation across the solidifying inner-core boundary. Our studies imply that the adiabatic core heat flow is low enough to enable thermal convection to drive the geodynamo over most and possibly all of Earth's history, while the strength of compositional convection increases with the inner-core growth and accounts for ∼83% of the buoyancy flux to the present-day geodynamo.

Graphene has great potential for fabrication of ultrafast opto-electronics, in which relaxation and transport of photoexcited carriers determine device performance. Even though ultrafast carrier relaxation in graphene has been studied vigorously, transport properties of photoexcited carriers in graphene are largely unknown. In this work, we utilize an ultrafast grating imaging technique to measure lifetime (τr), diffusion coefficient (D), diffusion length (L) and mobility (μ) of photoexcited carriers in mono- and multi-layer graphene non-invasively. In monolayer graphene, D∼10,000 cm2/s and μ∼120,000 cm2/V have been observed, both of which decrease drastically in multilayer graphene, indicating that the remarkable transport properties in monolayer graphene originate from its unique Dirac-Cone energy structure. Mobilities of photoexcited carriers measured here are several times larger than the Hall and Field-Effect mobilities reported in literature (

Hydrous magnesium silicate phase D plays a key role in the transport of water from the upper to the lower mantle via subducted slabs. Here we report pressure dependence hyperfine and lattice parameters of FeAl-bearing phase D up to megabar pressures using synchrotron nuclear forward scattering and X-ray diffraction in a diamond anvil cell at room temperature. FeAl-bearing phase D undergoes a two-stage high-spin to low-spin transition of iron for Fe2+ at 37–41 GPa and for Fe3+ at 64–68 GPa. These transitions are accompanied by an increase in density and a significant softening in the bulk modulus and bulk velocity at their respective pressure range. The occurrence of the dense low-spin FeAl-bearing phase D with relatively high velocity anisotropies in deep-subducted slabs can potentially contribute to small-scale seismic heterogeneities in the middle-lower mantle beneath the circum-Pacific area.

We investigated the combined effect of pressure and temperature on the elasticity of single-crystal superhydrous phase B (Shy-B) using Brillouin scattering and X-ray diffraction up to 12 GPa and 700 K. Using the obtained elasticity, we modeled the anisotropy of Shy-B along slab geotherms, showing that Shy-B has a low anisotropy and cannot be the major cause of the observed anisotropy in the region. Modeled velocities of Shy-B show that Shy-B will be shown as positive velocity anomalies at the bottom transition zone. Once Shy-B is transported to the topmost lower mantle, it will exhibit a seismic signature of lower velocities than topmost lower mantle. We speculate that an accumulation of hydrous phases, such as Shy-B, at the topmost lower mantle with a weight percentage of ~17–26% in the peridotite layer as subduction progresses could help explain the observed 2–3% low shear velocity anomalies in the region.

The new hexagonal aluminous phase, named the NAL phase, is expected to be stable at depths of

Spin and valence states of iron in single-crystal bridgmanite (Mg0.89Fe0.12Al0.11Si0.89O3) are investigated using X-ray emission and Mossbauer spectroscopies with laser annealing up to 115 GPa. The results show that Fe predominantly substitutes for Mg2+ in the pseudo-dodecahedral A site, in which 80% of the iron is Fe3+ that enters the lattice via the charge-coupled substitution with Al3+ in the octahedral B site. The total spin momentum and hyperfine parameters indicate that these ions remain in the high-spin state with Fe2+ having extremely high quadrupole splitting due to lattice distortion. (Al,Fe)-bearing bridgmanite is expected to contain mostly high-spin, A-site Fe3+, together with a smaller amount of A-site Fe2+, that remains stable throughout the region. Even though the spin transition of B-site Fe3+ in bridgmanite was reported to cause changes in its elasticity at high pressures, (Fe,Al)-bearing bridgmanite with predominantly A-site Fe will not exhibit elastic anomalies associated with the spin transition.

In this study, we have examined the lower mantle composition and mineralogy by modeling the density (ρ), bulk sound velocity (VΦ), and dlnρ/dlnVΦ profiles of candidate lower mantle minerals using literature and new experimental equation of state (EoS) results. For CaSiO3 perovskite, complimentary synchrotron X-ray diffraction measurements in a laser-heated diamond anvil cell were conducted up to 156 GPa between 1200 K and 2600 K to provide more reliable constraints on the thermal EoS parameters. These new experimental results as well as literature P-V-T data sets are systematically analyzed using an internally self-consistent pressure scale. We have modeled ρ, VΦ, and dlnρ/dlnVΦ profiles of the lower mantle with representative pyrolitic and chondritic compositional models in which the effect of Fe spin transition in ferropericlase is also taken into account. Our modeling results show that a pyrolitic lower mantle with an aggregate mineralogy of 75 vol % bridgmanite, 17 vol % ferropericlase, and 8 vol % CaSiO3 perovskite produces ρ and VΦ profiles in better agreement with preliminary reference Earth model than a lower mantle with a chondritic composition. The modeled ρ, VΦ, and dlnρ/dlnVΦ are mainly affected by the relative ratio of bridgmanite and ferropericlase but are not sensitive to the variation of the CaSiO3 perovskite content. In addition, the spin crossover of Fe in ferropericlase can greatly raise the value of dlnρ/dlnVΦ in the middle lower mantle, which is useful in detecting the presence of ferropericlase in the region. Based on these new mineral physical constraints and radial seismic structure, our study suggests the lower mantle is pyrolitic, which is chemically indistinguishable from the upper mantle.