Papers by Russell J. Hemley
Frontiers in Electronic Materials, 2022
The search for room temperature superconductivity has accelerated in the last few years driven by... more The search for room temperature superconductivity has accelerated in the last few years driven by experimentally accessible theoretical predictions that indicated alloying dense hydrogen with other elements could produce conventional superconductivity at high temperatures and pressures. These predictions helped inform the synthesis of simple binary hydrides that culminated in the discovery of the superhydride LaH10with a superconducting transition temperatureTcof 260 K at 180 GPa. We have now successfully synthesized a metallic La-based superhydride with an initialTcof 294 K. When subjected to subsequent thermal excursions that promoted a chemical reaction to a higher order system, theTconset was driven irreversibly to 556 K. X-ray characterization confirmed the formation of a distorted LaH10based backbone that suggests the formation of ternary or quaternary compounds with substitution at the La and/or H sites. The results provide evidence for hot superconductivity, aligning with re...
The Journal of Chemical Physics, 2021
X-ray diffraction indicates that the structure of the recently discovered carbonaceous sulfur hyd... more X-ray diffraction indicates that the structure of the recently discovered carbonaceous sulfur hydride (C–S–H) room-temperature superconductor is derived from previously established van der Waals compounds found in the H2S–H2 and CH4–H2 systems. Crystals of the superconducting phase were produced by a photochemical synthesis technique, leading to the superconducting critical temperature Tc of 288 K at 267 GPa. X-ray diffraction patterns measured from 124 to 178 GPa, within the pressure range of the superconducting phase, are consistent with an orthorhombic structure derived from the Al2Cu-type determined for (H2S)2H2 and (CH4)2H2 that differs from those predicted and observed for the S–H system at these pressures. The formation and stability of the C–S–H compound can be understood in terms of the close similarity in effective volumes of the H2S and CH4 components, and denser carbon-bearing S–H phases may form at higher pressures. The results are crucial for understanding the very hig...
Physical Review Letters, 2019
Recent predictions and experimental observations of high T c superconductivity in hydrogen-rich m... more Recent predictions and experimental observations of high T c superconductivity in hydrogen-rich materials at very high pressures are driving the search for superconductivity in the vicinity of room temperature. We have developed a novel preparation technique that is optimally suited for megabar pressure syntheses of superhydrides using modulated laser heating while maintaining the integrity of sample-probe contacts for electrical transport measurements to 200 GPa. We detail the synthesis and characterization of lanthanum superhydride samples, including fourprobe electrical transport measurements that display significant drops in resistivity on cooling up to 260 K and 180-200 GPa, and resistivity transitions at both lower and higher temperatures in other experiments. Additional current-voltage measurements, critical current estimates, and lowtemperature x-ray diffraction are also obtained. We suggest that the transitions represent signatures of superconductivity to near room temperature in phases of lanthanum superhydride, in good agreement with density functional structure search and BCS theory calculations.
Physical Review B, 2017
For decades, numerous attempts have been made to produce polar oxynitride perovskites, where some... more For decades, numerous attempts have been made to produce polar oxynitride perovskites, where some of the oxygen are replaced by nitrogen, but a polar ordered oxynitride has never been demonstrated. Caracas and Cohen studied possible ordered polar oxynitrides within density functional theory (DFT) and found a few candidates that were predicted to be insulating and at least metastable. YSiO 2 N stood out with huge predicted polarization and nonlinear optic coefficients. In this study, we demonstrate the synthesis of perovskite-structured YSiO 2 N by using a combination of a diamond anvil cell and in-situ laser heating technique. Subsequent in-situ X-ray diffraction, second harmonic generation, and Raman scattering measurements confirm that it is polar and a strong nonlinear optical material, with structure and properties similar to those predicted by DFT.
Physical Review Letters, 2017
High P-T Raman spectra of hydrogen in the vibron and lattice mode regions were measured up to 300... more High P-T Raman spectra of hydrogen in the vibron and lattice mode regions were measured up to 300 GPa and 900 K using externally heated diamond anvil cell techniques. A new melting line determined from the disappearance of lattice mode excitations was measured directly for the first time above 140 GPa. The results differ from theoretical predictions and extrapolations from lower pressure melting relations. In addition, discontinuities in Raman frequencies are observed as a function of pressure and temperature indicative of phase transition at these conditions. The appearance of a new Raman feature near 2700 cm-1 at ~300 GPa and 370 K indicates the transformation to a new crystalline phase. Theoretical calculations of the spectrum suggest the new phase is the proposed Cmca-4 metallic phase. The transition pressure is close to that of a recently reported transition observed on dynamic compression.
Treatise on Geophysics, 2015
Nonlinear optical methods 2.13.4.2 M€ ossbauer Spectroscopy 2.13.4.3 x-Ray and Neutron Diffractio... more Nonlinear optical methods 2.13.4.2 M€ ossbauer Spectroscopy 2.13.4.3 x-Ray and Neutron Diffraction 2.13.4.3.1 x-Ray diffraction 2.13.4.3.2 Neutron diffraction 2.13.4.4 Inelastic x-Ray Scattering and Spectroscopy 2.13.4.4.1 x-Ray absorption spectroscopy 2.13.4.4.2 x-Ray emission spectroscopy 2.13.4.4.3 x-Ray inelastic near-edge spectroscopy 2.13.4.4.4 x-Ray magnetic circular dichroism 2.13.4.4.5 Electronic inelastic x-ray scattering 2.13.4.4.6 Resonant inelastic x-ray spectroscopy 2.13.4.4.7 IXS spectroscopy 2.13.4.4.8 Compton scattering 2.13.4.4.9 Nuclear resonance forward scattering 2.13.4.4.10 Nuclear resonant inelastic x-ray scattering 2.13.4.4.11 Phonon inelastic x-ray scattering 2.13.4.5 Transport Measurements 2.13.4.5.1 Electrical conductivity 2.13.4.5.2 Magnetic susceptibility 2.13.4.6 Resonance Methods 2.13.4.6.1 Electron paramagnetic and electron spin resonance 2.13.4.6.2 Nuclear magnetic resonance 2.13.4.6.
Advances in High-Pressure Technology for Geophysical Applications, 2005
The laser-heated diamond anvil cell (LHDAC) technique is a uniquely powerful method for generatin... more The laser-heated diamond anvil cell (LHDAC) technique is a uniquely powerful method for generating the ultrahigh static pressures and temperatures (P. 100 GPa and T. 3000 K) found deep within planetary interiors. Here we show that the LHDAC technique can be used in conjunction with nuclear resonant inelastic X-ray scattering and synchrotron Mössbauer spectroscopy for studying magnetic, elastic, thermodynamic, and vibrational properties of 57 Fe-containing materials under high pressures and temperatures. A Nd:YLF laser, operating in continuous donut mode (TEM 01), has been used to heat a sample of 57 Fe-enriched hematite (Fe 2 O 3) and iron from both sides of a diamond cell. Temperatures of the laser-heated sample are measured by means of spectroradiometry and the detailed balance principle of the energy spectra. The detailed balance principle applied to the inelastic X-ray scattering spectra provides absolute temperatures of the laser-heated sample. When the sample was heated evenly on both sides, these temperatures were in very good agreement with values determined from the thermal radiation spectra fitted to the Planck radiation function. Synchrotron Mössbauer spectra and partial phonon density of states (PDOS) of Fe 2 O 3 have been obtained with these techniques up to 24 GPa and 1400 K, providing rich information for understanding physical properties of the sample under high pressures and temperatures. Time spectra of the synchrotron Mössbauer spectroscopy at 10 and 24 GPa upon laser heating reveal that Fe 2 O 3 undergoes a magnetic-to-nonmagnetic transition at 900 (^100) K and 1000 (^100) K, respectively, from a room-temperature magnetic state to a high-temperature nonmagnetic state. The PDOS of Fe 2 O 3 are shifted to lower energies at 1400 K, indicating the softening of the lattice vibrations at high temperatures. This study demonstrates a new arsenal of in situ probes to study magnetic, vibrational, elastic, and thermodynamic properties of 57 Fecontaining materials, such as metallic Fe, Fe alloys, and iron-bearing oxides and silicates [(Mg,Fe)O and (Mg,Fe)SiO 3 ], in the Earth's interior.
Physical Review B, 1993
Rotational and lattice phonon excitations in the Raman spectrum of solid molecular deuterium have... more Rotational and lattice phonon excitations in the Raman spectrum of solid molecular deuterium have been measured from 1.8 GPa to-200 GPa at 77-295 K to study pressure-induced changes in structural and dynamical properties of the dense solid. Continuous and discontinuous changes in three distinct pressure ranges are observed. At lower pressures ((30 GPa), there is a gradual increase in the linewidth of the So(0) band, together with gradually decreasing resolution of the higher-energy So(1) and So(2) features. At intermediate pressures (60-100 GPa), a change in the pressure dependence and linewidth of So(0), and a linewidth and intensity decrease in the E2~p honon band occur. This is interpreted as evidence for a phase transformation in the molecular solid beginning at-65 GPa (77 K). At the highest pressures (-160 GPa), abrupt changes in the low-frequency excitations suggest either an expansion of the molecular bond or a change in ordering at the vibron discontinuity. Evidence from the lowfrequency spectra for interaction between the deuterium sample and diamond anvil is also examined. ' ' In addition, a distinct isotope effect is observed in the pressure of the low-temperature 150-GPa
Advances in High-Pressure Technology for Geophysical Applications, 2005
We have built a micro-optical spectroscopy system coupled with a Nd:YLF laser heating system for ... more We have built a micro-optical spectroscopy system coupled with a Nd:YLF laser heating system for performing high pressure-temperature in situ Raman measurements in diamond anvil cells (DAC). A variety of materials can be investigated, providing information about structural and dynamical properties of condensed matter under extreme conditions. We report on a method for laser heating transparent samples using a metallic foil (Pt, Re, Mo, or W) as the infrared laser absorber (internal heating furnace) in the DAC. Metal foils of 5-15 mm in thickness with a small hole of 10-20 mm at the center are irradiated by the Nd:YLF laser beam directed into one side of the cell; the transparent sample in the small hole is uniformly heated and the Raman signals excited by an Ar þ or Kr þ laser are measured from the opposite side of the cell. The temperature of foil is measured by means of spectroradiometry, whereas the average temperature of sample is determined from the intensity ratios of Stokes/anti-Stokes pairs according to the principle of detailed balance. The average overall pairs give the sample temperature with the statistical accuracy of the Raman spectra, which is about 50-100 K. Transparent samples such as CO 2 have been heated up to 1600 K and 65 GPa, indicating the high efficiency of the internal metal furnace method. In situ Raman spectroscopy in the laser-heated DAC represents a powerful technique to characterize high P-T properties of materials including dense planetary gases and ices.
Physical Review Letters, 2013
Phase IV of dense solid hydrogen has been identified by its infrared spectrum using highpressure ... more Phase IV of dense solid hydrogen has been identified by its infrared spectrum using highpressure 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 (above 200 K), with the I-III-IV triple point located. Infrared transmission observed to the lowest photon energies measured place constraints on the electronic properties of the phase.
Reviews of Modern Physics, 1994
During the past five years, major progress has been made in the experimental study of solid hydro... more During the past five years, major progress has been made in the experimental study of solid hydrogen at ultrahigh pressures as a result of developments in diamond-cell technology. Pressures at which metallization has been predicted to occur have been reached (250-300 Gigapascals). Detailed studies of the dynamic, structural, and electronic properties of dense hydrogen reveal a system unexpectedly rich in physical phenomena, exhibiting a variety of transitions at ultrahigh pressures. This colloquium explores the study of dense hydrogen as an archetypal problem in condensed-matter physics.
Reviews in Mineralogy and Geochemistry, 2013
We briefly review theoretical methods used to examine dense carbon-bearing minerals, focusing on ... more We briefly review theoretical methods used to examine dense carbon-bearing minerals, focusing on first-principles or ab initio approaches. To compute the energies, one can choose one among a hierarchy of theoretical approximations. The energetics of the phases considered in this chapter have also been studied using quantum chemistry methods. These approaches have been applied, for example, to pure carbon phases (Guth 1990; Che et al. 1999). A leading approach is density functional theory (Hohenberg and Kohn 1964; Kohn and Sham 1965), which in principle is an exact quantum-mechanical theory, but in practice requires approximations, such as the LDA (local density approximation: Perdew and Wang 1992). Early LDA calculations proved successful in predicting the high-pressure behavior of carbon (e.g., Fahy et al. 1986; Fahy and Louie 1977). Recent extensions of the LDA include GGA (generalized gradient approximation: Perdew et al. 1996), meta-GGA (Tao et al. 2003), or higher-level approximations currently under development. The only approximate term in the equations is the exchange-correlation energy (the non-classical part of the electron-electron interaction energy), the most successful approximations of which are based on the properties of the electron gas, with more advanced approximations taking into account more non-local features, for example, gradient, Laplacian, or orbital kinetic energy density. The usual accuracy of such approximations as LDA and GGA is on the order of 1-2% for bond lengths and unit-cell parameters, where LDA usually underestimates and GGA overestimates bond lengths; ~15% for the elastic constants; and ~5% for vibrational frequencies. For phase transitions and chemical reactions, the GGA seems to perform much better than the LDA, with phase transition pressures accurate to within ~5 GPa (usually overestimated); however, for metal-insulator transitions errors of both approximations are typically much larger. For ionic and covalent materials and for normal metals (carbon allotropes and most carbonates and perhaps carbides belong to these classes) both LDA and GGA give good description of the structural properties and thermodynamics. Large errors in all compounds are documented for calculations of electronic excitation energies and band gaps (both LDA and GGA significantly underestimate band gaps); one must employ special methods, such as the GW method (Aryasetiawan and Gunnarsson 1998), to compute these parameters. Mott insulators represent a particular pathological case, where today's DFT too often gives unreasonable results. Until recently, DFT calculations could not adequately account for van der Waals interactions, but ways for incorporating these effects are now possible (Dion et al. 2004; Roman-Perez and Soler 2009).
Review of Scientific Instruments, 2004
We describe an in situ high pressure-temperature Raman technique for studying materials in laser-... more We describe an in situ high pressure-temperature Raman technique for studying materials in laser-heated diamond anvil cells using a Nd:YLF laser (1053 nm) as the heating source and an ion laser as the Raman exciting source. Here we introduce the method of laser heating transparent samples using a metallic foil (Pt,Re, or W) as the laser absorber (internal heating furnace) in a diamond cell. The YLF laser is used to effectively laser-heat one side of a metal foil 5–15 μm thick with a small hole of 10–20 μm in diameter at the center. The foil, in turn, heats a transparent sample while the Raman signals excited by an Ar+ or Kr+ laser are measured. Temperature of the laser-heated foil is measured by means of spectroradiometry whereas the average temperature of the heated sample is independently determined from the intensity ratios of the anti-Stokes/Stokes excitation pairs. The intrinsic temperature-dependent asymmetry of the Raman spectra arises from the principle of the detailed balan...
Proceedings of the National Academy of Sciences, 2010
The use of nanoscale x-ray probes overcomes several key limitations in the study of materials up ... more The use of nanoscale x-ray probes overcomes several key limitations in the study of materials up to multimegabar (> 200) pressures, namely, the spatial resolution of measurements of multiple samples, stress gradients, and crystal domains in micron to submicron size samples in diamond-anvil cells. Mixtures of Fe, Pt, and W were studied up to 282 GPa with 250–600 nm size synchrotron x-ray absorption and diffraction probes. The probes readily resolve signals from individual materials, between sample and gasket, and peak pressures, in contrast to the 5-μm-sized x-ray beams that are now becoming routine. The use of nanoscale x-ray beams also enables single-crystal x-ray diffraction studies in nominally polycrystalline samples at ultrahigh pressures, as demonstrated in measurements of (Mg,Fe)SiO 3 postperovskite. These capabilities have potential for driving a push toward higher maximum pressures and further miniaturization of high-pressure devices, in the process advancing studies at ...
Proceedings of the National Academy of Sciences, 2000
Mineral inclusions in diamonds provide an important source of information about the composition o... more Mineral inclusions in diamonds provide an important source of information about the composition of the continental lithosphere at depths exceeding 120–150 km, i.e., within the diamond stability field. Fossilized high pressures in coesite inclusions from a Venezuela diamond have been identified and measured by using laser Raman and synchrotron x-ray microanalytical techniques. Micro-Raman measurements on an intact inclusion of remnant vibrational band shifts give a high confining pressure of 3.62 (±0.18) GPa. Synchrotron single-crystal diffraction measurements of the volume compression are in accord with the Raman results and also revealed direct structural information on the state of the inclusion. In contrast to olivine and garnet inclusions, the thermoelasticity of coesite favors accurate identification of pressure preservation. Owing to the unique combination of physical properties of coesite and diamond, this “coesite-in-diamond” geobarometer is virtually independent of temperat...
Proceedings of the National Academy of Sciences, 2007
There is a great interest in electronic transitions in hydrogen-rich materials under extreme cond... more There is a great interest in electronic transitions in hydrogen-rich materials under extreme conditions. It has been recently suggested that the group IVa hydrides such as methane (CH 4 ), silane (SiH 4 ), and germane (GeH 4 ) become metallic at far lower pressures than pure hydrogen at equivalent densities because the hydrogen is chemically compressed in group IVa hydride compounds. Here we report measurements of Raman and infrared spectra of silane under pressure. We find that SiH 4 undergoes three phase transitions before becoming opaque at 27–30 GPa. The vibrational spectra indicate the material transforms to a polymeric (framework) structure in this higher pressure range. Room-temperature infrared reflectivity data reveal that the material exhibits Drude-like metallic behavior above 60 GPa, indicating the onset of pressure-induced metallization.
Proceedings of the National Academy of Sciences, 2011
Raman spectroscopy in laser-heated diamond anvil cells has been employed to probe the bonding sta... more Raman spectroscopy in laser-heated diamond anvil cells has been employed to probe the bonding state and phase diagram of dense hydrogen up to 140 GPa and 1,500 K. The measurements were made possible as a result of the development of new techniques for containing and probing the hot, dense fluid, which is of fundamental importance in physics, planetary science, and astrophysics. A pronounced discontinuous softening of the molecular vibron was found at elevated temperatures along with a large broadening and decrease in intensity of the roton bands. These phenomena indicate the existence of a state of the fluid having significantly modified intramolecular bonding. The results are consistent with the existence of a pressure-induced transformation in the fluid related to the presence of a temperature maximum in the melting line as a function of pressure.
Physics Letters A, 1992
We report observations ofan intense Raman peak at 240 cm-'that appears abruptly above 150 GPa at ... more We report observations ofan intense Raman peak at 240 cm-'that appears abruptly above 150 GPa at low temperatures (e.g., 77 K) in diamond-cell samples of hydrogen and deuterium. The band disappears above a critical temperature. The spectral changes are interpreted as evidence for a new high-pressure transformation.
Physical Review Letters, 2012
Diamond anvil cell techniques have been developed to confine and measure hydrogen samples under s... more Diamond anvil cell techniques have been developed to confine and measure hydrogen samples under static conditions to pressures above 300 GPa from 12 to 300 K using synchrotron infrared and optical absorption techniques. A decreasing absorption threshold in the visible is observed, but the material remains transparent down to 0.1 eV to 360 GPa over a broad temperature range. The persistence of the strong infrared absorption of the vibron characteristic of phase III indicates stability of the paired state of hydrogen. There is no evidence for the predicted metallic state over these conditions, in contrast to recent reports, but electronic properties consistent with semimetallic behavior are observed in phase III.
Physical Review Letters, 1993
High-pressure optical and x-ray studies of H2-H20 mixtures have revealed the formation of the fir... more High-pressure optical and x-ray studies of H2-H20 mixtures have revealed the formation of the first hydrogen clathrate hydrates. A rhombohedral hydrate with a H20 sublattice similar to ice II and a H2. HzO 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 H20 molecules in a cubic diamond structure and with a very high H2.'H20 stoichiometry of I:l. The H2 molecules occupy voids in the H20 framework, thus improving the packing eSciency and stabilizing this hydrate to very high pressures of at least 30 GPa.
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Papers by Russell J. Hemley