Russell J Hemley

• Ph.D.
• Professor at George Washington University

We present the successful synthesis and characterization of a one-dimensional high-entropy oxide (1D-HEO) exhibiting nanoribbon morphology. These 1D-HEO nanoribbons exhibit high structural stability at elevated temperatures (to 1000°C), elevated pressures (to 12 gigapascals), and long exposure to harsh acid or base chemical environments. Moreover,...

The high pressure behavior of hydrazine, N2H4, has been investigated to 50 GPa at room temperature using infrared and Raman spectroscopy to explore pressure induced phase transitions and changes in hydrogen bonding. Three solid–solid phase transitions were detected at 11, 21, and 32 GPa on room temperature compression through dramatic changes in th...

In light of breakthroughs in superconductivity under high pressure, and considering that record critical temperatures (T c s) across various systems have been achieved under high pressure, the primary challenge for higher T c should no longer solely be to increase T c under extreme conditions but also to reduce, or ideally eliminate, the need for a...

In light of breakthroughs in superconductivity under high pressure, and considering that record critical temperatures (T$_c$s) across various systems have been achieved under high pressure, the primary challenge for higher Tc should no longer solely be to increase T$_c$ under extreme conditions but also to reduce, or ideally eliminate, the need for...

Y$_2$Co$_3$ is a newly discovered antiferromagnetic (AFM) compound with distorted kagome layers. Previous investigations via bulk magnetization measurements suggested a complex noncollinear magnetic behavior, with magnetic moments primarily anti-aligned along the $b$ axis and some canting towards the $ac$ plane. In this study, we report the magneti...

Y2⁢Co3 is a newly discovered antiferromagnetic (AFM) compound with distorted kagome layers. Previous investigations via bulk magnetization measurements suggested a complex noncollinear magnetic behavior, with magnetic moments primarily anti-aligned along the b axis and some canting towards the ac plane. In this study, we report the magnetic structu...

The abrupt drop of resistance to zero at a critical temperature is a key signature of the current paradigm of the metal–superconductor transition. However, the emergence of an intermediate bosonic insulating state characterized by a resistance peak preceding the onset of the superconducting transition has challenged this traditional understanding....

We study ferroelectricity in the classic perovskite ferroelectric PbTiO_{3} to high pressures with density functional theory (DFT) and experimental diamond-anvil techniques. We use second harmonic generation spectroscopy to detect lack of inversion symmetry. Consistent with early understanding and experiments, we find that ferroelectricity disappea...

Recent band structure calculations have suggested the potential for band tuning in the chiral semiconductor Ag3AuTe2 to zero upon application of negative strain. In this study, we report on the synthesis of polycrystalline Ag3AuTe2 and investigate its transport and optical properties and mechanical compressibility. Transport measurements reveal the...

While hydrogen-rich materials have been demonstrated to exhibit high T c superconductivity at high pressures, there is an ongoing search for ternary, quaternary, and more chemically complex hydrides that achieve such high critical temperatures at much lower pressures. First-principles searches are impeded by the computational complexity of solving...

Rare-earth trihydrides ($R$H$_3$) exhibit intriguing coupled electronic and structural properties as a function of doping, hydrogen vacancies, and thermodynamic conditions. Theoretical studies of these materials typically rely on density functional theory (DFT), including the use of small supercells that may underestimate strong correlation effects...

The recent surprising discovery of superconductivity with critical temperature $T_c$ = 32 K in MoB$_2$ above 70 GPa has led to the search for related materials that may superconduct at similarly high $T_c$ values and lower pressures. We have studied the superconducting and structural properties of Re$_{0.10}$Mo$_{0.90}$B$_2$ to 170 GPa. A structura...

The superconducting and structural properties of elemental strontium metal were investigated under pressures up to 60 GPa while maintaining cryogenic conditions during pressure application. Applying pressure at low temperatures reveals differences in superconducting and structural phases compared to previous reports obtained at room temperatures. N...

Amber is a unique example of a fragile glass that has been extensively aged below its glass transition temperature, thus reaching a state that is not accessible under normal experimental conditions. We studied the medium-range order of Baltic amber by x-ray diffraction (XRD) at high pressures. The pressure dependences of the low-angle XRD intensity...

X-ray diffraction measurements of equiatomic B2-ordered FeV were performed in a diamond-anvil cell at room temperature at several pressure points up to 80 GPa that showed the cubic phase to be stable with no indication of structural phase transitions. Density functional theory at 0 K predicts Fermi surface nesting, an electronic topological transit...

The recent theory-driven discovery of a class of clathrate hydrides (e.g., CaH 6 , YH 6 , YH 9 , and LaH 10 ) with superconducting critical temperatures ( T c ) well above 200 K has opened the prospects for “hot” superconductivity above room temperature under pressure. Recent efforts focus on the search for superconductors among ternary hydrides th...

Recent band structure calculations have suggested the potential for band tuning in a chiral semi-conductor, Ag$_3$AuTe$_2$, to zero upon application of negative strain. In this study, we report on the synthesis of polycrystalline Ag$_3$AuTe$_2$ and investigate its transport, optical properties, and pressure compatibility. Transport measurements rev...

Symmetry-protected band degeneracy, coupled with a magnetic order, is the key to realizing novel magnetoelectric phenomena in topological magnets. While the spin-polarized nodal states have been identified to introduce extremely-sensitive electronic responses to the magnetic states, their possible role in determining magnetic ground states has rema...

Hydrogen (H2) and helium (He), the most abundant elements in the universe, pose a unique challenge in measuring the equation of state of the mixture, owing to their differing physical properties. There remains a need for data with high enough precision to discriminate between existing equation of state (EOS) mix models in order to understand the in...

First-principles density functional theory (DFT) calculations of supercell structures based on N-doped Fm3¯mLuH3 reveal configurations of Fm3¯mLu8H23−xN that exhibit novel electronic properties such as flat bands, sharply peaked densities of states (van Hove singularities, vHs), and intersecting Dirac cones near the Fermi energy (EF). These electro...

High-pressure studies on elements play an essential role in superconductivity research, with implications for both fundamental science and applications. Here we report the experimental discovery of surprisingly low pressure driving a novel germanium allotrope into a superconducting state in comparison to that for α-Ge. Raman measurements revealed...

Brillouin scattering spectroscopy has been used to obtain an accurate (<1%) ρ−P equation of state (EOS) of 1:1 and 9:1 H2−He molar mixtures from 0.5 to 5.4 GPa at 296 K. Our calculated equations of state indicate close agreement with the experimental data right to the freezing pressure of hydrogen at 5.4 GPa. The measured velocities agree on averag...

A recent study has demonstrated that MoB2, transforming to the same structure as MgB2 (P6/mmm), superconducts at temperatures above 30 K near 100 GPa [C. Pei et al., Natl. Sci. Rev. 10, nwad034 (2023)], and Nb substitution in MoB2 stabilizes the P6/mmm structure down to ambient pressure [A. C. Hire et al., Phys. Rev. B 106, 174515 (2022)]. The curr...

We report the P-V-T equation of state measurements of B4C to 50 GPa and approximately 2500 K in laser-heated diamond anvil cells. We obtain an ambient temperature, third-order Birch–Murnaghan fit to the P-V data that yields a bulk modulus K0 of 221(2) GPa and derivative, (dK/dP)0 of 3.3(1). These were used in fits with both a Mie–Grüneisen–Debye mo...

We employed high-pressure Brillouin scattering to study the pressure dependencies of acoustic modes of glycerol up to 14 GPa at 300 K. We observed longitudinal acoustic velocities and transverse acoustic velocities for the first time from 5 to 14 GPa. The results allow the determination of a complete set of elastic properties and an accurate determ...

Pressure is a unique tuning parameter for probing the properties of materials, and it has been particularly useful for studies of electronic materials such as high-temperature cuprate superconductors. Here we report the effects of quasihydrostatic compression produced by a neon pressure medium on the structures of bismuth-based high-Tc cuprate supe...

The recent report of superconductivity in nitrogen-doped lutetium hydride at near-ambient pressures and temperatures has attracted great attention but also continuing controversy. Several experimental groups have reported no observation of superconductivity at these conditions in Lu-N-H samples they have prepared. To address this issue, we have car...

First-principles density functional theory (DFT) calculations of Lu-H-N compounds reveal low-energy configurations of Fm$\overline{3}$m Lu$_{8}$H$_{23-x}$N structures that exhibit novel electronic properties such as flat bands, sharply peaked densities of states (van Hove singularities), and intersecting Dirac cones near the Fermi energy (E$_F$). T...

The strongly correlated material FeSi exhibits several unusual thermal, magnetic, and structural properties under varying pressure-temperature (P-T) conditions. It is a potential thermoelectric alloy and a materials of several geological implications as a possible constituent at the Earth's core mantle boundary (CMB). The phase transition behavior...

Recent work has demonstrated that MoB$_2$, transforming to the same structure as MgB$_2$ ($P6/mmm$), superconducts at temperatures above 30 K near 100 GPa [C. Pei $et$ $al$. Natl. Sci. Rev., nwad034 (2023)], and Nb-substitution in MoB$_2$ stabilizes the $P6/mmm$ structure down to ambient pressure [A. C. Hire $et$ $al$. Phys. Rev. B 106, 174515 (202...

The phenomenon of high temperature superconductivity, approaching room temperature, has been realized in a number of hydrogen-dominant alloy systems under high pressure conditions1-12. A significant discovery in reaching room temperature superconductivity is the photo-induced reaction of sulfur, hydrogen, and carbon that initially forms of van der...

Most metals adopt simple structures such as body-centered cubic (BCC), face-centered cubic (FCC), and hexagonal close-packed (HCP) structures in specific groupings across the periodic table, and many undergo transitions to surprisingly complex structures on compression, not expected from conventional free-electron-based theories of metals. First-pr...

High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB 2 during compression induces superconductivity above 50 GPa with a maximum superconducting critical temperature, T c of 17 K at 91 GPa. Upon further compression up to 187 GPa, the T c gradually decreases. Theoretical calculations show that elec...

Pressure is a unique tuning parameter for probing the properties of materials and has been particularly useful for studies of electronic materials such as high-temperature cuprate superconductors. Here we report the effects of quasi-hydrostatic compression produced by a neon pressure-medium on the structures of bismuth-based high $\mathit{T_c}$ cup...

Recently, room temperature superconductivity was measured in a carbonaceous sulfur hydride material whose identity remains unknown. Herein, first-principles calculations are performed to provide a chemical basis for structural candidates derived by doping H 3 S with low levels of carbon. Pressure stabilizes unusual bonding configurations about the...

Achieving room-temperature superconductivity has been an enduring scientific pursuit driven by broad fundamental interest and enticing potential applications. The recent discovery of high-pressure clathrate superhydride LaH10 with superconducting critical temperatures (Tc) of 250-260 K made it tantalizingly close to realizing this long-sought goal....

Solid molecular hydrogen has been predicted to be metallic and high-temperature superconducting at ultrahigh hydrostatic pressures that push current experimental limits. Meanwhile, little is known about the influence of nonhydrostatic conditions on its electronic properties at extreme pressures where anisotropic stresses are inevitably present and...

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 re...

Despite years of intense study, the superconducting mechanism seen in the cuprates is not yet fully understood. In recent years pressure has been used as a tuning parameter to probe the unusual electronic properties of cuprate superconductors, such as anomalous changes in Tc observed in the bismuth-based cuprates. We report the effects of quasi-hyd...

Designing materials with advanced functionalities is the main focus of contemporary solid-state physics and chemistry. Research efforts worldwide are funneled into a few high-end goals, one of the oldest, and most fascinating of which is the search for an ambient temperature superconductor (A-SC). The reason is clear: superconductivity at ambient c...

Terapascal iron-melting temperature The pressure and temperature conditions at which iron melts are important for terrestrial planets because they determine the size of the liquid metal core, an important factor for understanding the potential for generating a radiation-shielding magnetic field. Kraus et al . used laser-driven shock to determine th...

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 c...

Recent theoretical and experimental studies of hydrogen-rich materials at megabar pressures (i.e., >100 GPa) have led to the discovery of very high-temperature superconductivity in these materials. Lanthanum superhydride LaH10 has been of particular focus as the first material to exhibit a superconducting critical temperature (Tc) near room tempera...

Significance Superhydrides are a materials system where near–room-temperature superconductivity has been achieved but only at very high (megabar) pressures. This work proposes an approach that combines pressure and electrochemistry to stabilize superhydrides at moderate pressures. Through a computational study of the palladium–hydrogen system, we c...

The study of superconductivity in compressed hydrides is of great interest due to measurements of high critical temperatures (Tc) in the vicinity of room temperature, beginning with the observations of LaH10 at 170–190 GPa. However, the pressures required for synthesis of these high-Tc superconducting hydrides currently remain extremely high. Here...

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 th...

First-principles calculations were carried out to provide a chemical basis for proposed structures associated with the recently reported room-temperature superconductivity in a carbonaceous sulfur hydride material under pressure. Calculations were performed on supercells of H$_3$S doped with 1.85-25\% carbon, corresponding to SH$_3$~$\rightarrow$~C...

The study of superconductivity in compressed hydrides is of great interest due to measurements of high critical temperatures (Tc) in the vicinity of room temperature, beginning with the observations of LaH10 at 170-190 GPa. However, the pressures required for synthesis of these high Tc superconducting hydrides currently remain extremely high. Here...

X-ray diffraction indicates that the structure of the recently discovered room temperature carbonaceous sulfur hydride (C-S-H) superconductor is derived from previously established van der Waals compounds found in the H$_2$S-H$_2$ and CH$_4$-H$_2$ systems. Crystals of the superconducting phase were produced by a photochemical synthesis technique le...

Evolutionary crystal structure prediction searches have been employed to explore the ternary Li–F–H system at 300 GPa. Metastable phases were uncovered within the static lattice approximation, with LiF3H2, LiF2H, Li3F4H, LiF4H4, Li2F3H, and LiF3H lying within 50 meV/atom of the 0 K convex hull. All of these phases contain HnFn+1− (n = 1, 2) anions...

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 an...

Palladium hydride alloys are superconductors and hydrogen storage materials. One synthesis route is compression of Pd to high pressure in a hydrogen-rich environment. Here we report the evolution of the unit cell volume of PdHx synthesized by compressing Pd in a pure H2 medium to pressures from 0.2 to 8 GPa in a diamond anvil cell at room temperatu...

Evolutionary crystal structure prediction searches have been employed to explore the ternary Li-F-H system at 300 GPa. Metastable phases were uncovered within the static lattice approximation, with LiF$_3$H$_2$, LiF$_2$H, Li$_3$F$_4$H, LiF$_4$H$_4$, Li$_2$F$_3$H and LiF$_3$H lying within 50 meV/atom of the 0 K convex hull. All of these phases conta...

Recent theoretical and experimental studies of hydrogen-rich materials at megabar pressures (i.e., >100 GPa) have led to the discovery of very high-temperature superconductivity in these materials. Lanthanum superhydride LaH$_{10}$ has been of particular focus as the first material to exhibit a superconducting critical temperature (T$_c$) near room...

Significance Diamonds are among the most important samples of the solid Earth owing to the unique information they provide about the planet’s interior. New analytical techniques have enabled the discovery of distinct inclusions in diamond hosts having mineral associations that constrain the mineralogy at great depths within the Earth. Currently, ex...

Semiconductor optical fibers encapsulated in a protective diamond coating can theoretically lead to immense power handling capabilities and infrared functionality. Here, silicon optical fibers are grown using high pressure chemical vapor deposition before being coated by 50 μm–300 μm of diamond by microwave plasma-assisted chemical vapor deposition...

Palladium-hydrogen is a widely-studied material system with the highest hydride phase being Pd$_3$H$_4$. Recently, superhydrides (MH$_n$ with $n>6$) have been computationally identified and synthesized with rare-earth and early transition metals under pressure. In this work, we evaluate the possibility of electrochemically synthesizing palladium su...

While exploring potential superconductors in the C-S-H ternary system using first-principles crystal structure prediction methods, we uncovered a class of hydride perovskites based on the intercalation of methane into an H3S framework. These intriguing H3S−CH4 structures emerge as metastable at ∼100 GPa. Electron-phonon coupling calculations indica...

Significance Understanding the interior structure and dynamics of outer Solar System planets in terms of their component materials is a major scientific challenge. A highly intriguing case concerns the anomalous nondipolar and nonaxisymmetric magnetic fields of Uranus and Neptune. A thin-shell dynamo model has been shown to capture observed phenome...

The use of high pressure to realize superconductivity in the vicinity of room temperature has a long history, much of it focused on achieving this in hydrogen rich materials. This paper provides a brief overview of the work presented at this May 2018 conference, together with background on motivation and techniques, the theoretical predictions of s...

In their comment, Desjarlais et al . claim that a small temperature drop occurs after isentropic compression of fluid deuterium through the first-order insulator-metal transition. We show that their calculations do not correspond to the experimental thermodynamic path, and that thermodynamic integrations with parameters from first-principles calcul...

Determining the effects of extreme conditions on proteins from “extremophilic” and mesophilic microbes is important for understanding how life adapts to living at extremes as well as how extreme conditions can be used for sterilization and food preservation. Previous molecular dynamics simulations of dihydrofolate reductase (DHFR) from a psychropie...

Determining how enzymes in piezophilic microbes function at high pressure can give insights into how life adapts to living at high pressure. Here, the effects of pressure and temperature on loop motions of Escherichia coli (Ec) and Moritella profunda (Mp) dihydrofolate reductase (DHFR) are compared via molecular dynamics simulations at combinations...

Recent predictions and experimental observations of high Tc 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 las...

Pressure-induced phase transitions in single-crystal PbZr0.54Ti0.46O3 are investigated with high-pressure Raman scattering and x-ray single crystal and powder diffraction. The appearance of a Raman peak near 380 cm⁻¹ indicates a structural transition at 3 GPa. A second transition, driven by an soft optical phonon, occurs at 9 GPa. A third transitio...

Recent computational studies have predicted that rare-earth superhydrides are promising high-temperature superconductors. Of these phases having very high hydrogen content (XHn,n>6, where X is the rare-earth atom) a cubic phase of lanthanum hydride, recently synthesized at 170 GPa and identified as LaH10±x, is in good agreement with theoretical pre...

Recent predictions and experimental observations of high Tc 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 pulsed laser...

Laser-shocking deuterium into metal The conditions in which hydrogen disassociates and becomes an atomic metal occur in high-energy-density environments, such as the interiors of giant planets and nuclear explosions. Celliers et al. trained 168 lasers on deuterium samples at the National Ignition Facility to measure the pressure and temperature con...

Two new polyhydrides of calcium have been synthesized at high pressures and high temperatures and characterized by Raman spectroscopy, infrared spectroscopy, and synchrotron x-ray diffraction. Above 20 GPa and 700 K, we synthesize a phase having a monoclinic (C2/m) structure with Ca2H5 composition, which is characterized by a distinctive vibration...

Brillouin scattering was performed on an elastomeric polyurea, an important technological polymer. Being widely used for impact modification, of particular interest is its response to extreme pressure conditions. We applied pressures up to 13.5 GPa using a diamond anvil cell and measured the longitudinal and transverse sound velocities via Brilloui...

The effects of hydrostatic pressure at ambient temperature on the structural and dielectric properties of PbFe0.5Nb0.5O3 (PFN) were investigated using second harmonic generation (SHG) and powder x-ray diffraction measurements to 31 GPa. The results demonstrate that PFN undergoes a pressure-induced structural transition from the R3m ferroelectric to...

A quasiharmonic analysis (QHA) method is used to compare the potential energy landscapes of dihydrofolate reductase (DHFR) from a piezophile (pressure-loving organism), Moritella profunda (Mp), and a mesophile, Escherichia coli (Ec). The QHA method considers atomic fluctuations of the protein as motions of an atom in a local effective potential cre...

High pressure x-ray diffraction of PdHx and PdDx demonstrate that these materials remain in a face-centered cubic (fcc, Fm3 ̅m) structure to these pressures at room temperature. The volumes indicate stoichiometric compositions under pressure with x = 1 for both materials. No indications of phase transitions were observed up to the highest pressures...

Recent theoretical calculations predict that megabar pressure stabilizes very hydrogen-rich simple compounds having new clathrate-like structures and remarkable electronic properties including room-temperature superconductivity. We report x-ray diffraction and optical studies demonstrating that superhydrides of lanthanum can be synthesized with La...

Recent theoretical calculations predict that megabar pressure stabilizes very hydrogen-rich simple compounds having new clathrate-like structures and remarkable electronic properties including room-temperature superconductivity. We report x-ray diffraction and optical studies demonstrating that superhydrides of lanthanum can be synthesized with La...

Positional fluctuations of an atom in a protein can be described as motion in an effective local energy minimum created by the surrounding protein atoms. The dependence of atomic fluctuations on both temperature (T) and pressure (P) has been used to probe the nature of these minima, which are generally described as harmonic in experiments such as x...

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 prediction...

Recent high-pressure studies have uncovered an alternative class of materials, insulating electride phases created by compression of simple metals. These exotic insulating phases develop an unusual electronic structure: the valence electrons move away from the nuclei and condense at interstitial sites, thereby acquiring the role of atomic anions or...

A systematic structure search in the La-H and Y-H systems under pressure reveals some hydrogen-rich structures with intriguing electronic properties. For example, LaH10 is found to adopt a sodalite-like face-centered cubic (fcc) structure, stable above 200 GPa, and LaH8 a C2/m space group structure. Phonon calculations indicate both are dynamically...

The pressure dependence of the Boson peak (BP) of glycerol, including its behavior across the liquid-glass transition, has been studied under pressure using Raman scattering. A significant hardening of the BP was observed with increasing pressure up to 11 GPa at room temperature. The pressure dependence of BP frequency νBP is proportional to (1+P/P...

The validity of simple effective oscillator models for extracting band gap energies from frequency-dependent refractive index data of high pressure ice and solid hydrogen was assessed through comparisons of theoretical dielectric response functions obtained from Bethe–Salpeter equation and band gaps calculated from the GW method. For model structur...