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R. J. Hemley

Publications and source records attributed to R. J. Hemley.

18 recordsLinked to original sources
arXiv2026

Amorphous High Density Plutonium

Metastable aluminum-alloyed $δ$-plutonium shrinks rapidly and pure $α$-plutonium swells rapidly at 4 K. At ambient temperature alloyed $δ$-plutonium swells about $10^{-3}$ as fast as it shrinks at 4 K, but its bulk density decreases more slowly than would be inferred from the increase in its lattice parameter determined by X-ray diffraction. These results might be explained as the result of ingrowth of the opposite phases, but they have not been found in X-ray diffraction. The cryogenic results may be explained by ingrowth of an amorphous phase with density intermediate between those of $α$ and $δ$. This phase, when formed from alloyed $δ$-plutonium, rapidly but not instantaneously anneals to $δ$-plutonium at temperatures $\gtrapprox 100\,$K; when formed from pure $α$-plutonium it anneals to $α$ at similar temperatures. The room temperature discrepancy between growth of lattice parameter and length of $δ$-plutonium is also explained by ingrowth of a denser amorphous phase that is continuously formed and annealed out.

cond-mat.mtrl-sci↗
arXiv2025

Pressure tuning of Kitaev spin liquid candidate Na$_3$Co$_2$SbO$_6$

The search for Kitaev's quantum spin liquid (KQSL) state in real materials has recently expanded with the prediction that honeycomb lattices of divalent, high-spin cobalt ions could host the dominant bond-dependent exchange interactions required to stabilize the elusive entangled quantum state. The layered honeycomb Na$_3$Co$_2$SbO$_6$ has been singled out as a leading candidate provided that the trigonal crystal field acting on Co $3d$ orbitals, which enhances non-Kitaev exchange interactions between $J_{\rm eff}=\frac{1}{2}$ spin-orbital pseudospins, is reduced. We find that applied pressure leads to anisotropic compression of the layered structure, significantly reducing the trigonal distortion of CoO$_6$ octahedra. A strong enhancement of ferromagnetic correlations between pseudospins is observed in the spin-polarized (3 Tesla) phase up to about 60 GPa. Higher pressures drive a spin transition into a low-spin state destroying the $J_{\rm eff}=\frac{1}{2}$ local moments required to map the spin Hamiltonian into Kitaev's model. The spin transition strongly suppresses the low-temperature magnetic susceptibility and appears to stabilize a paramagnetic phase driven by frustration. Although applied pressure fails to realize a KQSL state, the possible emergence of frustrated magnetism of localized, low-spin $S=\frac{1}{2}$ moments opens the door for exploration of novel magnetic quantum states in compressed honeycomb lattices of divalent cobaltates.

cond-mat.str-el↗
arXiv2025

Novel electronic state of honeycomb iridate Cu$_2$IrO$_3$ at high pressure

Cu$_2$IrO$_3$ has attracted recent interest due to its proximity to the Kitaev quantum spin liquid state and the complex structural response observed at high pressures. We use x-ray spectroscopy and scattering as well as electrical transport techniques to unveil the electronic structure of Cu$_2$IrO$_3$ at ambient and high pressures. Despite featuring a $\mathrm{Ir^{4+}}$ $J_{\rm{eff}}=1/2$ state at ambient pressure, Ir $L_{3}$ edge resonant inelastic x-ray scattering reveals broadened electronic excitations that point to the importance of Ir $5d$-Cu $3d$ interaction. High pressure first drives an Ir-Ir dimer state with collapsed $\langle \mathbf{L} \cdot \mathbf{S} \rangle$ and $\langle L_z \rangle/\langle S_z \rangle$, signaling the formation of $5d$ molecular orbitals. A novel $\mathrm{Cu \to Ir}$ charge transfer is observed at the onset of phase 5 above 30 GPa at low temperatures, leading to an approximate $\mathrm{Ir^{3+}}$ and $\mathrm{Cu^{1.5+}}$ valence, with persistent insulating electrical transport seemingly driven by charge segregation of Cu 1+/2+ ions into distinct sites. Concomitant x-ray spectroscopy and scattering measurements through different thermodynamic paths demonstrate a strong electron-lattice coupling, with $J_{\rm{eff}}=1/2$ and $\mathrm{Ir^{3+}}$/$\mathrm{Cu^{1.5+}}$ electronic states occurring only in phases 1 and 5, respectively. Remarkably, the charge-transferred state can only be reached if Cu$_2$IrO$_3$ is pressurized at low temperature, suggesting that phonons play an important role in the stability of this phase. These results point to the choice of thermodynamic path across interplanar collapse transition as a key route to access novel states in intercalated iridates.

cond-mat.str-el↗
arXiv2024

Superconductivity in pressurized Re$_{0.10}$Mo$_{0.90}$B$_2$

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 structural phase transition from R3m to P6/mmm commences at 48 GPa, with the first signatures of superconductivity appearing above 44 GPa. The critical temperature is observed to increase with pressure. A complete resistive transition is observed only above 150 GPa, where the highest onset $T_c$ of 30 K is also achieved. Upon releasing pressure, the high pressure superconducting phase is found to be metastable. During unloading, a complete resistive superconducting transition is observed all the way down to 20 GPa (with onset $T_c \sim 20$ K). Our results suggest that the P6/mmm structure is responsible for the observed superconductivity.

cond-mat.supr-con↗
arXiv2024

Effect of low-temperature compression on superconductivity and crystal structure in strontium metal

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. Notably, the superconducting critical temperature exhibits a twofold increase under compression after cryogenic cooling within the pressure range of 35-42 GPa, compared to cryogenic cooling after room-temperature compression. Subsequently, the transition width becomes significantly sharper above 42 GPa. Low-temperature X-ray diffraction measurements under pressure reveal that this change corresponds to the Sr-III to Sr-IV transition, with no evidence of any metastable structure. Furthermore, the monoclinic Sr-IV structure was observed to remain stable to much higher pressures - at least up to 60 GPa, without the appearance of the incommensurate Sr-V phase present at room temperature. This implies that thermal activation energy plays an important role in overcoming the presence of a kinetic barrier to the Sr-V phase at room temperature.

cond-mat.supr-con↗
arXiv2023

Nb-substitution suppresses the superconducting critical temperature of pressurized MoB$_2$

A 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 (2022)]. The current work explores the high pressure superconducting behavior of Nb-substituted MoB$_2$ (Nb$_{0.25}$Mo$_{0.75}$B$_2$). High pressure x-ray diffraction measurements show that the sample remains in the ambient pressure $P6/mmm$ structure to at least 160 GPa. Electrical resistivity measurements demonstrate that from an ambient pressure $T_c$ of 8 K (confirmed by specific heat to be a bulk effect), the critical temperature is suppressed to 4 K at 50 GPa, before gradually rising to 5.5 K at 170 GPa. The critical temperature at high pressure is thus significantly lower than that found in MoB$_2$ under pressure (30 K), revealing that Nb-substitution results in a strong suppression of the superconducting critical temperature. Our calculations indeed find a reduced electron-phonon coupling in Nb$_{0.25}$Mo$_{0.75}$B$_2$, but do not account fully for the observed suppression, which may also arise from inhomogeneity and enhanced spin fluctuations.

cond-mat.supr-con↗
arXiv2022

Creating superconductivity in WB2 through pressure-induced metastable planar defects

High-pressure electrical resistivity measurements reveal that the mechanical deformation of ultra-hard WB2 during compression induces superconductivity above 50 GPa with a maximum superconducting critical temperature, Tc of 17 K at 90 GPa. Upon further compression up to 190 GPa, the Tc gradually decreases. Theoretical calculations show that electron-phonon mediated superconductivity originates from the formation of metastable stacking faults and twin boundaries that exhibit a local structure resembling MgB2} (hP3, space group 191, prototype AlB2). Synchrotron x-ray diffraction measurements up to 145 GPa} show that the ambient pressure hP12 structure (space group 194, prototype WB2) continues to persist to this pressure, consistent with the formation of the planar defects above 50 GPa. The abrupt appearance of superconductivity under pressure does not coincide with a structural transition but instead with the formation and percolation of mechanically-induced stacking faults and twin boundaries. The results identify an alternate route for designing superconducting materials.

cond-mat.supr-con↗
arXiv2021

High pressure study of low-Z superconductor Be$_{22}$Re

With $T_c \sim 9.6~\mathrm{K}$, Be$_{22}$Re exhibits one of the highest critical temperatures among Be-rich compounds. We have carried out a series of high-pressure electrical resistivity measurements on this compound to 30 GPa. The data show that the critical temperature $T_c$ is suppressed gradually at a rate of $dT_c/dP = -0.05~\mathrm{K/GPa}$. Using density functional theory (DFT) calculations of the electronic and phonon density of states (DOS) and the measured critical temperature, we estimate that the rapid increase in lattice stiffening in Be$_{22}$Re overwhelms a moderate increase in the electron-ion interaction with pressure, resulting in the decrease in $T_c$. High pressure x-ray diffraction measurements show that the ambient pressure crystal structure of Be$_{22}$Re persists to at least 154 GPa. We discuss the relationship between low-Z Be-rich superconductors and the high-$T_c$ superhydrides.

cond-mat.supr-con↗
arXiv2020

Hot Hydride Superconductivity above 550 K

The search for room temperature superconductivity has accelerated dramatically in the last few years driven largely by theoretical predictions that first indicated alloying dense hydrogen with other elements could produce conventional phonon-mediated superconductivity at very high temperatures and at accessible pressures, and more recently, with the success of structure search methods that have identified specific candidates and pressure-temperature (P-T) conditions for synthesis. These theoretical advances have prompted improvements in experimental techniques to test these predictions. As a result, experimental studies of simple binary hydrides under pressure have yielded high critical superconducting transition temperatures (Tc), of 260 K in LaH10, close to the commonly accepted threshold for room temperature, 293 K, at pressures near 180 GPa. We successfully synthesized a metallic La-based superhydride from La metal and ammonia borane, NH3BH3, and find a multi-step transition with a Tc of 294 K for the highest onset. When subjected to subsequent thermal excursions to higher temperatures that promoted a chemical reaction to what we believe is a ternary or higher order system, the transition temperature was driven to higher temperatures. Although the reaction does not appear to be complete, the onset temperature was pushed from 294 K to 556 K before the experiments had to be terminated. The results provide evidence for hot superconductivity well above room temperature, in line with recent predictions for a higher order hydride under pressure.

cond-mat.supr-con↗
arXiv2016

A molecular perspective on the limits of life: Enzymes under pressure

From a purely operational standpoint, the existence of microbes that can grow under extreme conditions, or "extremophiles", leads to the question of how the molecules making up these microbes can maintain both their structure and function. While microbes that live under extremes of temperature have been heavily studied, those that live under extremes of pressure have been neglected, in part due to the difficulty of collecting samples and performing experiments under the ambient conditions of the microbe. However, thermodynamic arguments imply that the effects of pressure might lead to different organismal solutions than from the effects of temperature. Observationally, some of these solutions might be in the condensed matter properties of the intracellular milieu in addition to genetic modifications of the macromolecules or repair mechanisms for the macromolecules. Here, the effects of pressure on enzymes, which are proteins essential for the growth and reproduction of an organism, and some adaptations against these effects are reviewed and amplified by the results from molecular dynamics simulations. The aim is to provide biological background for soft matter studies of these systems under pressure.

cond-mat.soft↗
arXiv2013

Sound velocities of hexagonal close-packed H2 and He under pressure

Bulk, shear, and compressional aggregate sound velocities of hydrogen and helium in the close- packed hexagonal structure are calculated over a wide pressure range using two complementary approaches: semi-empirical lattice dynamics based on the many-body intermolecular potentials and density-functional theory in the generalized gradient approximation. The sound velocities are used to calculate pressure dependence of the Debye temperature. The comparison between experiment and first-principle and semi-empirical calculations provide constraints on the density dependence of intermolecular interactions in the zero-temperature limit.

cond-mat.other↗
arXiv2009

Origin of Diffuse Scattering in Relaxor Ferroelectrics

High-pressure and variable temperature single crystal synchrotron X-ray measurements combined with first-principles based molecular dynamics simulations study diffuse scattering in the relaxor ferroelectric system PSN (PbSc$_{1/2}$Nb$_{1/2}$O$_3$). Constant temperature experiments show pressure induced transition to the relaxor phase at different temperatures characterized by butterfly and rod shaped diffuse scattering around the $\{$h00$\}$ and $\{$hh0$\}$ Bragg spots, respectively. The simulations reproduce the observed diffuse scattering features as well as their pressure-temperature behavior, and show that they arise from polarization correlations between chemically-ordered regions, which in previous simulations were shown to behave as polar nanoregions. Simulations also exhibit radial diffuse scattering (elongated towards and away from {\bf Q}=(000)), that persists even in the paraelectric phase, consistent with previous neutron experiments on (PbMg$_{1/3}$Nb$_{2/3}$O$_3$) (PMN)

cond-mat.mtrl-sci↗
arXiv2008

Layered Structures Favor Superconductivity in Compressed Solid SiH$_{4}$

The electronic and lattice dynamical properties of compressed solid SiH$_{4}$ have been calculated in the pressure range up to 300 GPa with density functional theory. We find that structures having a layered network with eight-fold SiH$_{8}$ coordination favor metallization and superconductivity. SiH$_{4}$ in these layered structures is predicted to have superconducting transition temperatures ranging from 20 to 80 K, thus presenting new possibilities for exploring high temperature superconductivity in this hydrogen-rich system.

cond-mat.supr-con↗
arXiv2001

Phonon-assisted electronic topological transition in MgB2 under pressure

We report measurements of the superconducting critical temperature Tc of polycrystalline MgB2 samples containing isotopically pure (10)B and (11)B under quasi-hydrostatic pressure conditions in He pressure media up to 44 GPa. Measurements to volume compressions V/V_0 ~ 0.82 allow us to observe a kink in the volume dependence of Tc for Mg(10)B2 (at 20 GPa) and Mg(11)B2 (at 15 GPa). The pressure dependence of the E(2g) mode also changes abruptly around 20 GPa for the Mg(10)B2 sample. The anharmonic character of the E(2g) phonon mode and anomalies in Tc pressure dependence are interpreted as the result of a phonon-assisted Lifshitz electronic topological transition.

cond-mat.supr-con↗
arXiv2001

Pressure Dependence of the Raman Spectrum, Lattice Parameters and Superconducting Critical Temperature of MgB2

We have observed a strongly broadened Raman band of MgB2 that shows anomalously large pressure dependence of its frequency. This band and its pressure dependence can be interpreted as the E2g zone center phonon, which is strongly anharmonic because of coupling to electronic excitations. The pressure dependence of Tc was measured to 14 GPa in hydrostatic conditions and can be explained only when a substantial pressure dependence of the Hopfield parameter h=N(0) ~(V0/V)^2.3(6)is taken into account.

cond-mat.supr-con↗
arXiv1999

Pressure induced high-spin to low-spin transition in FeS evidenced by x-ray emission spectroscopy

We report the observation of the pressure-induced high-spin to low-spin transition in FeS using new high-pressure synchrotron x-ray emission spectroscopy techniques. The transition is evidenced by the disappearance of the low-energy satellite in the Fe K$β$ emission spectrum of FeS. Moreover, the phase transition is reversible and closely related to the structural phase transition from a manganese phosphide-like phase to a monoclinic phase. The study opens new opportunities for investigating the electronic properties of materials under pressure.

cond-mat.mtrl-sci↗
arXiv1997

Quantum and Classical Orientational Ordering in Solid Hydrogen

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

cond-mat↗