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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Cobalt-based pyroxenes: A new playground for Kitaev physics

Recent advances in the study of cobaltites have unveiled their potential as a promising platform for realizing Kitaev physics in honeycomb systems and the Ising model in weakly coupled chain materials. In this manuscript, we explore the magnetic properties of pyroxene SrCoGe 2 O 6 using a combination of neutron scattering, ab initio methods, and linear spin-wave theory. Through careful examination of inelastic neutron scattering powder spectra, we propose a modified Kitaev model to accurately describe the twisted chains of edge-sharing octahedra surrounding Co 2+ ions. The extended Kitaev–Heisenberg model, including a significant anisotropic bond-dependent exchange term with K/|J| = 0.96, is identified as the key descriptor of the magnetic interactions in SrCoGe 2 O 6 . Furthermore, our heat capacity measurements reveal an effect of an external magnetic field (approximately 13 T) which shifts the system from a fragile antiferromagnetic ordering with T N = 9 K to a field-induced state. We argue that pyroxenes, particularly those modified by substituting Ge with Si and its less extended p orbitals, emerge as a platform for the Kitaev model. This opens up possibilities for advancing our understanding of Kitaev physics.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Non-magnetic ion site disorder effects on the quantum magnetism of a spin-1/2 equilateral triangular lattice antiferromagnet

Here, with the motivation to study how non-magnetic ion site disorder affects the quantum magnetism of Ba 3 CoSb 2 O 9 , a spin-1/2 equilateral triangular lattice antiferromagnet, we performed DC and AC susceptibility, specific heat, elastic and inelastic neutron scattering measurements on single crystalline samples of Ba 2.87 Sr 0.13 CoSb 2 O 9 with Sr doping on non-magnetic Ba 2+ ion sites. The results show that Ba 2.87 Sr 0.13 CoSb 2 O 9 exhibits (i) a two-step magnetic transition at 2.7 K and 3.3 K, respectively; (ii) a possible canted 120 degree spin structure at zero field with reduced ordered moment as 1.24 μ B /Co; (iii) a series of spin state transitions for both H∥ab-plane and H∥c-axis. For H∥ab-plane, the magnetization plateau feature related to the up–up–down phase is significantly suppressed; (iv) an inelastic neutron scattering spectrum with only one gapped mode at zero field, which splits to one gapless and one gapped mode at 9 T. All these features are distinctly different from those observed for the parent compound Ba 3 CoSb 2 O 9 , which demonstrates that the non-magnetic ion site disorder (the Sr doping) plays a complex role on the magnetic properties beyond the conventionally expected randomization of the exchange interactions. We propose the additional effects including the enhancement of quantum spin fluctuations and introduction of a possible spatial anisotropy through the local structural distortions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

High-pressure synthesis and neutron scattering study of tantalum hydride TaH 1.23(5) and a tantalum polymorph with A15-type structure

A unique type of tantalum hydride was synthesized by exposing tantalum dihydride to the high hydrogen pressure of 9 GPa and a temperature of using toroid-type high-pressure chambers. The samples of this hydride were cooled down to 100 K, recovered to ambient pressure, and studied in a metastable state by hot extraction, powder x-ray and neutron diffraction, and inelastic neutron scattering. X-ray diffraction demonstrated that this hydride had an A15-type crystal structure of metal lattice (space group , Ta atoms at the 2a and 6c Wyckoff positions) and a lattice parameter of a = 5.150(5) Å at T = 85 K. The hydrogen content determined by hot extraction was H/Ta - 1.23(5). Hydrogen desorption during heating the sample in vacuum proceeded in two steps—first, Δ⁢H⁡/Ta=0.2 was desorbed at around -70 °⁢C, and then the rest of the hydrogen was desorbed between 100 ⁢°C and 390 ⁢°⁢C. The A⁢15-type metal lattice was preserved upon hydrogen removal, leaving a unique polymorph of tantalum. Neutron diffraction of A⁢15-TaH 1.23⁢(5) demonstrated that hydrogen atoms occupy the 24⁢k and 16⁢i Wyckoff sites in the crystal structure, and annealing at 250 K resulted in a decrease of the 24⁢k and an increase of the 16⁢i site occupancies. Inelastic neutron scattering revealed four vibrational modes in the fundamental band of A⁢15-TaH 1.23⁢(5) at 72, 135, 145, and 166 meV, the first three and the last one of which were tentatively assigned to the vibrations of H atoms at the 24⁢k and 16⁢i sites, respectively. No superconductivity was found in A⁢15-TaH 1.1 and hydrogen-free A15-Ta at temperatures down to 1.5 K.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Magnetic fluctuations and the spin–orbit interaction in Mott insulating CoO

Motivated by the presence of an unquenched orbital angular momentum in CoO, a team at Chalk River, including a recently hired research officer Roger Cowley, performed the first inelastic neutron scattering experiments on the classic Mott insulator. Despite identifying two magnon modes at the zone boundary, the team was unable to parameterise the low energy magnetic excitation spectrum below TN using conventional pseudo-bosonic approaches, instead achieving only qualitative agreement. It would not be for another 40 years that Roger, now at Oxford and motivated by the discovery of the high-Tc cuprate superconductors, would make another attempt at the parameterisation of the magnetic excitation spectrum that had previously alluded him at the start of his career. Upon his return to CoO, Roger found a system embroiled in controversy, with some of its most fundamental parameters still remaining undetermined. Faced with such a formidable task, Roger performed a series of inelastic neutron scattering experiments in the early 2010s on both CoO and a magnetically dilute structural analogue Mg 0.97 Co 0.03 O. These experiments would prove instrumental in the determination of both single-ion and cooperative magnetic parameters for CoO. Both these sets of parameters would eventually be used in a spin–orbit exciton model, developed by his longtime friend and collaborator Bill Buyers, to successfully parameterise the complex spectrum that both measured at Chalk River almost 50 years prior. The story of CoO is of one that has come full circle, one filled with both spectacular failures and intermittent, yet profound, little victories.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Extreme confinement of hydrogen gas within fullerenelike nanoporous carbon

Nanoporous carbons and carbon nanostructures can store hydrogen at cryogenic temperatures but lack the volumetric and gravimetric capacity to be industrially significant. Recent inelastic neutron scattering experiments suggest a highly dense phase of hydrogen at temperatures well above the melting point of solid hydrogen. However, it remains unclear how pore geometry and intermolecular interactions enable these dense phases to exist, with dispersion (van der Waals) or electrostatic/induction suggested to be the key effects in slit and curved pores but their relative contributions have yet to be quantified. In this paper, we perform benchmark electronic structure calculations allowing the interactions between planar and curved aromatic molecules with hydrogen to be accurately determined. Dispersion was found to dominate over electrostatic and inductive effects with some many-body charge transfer (Dobson type-A) effects needed to capture the most highly curved structures. Density functional methods that include type-A many-body effects were found to accurately describe the intermolecular interactions at a fraction of the cost of coupled-cluster simulations and these approaches were used to calculate the energies inside large carbon bowl and slit pores. The interaction energies inside the bowl pores were found to depend on the orientation of the hydrogen molecule. This rotational barrier, modeled as a quantum hindered rotor, could reproduce the peak splitting observed in inelastic neutron scattering experiments, with weak splitting arising from bowl-like fullerene pores and strong splitting from highly confining nanotubelike pores. Increasing the fraction of such curved pores in nanoporous carbons may therefore offer a pathway to enhance their hydrogen-storage capacity. Moreover, the preferential adsorption of ortho hydrogen on nanotubelike pores could enable the storage of high-density hydrogen without the need to remove heat produced during the ortho-para hydrogen conversion.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Evolution of magnetic interactions in Sb-substituted MnBi 2 Te 4

The Mn(Bi 1–x Sb x ) 2 Te 4 series is purported to span from an antiferromagnetic (AF) topological insulator at x = 0 to a trivial AF insulator at x = 1. Here we report on neutron diffraction and inelastic neutron scattering studies of the magnetic interactions across this series. All compounds measured possess ferromagnetic (FM) triangular layers and we find a crossover from AF to FM interlayer coupling near x = 1 for our samples. The large spin gap at x = 0 closes rapidly and the average FM exchange interactions within the triangular layer increase with Sb substitution. Similar to a previous study of MnBi 2 Te 4 , we find severe spectral broadening which increases dramatically across the compositional series. In addition to broadening, we observe an additional sharp magnetic excitation in MnSb 2 Te 4 that may indicate the development of local magnetic modes originating from antisite disorder between Mn and Sb sublattices. Here, the results suggest that both substitutional and antisite disorder contribute substantially to the magnetism in Mn(Bi 1–x Sb x ) 2 Te 4 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Vibrations and Phase Stability in Mixed Valence Antimony Oxide

α-Sb 2 O 4 (cervantite) and β-Sb 2 O 4 (clinocervantite) are mixed valence compounds with equal proportions of Sb III and Sb V as represented in the formula Sb III Sb V O 4 . Their structure and properties can be difficult to calculate owing to the Sb III lone-pair electrons. Here, we present a study of the lattice dynamics and vibrational properties using a combination of inelastic neutron scattering, Mössbauer spectroscopy, nuclear inelastic scattering, and density functional theory (DFT) calculations. DFT calculations that account for lone-pair electrons match the experimental densities of phonon states. Mössbauer spectroscopy reveals the β phase to be significantly harder than the α phase. Calculations with O vacancies reveal the possibility for nonstoichiometric proportions of Sb III and Sb V in both phases. An open question is what drives the stability of the α phase over the β phase, as the latter shows pronounced kinetic stability and lower symmetry despite being in the high-temperature phase. Since the vibrational entropy difference is small, it is unlikely to stabilize the α phase. In conclusion, our results suggest that the α phase is more stable only because the material is not fully stoichiometric.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Water Migration and Swelling in Engineered Barrier Materials for Radioactive Waste Disposal

Deep, underground repositories are needed to isolate radioactive waste from the biosphere. Because bentonite is an integral component of many multibarrier repository systems, information on the hydraulic behavior of bentonite is crucial for modeling the long-term viability of such systems. In this paper the hydraulic behavior of bentonite samples was analyzed as a function of aggregate size, and samples were subjected to hydrothermal treatments involving contact with NaCl, KCl, and deionized water. Neutron and X-ray imaging were used to quantify water sorption into packed bentonite samples and bentonite swelling into the water column. The distance between the original clay-water interface and the wetting front was determined as a function of time. Average water uptake exhibited a square-root-of-time dependence in freshly prepared samples, but more variable rates were observed for samples previously in contact with water. The radiography was supported by small-angle neutron scattering analysis and ultra-small-angle neutron scattering analysis of aggregate size distributions and by inelastic neutron scattering to understand the physicochemical environment of the sorbed water. Results showed that hydrothermal treatment with KCl had the greatest effect of increased water transport in the bentonite, possibly as a result of the interaction of K + with smectite layers in the clay.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2022

Frustration-induced diffusive scattering anomaly and dimension change in FeGe 2

Magnetic frustration, arising from the competition of exchange interactions, has received great attention because of its relevance to exotic quantum phenomena in materials. In the current work, we report an unusual checkerboard-shaped scattering anomaly in FeGe 2 , far from the known incommensurate magnetic satellite peaks, by inelastic neutron scattering. More surprisingly, such phenomenon appears as spin dynamics at low temperature, but it becomes prominent above Néel transition as elastic scattering. Here, a model Hamiltonian that includes an intraplane next-nearest neighbor was proposed and such anomaly is attributed to the near-perfect magnetic frustration and the emergence of unexpected two-dimensional magnetic order in the quasi-one-dimensional FeGe 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Hydrogen Dynamics in Supercritical Water Probed by Neutron Scattering and Computer Simulations

In this work, an investigation of supercritical water is presented combining inelastic and deep inelastic neutron scattering experiments and molecular dynamics simulations based on a machine-learned potential of ab initio quality. The local hydrogen dynamics is investigated at 250 bar and in the temperature range of 553–823 K, covering the evolution from subcritical liquid to supercritical gas-like water. The evolution of libration, bending, and stretching motions in the vibrational density of states is studied, analyzing the spectral features by a mode decomposition. Moreover, the hydrogen nuclear momentum distribution is measured, and its anisotropy is probed experimentally. It is shown that hydrogen bonds survive up to the higher temperatures investigated, and we discuss our results in the framework of the coupling between intramolecular modes and intermolecular librations. Results show that the local potential affecting hydrogen becomes less anisotropic within the molecular plane in the supercritical phase, and we attribute this result to the presence of more distorted hydrogen bonds.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Neutron scattering study of the kagome metal Sc 3 Mn 3 Al 7 Si 5

Sc 3 Mn 3 Al 7 Si 5 is a rare example of a correlated metal in which the Mn moments form a kagome lattice. The absence of magnetic ordering to the lowest temperatures suggests that geometrical frustration of magnetic interactions may lead to strong magnetic fluctuations. We have performed inelastic neutron scattering measurements on Sc 3 Mn 3 Al 7 Si 5 , finding that phonon scattering dominates for energies from ~20–50 meV. These results are in good agreement with ab initio calculations of the phonon dispersions and densities of states, and as well reproduce the measured specific heat. A weak magnetic signal was detected at energies less than ~10 meV, present only at the lowest temperatures. The magnetic signal is broad and quasielastic, as expected for metallic paramagnets.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Magnetism of Nd 2 O 3 single crystals near the Néel temperature

Single crystals of Nd 2 O 3 were grown and characterized using neutron scattering and thermodynamic measurements. Nd 2 O 3 has long-range antiferromagnetic order below T N =0.55 K and specific-heat measurements have demonstrated that a significant amount of the magnetic entropy is released above T N . Inelastic neutron scattering experiments reveal a magnetic mode(s) with little dispersion peaked at ≈0.37meV that is of greatest intensity below T N but persists above 2T N . This persistence of dynamic correlations is likely related to frustrated interactions associated with the nearly ideal stacked triangular lattice geometry of J eff =12 spins on Nd 3+ ions. Furthermore, the magnetization is observed to be strongly anisotropic at all temperatures due to crystal field effects, with easy-plane anisotropy observed. Additionally, a noncompensated magnetic structure is inferred from the temperature dependence of the magnetization when a magnetic field of sufficient strength is applied within the basal plane near T N , and the evolution of the long-range order is summarized in a temperature-field phase diagram.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Geometrical frustration versus Kitaev interactions in BaCo 2 (AsO 4 ) 2

Recently, Co-based honeycomb magnets have been proposed as promising candidate materials to host the Kitaev spin liquid (KSL) state. One of the front-runners is BaCo 2 (AsO 4 ) 2 (BCAO), where it was suggested that the exchange processes between Co 2+ ions via the surrounding edge-sharing oxygen octahedra could give rise to bond-dependent Kitaev interactions. In this work, we present and analyze a comprehensive inelastic neutron scattering (INS) study of BCAO with fields in the honeycomb plane. Combining the constraints from the magnon excitations in the high-field polarized state and the inelastic spin structure factor measured in zero magnetic field, we examine two leading theoretical models: the Kitaev-type JK Γ Γ ′ model and the XXZ - J 1 - J 3 model. We show that the existing experimental data can be consistently accounted for by the XXZ - J 1 - J 3 model but not by the JK Γ Γ ′ model, and we discuss the implications of these results for the realization of a spin liquid phase in BCAO and more generally for the realization of the Kitaev model in cobaltates.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Neutron scattering off Weyl semimetals

We present how to detect type-I Weyl nodes in a material by inelastic neutron scattering. Such an experiment first of all allows one to determine the dispersion of the Weyl fermions. We extend the reasoning to produce a quantitative test of the Weyl equation, taking into account realistic anisotropic properties. These anisotropies are mostly contained in the form of the emergent magnetic moment of the excitations, which determines how they couple to the neutrons. Although there are many material parameters, we find several quantitative predictions that are universal and demonstrate that the excitations are described by solutions to the Weyl equation. The anisotropic coupling between electrons and neutrons implies that even fully unpolarized neutrons can reveal the spin-momentum locking of the Weyl fermions because the neutrons will couple to some components of the Weyl fermion pseudospin more strongly. On the other hand, in an experiment with polarized neutrons, the scattered neutron beam remains fully polarized in a direction that varies as a function of momentum transfer (within the range of validity of the Weyl equation). This allows measurement of the chirality of Weyl fermions for inversion-symmetric nodes. Furthermore, in this work we estimate that the scattering rate may be large enough for such experiments to be practical; in particular, the magnetic moment may be larger than the ordinary Bohr magneton, compensating for a small density of states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗