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At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Temperature-dependent lattice dynamics in iridium

The characterization of simple elemental systems is key to benchmarking first-principles modeling of electronic and vibrational behaviors of materials. A large body of literature has been built for most elemental systems in the periodic table. However, surprisingly little neutron work has been performed to probe the vibrational properties of iridium, likely due to its large neutron absorption cross section. Nonetheless, iridium is of significant scientific and technological interest due to large relativistic electron effects and electron-phonon coupling, particularly in strongly correlated iridate compounds. In this paper, we report temperature-dependent inelastic neutron scattering measurements of the vibrational properties of iridium, from which we extract key thermodynamic properties. To overcome the challenge of the large neutron absorption of iridium, we developed a simple post-processing correction procedure. The measured densities of phonon states compare well with quasiharmonic density functional theory calculations, although the obtained experimental phonon Grüneisen parameters are much larger than expected, reaching as high as γ=4.5, indicating substantial anharmonicity.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Low-Temperature Competing Magnetic Energy Scales in the Topological Ferrimagnet TbMn 6 Sn 6

TbMn 6 Sn 6 is a metallic ferrimagnet displaying signatures of both topological electrons and topological magnons arising from ferromagnetism and spin-orbit coupling within its Mn kagome layers. Inelastic neutron scattering measurements find strong ferromagnetic (FM) interactions within the Mn kagome layer and reveal a magnetic bandwidth of ~230 meV. The low-energy magnetic excitations are characterized by strong FM Mn-Mn and antiferromagnetic (AFM) Mn-Tb interlayer magnetic couplings. We observe weaker, competing long-range FM and AFM Mn-Mn interlayer interactions similar to those driving helical magnetism in the YMn 6 Sn 6 system. Combined with density-functional theory calculations, we find that competing Mn-Mn interlayer magnetic interactions occur in all RMn 6 Sn 6 compounds with R=Y, Gd-Lu, resulting in magnetic instabilities and tunability when Mn-R interactions are weak. In the case of TbMn 6 Sn 6 , strong AFM Mn-Tb coupling ensures a highly stable three-dimensional ferrimagnetic network.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

First Measurement of $\Lambda$ Electroproduction off Nuclei in the Current and Target Fragmentation Regions

We report results of $\Lambda$ hyperon production in semi-inclusive deep-inelastic scattering off deuterium, carbon, iron, and lead targets obtained with the CLAS detector and the CEBAF 5.014 GeV electron beam. These results represent the first measurements of the $\Lambda$ multiplicity ratio and transverse momentum broadening as a function of the energy fraction ($z$) in the current and target fragmentation regions. The multiplicity ratio exhibits a strong suppression at high $z$ and an enhancement at low $z$. The measured transverse momentum broadening is an order of magnitude greater than that seen for light mesons. This indicates that the propagating entity interacts very strongly with the nuclear medium, which suggests that propagation of di-quark configurations in the nuclear medium takes place at least part of the time, even at high $z$. The trends of these results are qualitatively described by the GiBUU transport model, particularly for the multiplicity ratios. These observations will potentially open a new era of studies of the structure of the nucleon as well as of strange baryons.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

High-resolution Compton spectroscopy using x-ray microcalorimeters

X-ray Compton spectroscopy is one of the few direct probes of the electron momentum distribution of bulk materials in ambient and operando environments. Herein, we report high-resolution inelastic x-ray scattering experiments with high momentum and energy transfer performed at a storage-ring-based high-energy x-ray light source facility using an x-ray transition-edge sensor (TES) microcalorimeter detector. The performance was compared with a silicon drift detector (SDD), an energy-resolving semiconductor detector, and Compton profiles were measured for lithium and cobalt oxide powders relevant to lithium-ion battery research. Spectroscopic analysis of the measured Compton profiles demonstrates the high-sensitivity to the low- Z elements and oxidation states. The line shape analysis of the measured Compton profiles in comparison with computed Hartree–Fock profiles is usually limited by the resolution of the semiconductor detector. We have characterized an x-ray TES microcalorimeter detector for high-resolution Compton scattering experiments using a bending magnet source at the Advanced Photon Source with a double crystal monochromator, providing monochromatic photon energies near 27.5 keV. The momentum resolution below 0.16 atomic units (a.u.) was measured, yielding an improvement of more than a factor of 7 over a state-of-the-art SDD for the same scattering geometry. Furthermore, the lineshapes of narrow valence and broad core electron profiles of sealed lithium metal were clearly resolved using an x-ray TES compared to smeared and broadened lineshapes observed when using the SDD. High-resolution Compton scattering using the energy-resolving area detector shown here presents new opportunities for spatial imaging of electron momentum distributions for a wide class of materials with applications ranging from electrochemistry to condensed matter physics.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Temperature-induced valence-state transition in double perovskite Ba 2 - x Sr x TbIrO 6

In this paper, a temperature-induced valence-state transition is studied in a narrow composition range 0.2 ≤ x ≤ 0.375 of Ba 2 - x Sr x TbIrO 6 by means of x-ray and neutron powder diffraction, resonant inelastic x-ray scattering, magnetic susceptibility, electrical resistivity, and specific heat measurements. The valence-state transition involves an electron transfer between Tb and Ir leading to the valence-state change between Tb 3 + / Ir 5 + and Tb 4 + / Ir 4 + phases. This first-order transition has a dramatic effect on the lattice, transport properties, and the long-range magnetic order at low temperatures for both Tb and Ir ions. Ir 5 + ion has an electronic configuration of 5 d 4 ( J eff = 0 ), which is expected to be nonmagnetic. In contrast, Ir 4 + ion with a configuration of 5 d 5 ( J eff = 1/2) favors a long-range magnetic order. For x = 0.1 with Tb 3 + / Ir 5 + configuration to the lowest temperature (2 K) investigated in this paper, a spin-glass behavior is observed around 5 K indicating Ir 5 + ( J eff = 0 ) ions act as a spacer reducing the magnetic interactions between Tb 3 + ions. For x = 0.5 with Tb 4 + / Ir 4 + configuration below the highest temperature 400 K of this paper, a long-range antiferromagnetic order at T N = 40 K is observed highlighting the importance of Ir 4 + ( J eff = 1/2) ions in promoting the long-range magnetic order of both Tb and Ir ions. For 0.2 ≤ x ≤ 0.375, a temperature-induced valence-state transition from high-temperature Tb 3 + / Ir 5 + phase to low-temperature Tb 4 + / Ir 4 + phase occurs in the temperature range 180 K ≤ T ≤ 325 K and the transition temperature increases with x . The compositional dependence demonstrates the ability to tune the the valence state for a critical region of x that leads to a concurrent change in magnetism and structure. Furthermore, this tuning ability could be employed with suitable strain in thin films to act as a switch as the magnetism is manipulated.

36 MATERIALS SCIENCE↗
DOE OSTI2023

X-ray Thomson scattering spectra from density functional theory molecular dynamics simulations based on a modified Chihara formula

Here, we study ab initio approaches for calculating x-ray Thomson scattering spectra from density functional theory molecular dynamics simulations based on a modified Chihara formula that expresses the inelastic contribution in terms of the dielectric function. We study the electronic dynamic structure factor computed from the Mermin dielectric function using an ab initio electron-ion collision frequency in comparison to computations using a linear-response time-dependent density functional theory (LR-TDDFT) framework for hydrogen and beryllium and investigate the dispersion of free-free and bound-free contributions to the scattering signal. A separate treatment of these contributions, where only the free-free part follows the Mermin dispersion, shows good agreement with LR-TDDFT results for ambient-density beryllium, but breaks down for highly compressed matter where the bound states become pressure ionized. LR-TDDFT is used to reanalyze x-ray Thomson scattering experiments on beryllium demonstrating strong deviations from the plasma conditions inferred with traditional analytic models at small scattering angles.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2025

Flavor Dependence of Charged Pion Fragmentation Functions

We have measured the flavor dependence of multiplicities for π + and π - production in semi-inclusive deep-inelastic scattering (SIDIS) on proton and deuteron to explore a possible charge symmetry violation in fragmentation functions. The experiment used an electron beam with energies of 10.2 and 10.6 GeV at Jefferson Lab and the Hall-C spectrometers. The electron kinematics spanned the range 0.3 < x < 0.6, 2 < Q 2 < 5.5 GeV2, and 2.2 < W < 3.2 GeV. The pion fractional momentum range was 0.3 < z < 0.7, and the transverse momentum range was 0 < p T < 0.25 GeV/c. Assuming factorization and allowing for isospin breaking, the results can be described by two “favored” and two “unfavored” effective low p T fragmentation functions that are flavor-dependent. We find each pair converges to a common flavor-independent fragmentation function at the highest W, where factorization is most applicable.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Following Metal-to-Ligand Charge-Transfer Dynamics with Ligand and Spin Specificity Using Femtosecond Resonant Inelastic X-ray Scattering at the Nitrogen K-Edge

We demonstrate for the case of photoexcited [Ru(2,2'-bipyridine) 3 ] 2+ how femtosecond resonant inelastic X-ray scattering (RIXS) at the ligand K-edge allows one to uniquely probe changes in the valence electronic structure following a metal-to-ligand charge-transfer (MLCT) excitation. Metal–ligand hybridization is probed by nitrogen-1s resonances providing information on both the electron-accepting ligand in the MLCT state and the hole density of the metal center. By comparing to spectrum calculations based on density functional theory, we are able to distinguish the electronic structure of the electron-accepting ligand and the other ligands and determine a temporal upper limit of (250 ± 40) fs for electron localization following the charge-transfer excitation. The spin of the localized electron is deduced from the selection rules of the RIXS process establishing new experimental capabilities for probing transient charge and spin densities.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Orbital selective spin waves in detwinned NaFeAs

The existence of orbital-dependent electronic correlations has been recognized as an essential ingredient to describe the physics of iron-based superconductors. NaFeAs, a parent compound of iron-based superconductors, exhibits a tetragonal-to-orthorhombic lattice distortion below T s ≈ 60 K, forming an electronic nematic phase with two 90° rotated (twinned) domains, and orders antiferromagnetically below T N ≈ 42 K. We use inelastic neutron scattering to study spin waves in uniaxial pressure-detwinned NaFeAs. By comparing the data with combined density functional theory and dynamical mean-field theory calculations, we conclude that spin waves up to an energy scale of E crossover ≈ 100 meV are dominated by d yz -d yz intraorbital scattering processes, which have the twofold (C 2 ) rotational symmetry of the underlying lattice. On the other hand, the spin wave excitations above E crossover , which have approximately fourfold (C 4 ) rotational symmetry, arise from the d xy -d xy intraorbital scattering that controls the overall magnetic bandwidth in this material. In addition, we find that the low-energy (E ≈ 6 meV) spin excitations change from approximate C 4 to C 2 rotational symmetry below a temperature T* (>T s ), while spin excitations at energies above E crossover have approximate C 4 rotational symmetry and are weakly temperature dependent. Here, these results are consistent with angle-resolved photoemission spectroscopy measurements, where the presence of a uniaxial strain necessary to detwin NaFeAs also raises the onset temperature T* of observable orbital-dependent band splitting to above T s , thus supporting the notion of orbital selective spin waves in the nematic phase of iron-based superconductors.

36 MATERIALS SCIENCE↗
NASA NTRS1980

Quasi-projection-operator calculation of autoionization states of O5/+/ /O VI/ and Si11/+/ /Si XII/

A practical approach to calculating diagnostic parameters includes both scattering and statelike (autoionization) elements. The paper presents inelastic quasi-projection-operator calculations of autoionization states of the three-electron ions of oxygen and silicon for states below the n = 3 manifod of the respective two-electron parent (O VI and Si XII). Wave functions containing up to 40 configurations are calculated. The O VI calculated results agree with experimental results within the experimental error (about + or - 2 eV) while for the Si XII case, it was necessary to include relativistic effects. Where formulas are available (states for which p + q = 2), the Si XII results are also in good agreement with experimental results for 2Se, 2Po, and 2De states. Since the calculated results for some of the higher states did not agree with identifications proposed by Trabert et al. (1979), alternative identifications are suggested.

Wakid, S.↗
DOE OSTI2026

The chemRIXS Instrument for Solution-Phase Resonant Inelastic X-ray Scattering at the Linac Coherent Light Source

The chemRIXS instrument at the Linac Coherent Light Source (LCLS) offers new opportunities for studying solution-phase systems with time-resolved soft X-ray spectroscopic methods such as resonant inelastic X-ray scattering (RIXS) using the recently commissioned high-repetition-rate LCLS-II X-ray free electron laser. The orders-of-magnitude X-ray flux improvement provided by the superconducting accelerator, combined with corresponding advances in the optical laser system and the liquid jet recirculation system, enables studies on dilute systems with high signal-to-noise compared to what was possible with the LCLS-I copper accelerator. These capabilities open up time-resolved X-ray absorption spectroscopy and RIXS to entirely new classes of samples, as well as enabling the development of new soft X-ray spectroscopic methods on liquid samples. An overview of the beamline components and the first LCLS-II commissioning results are presented.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Connection between f -electron correlations and magnetic excitations in UTe 2

The detailed anisotropic dispersion of the low-temperature, low-energy magnetic excitations of the candidate spin-triplet superconductor UTe 2 is revealed using inelastic neutron scattering. The magnetic excitations emerge from the Brillouin zone boundary at the high symmetry Y and T points and disperse along the crystallographic $\hat{b}$ -axis. In applied magnetic fields to at least μ 0 H = 11 T along the $\hat{c}-{\rm{axis}}$ , the magnetism is found to be field-independent in the (hk0) plane. The scattering intensity is consistent with that expected from U 3+ /U 4+ f-electron spins with preferential orientation along the crystallographic $\hat{a}$ -axis, and a fluctuating magnetic moment of μ eff =1.7(5) μ B . We propose interband spin excitons arising from f-electron hybridization as a possible origin of the magnetic excitations in UTe 2 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2026

Light-Induced Charge Order Mode in a Metastable Cuprate Ladder

We report the observation of an emergent charge order mode in the optically excited cuprate ladder Sr 14⁢ Cu 24⁢ O 41 . Near-infrared light in the ladder plane drives a symmetry-protected electronic metastable state together with a partial melting of the equilibrium charge order. Our time-resolved resonant inelastic x-ray scattering measurements at the upper Hubbard band reveal a collective excitation dispersing from the charge order wave vector up to 0.8 eV with a slope on the order of the quasiparticle velocity. These findings reveal a regime where correlated carriers acquire itinerant character at finite momentum, and charge order becomes dynamically fluctuating, offering a platform to explore light-induced pairing instabilities.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Dynamics of collision-induced energy transfer

Collisions are of fundamental importance in many relevant physico-chemical phenomena in the gas phase and at the gas-surface interface. Collision-induced energy transfer not only offers a sensitive probe of interatomic interactions, but also impacts an array of processes ranging from transport to reactivity. This perspective highlights some recent advances on collision-induced energy transfer dynamics. In the gas phase, cold collisions revealed remarkable quantum effects and stereodynamics of bimolecular inelastic scattering, which motivated development of new state-to-state quantum scattering methods. For surface processes, experiments of hydrogen atom scattering from various surfaces combined with first-principles molecular dynamics simulations shed valuable light on the relative importance of adiabatic energy transfer to surface phonons and nonadiabatic energy transfer to surface electrons. We outline further challenges in these fields and expect more fruitful interplay between experiment and theory.

Cold collision↗
DOE OSTI2023

Description of quantum interference using mixed quantum/classical theory of inelastic scattering

Manifestation of the quantum interference effect in the oscillation of scattering cross section is explored using the N 2 + O system as a case study. Calculations are carried out for two electronic PESs of the system, for various initial rotational states of N 2 , in a broad range of N 2 + O collision energies and using three theoretical methods: two versions of the approximate mixed quantum/classical theory (MQCT and AT-MQCT) and the accurate full-quantum coupled-channel method (implemented in MOLSCAT). A good agreement between different methods is observed, especially at high energies. Elastic scattering cross-sections oscillate as a function of collision energy, which is the result of quantum interference. The effects of initial rotational excitation and of the PES properties are studied in detail. For the final (thermally averaged) cross sections, both MOLSCAT and MQCT calculations predict a rather regular pattern of quantum oscillations that persist through a broad range of collision energies and expand into the low-energy regime where quantum scattering resonances are common. The difference between cross sections predicted by MQCT and MOLSCAT decreases from ~8% at low energies to ~2% at high energies. Finally, experimental data available at high collision energies are well reproduced.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Resonant x-ray emission across the L 3 edge of uranium compounds

A narrow bandwidth x-ray beamline and a multi-crystal von Hamos spectrometer were used to record x-ray absorption and x-ray emission (XES) across the uranium L 3 edge of UO 2 , UO 3 , Cs 2 UO 2 Cl 4 , and Cs 2 UCl 6 . Measurements were made over 17150–17250 eV with an instrumental resolution of ∼2 eV. This resolution allowed the use of Lorentzian peak fits to the L 3 N 4 and L 3 N 5 characteristic x-ray fluorescence lines that have ∼12–13 eV lifetime widths. The fluorescence yields of the four compounds display strong white lines near threshold and multiple scattering features at higher energies. The XES spectra were fit with three Lorentzians, two for the L 3 N 4 and L 3 N 5 lines and one for inelastic scattering by 2p–6d excitations and 4d–6d final states. The intensities of the white lines were largely due to the inelastic scattering component. To support the measurements, relativistic equation-of-motion coupled-cluster and restricted active space configuration interaction calculations were performed for uranium core-excitation energies in Cs 2 UO 2 Cl 4 and Cs 2 UCl 6 . These calculations with rigorous treatments of relativistic effects are shown to provide accurate uranium L 3 -edge binding energies, L 3 N 5 emission energies, as well as the energy losses in the inelastic scattering process. The measured and calculated results show variations between compounds with U(IV) oxidation states that contain 6d 0 5f 2 electrons in their ground configurations (UO 2 and Cs 2 UCl 6 ) compared with U(VI) compounds (UO 3 and Cs 2 UO 2 Cl 4 ) with empty 5f and 6d configurations.

74 ATOMIC AND MOLECULAR PHYSICS↗