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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Pressure control of the spin reorientation transition in the rare-earth orthoferrite YbFeO 3

YbFeO 3 is exceptional among the rare-earth orthoferrites for having the lowest spin-reorientation transition (SRT) temperature, T SRT ≃8K, which makes it particularly appealing to examine the interplay between noncollinear magnetism of the Fe sublattice and quasi-one-dimensional XXZ effective S = 1/2 chains of Yb 3+ moments. Our paper focuses on the magnetic dynamics of YbFeO 3 using inelastic neutron scattering (INS), at temperatures below and above the SRT, under an applied hydrostatic pressure of 2 GPa, and in magnetic fields up to 4 T. The low-energy zero-field excitation spectrum at ambient pressure and temperatures below the SRT is dominated by a gapped magnon mode of the Yb subsystem at 0.84 meV with a dispersion only in the [00L] direction. Above T SRT , a continuum appears on top of the magnon mode because of temperature population of the magnon band, and the gap decreases to around 0.4 meV. The INS spectra in the magnetic field, both above and below T SRT , are characterized by two well-separated gapped modes. The SRT is clearly visible at low fields B < 1T, but it gradually disappears at higher magnetic fields. The hydrostatic pressure of p = 2GPa effectively reduces the transition width, ΔT SRT , and keeps the SRT at higher fields up to B≃3T. Here, we discuss the effect of the applied pressure in the frame of the modified mean-field theory and show that in the vicinity of the T SRT the pressure tunes the fourth-order anisotropy constant that effectively reduces the ΔT SRT .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Magnetic excitations and absence of charge order in the van der Waals ferromagnet Fe 4.75 GeTe 2

Understanding the ground state of van der Waals (vdW) magnets is crucial for designing devices leveraging these platforms. Here, we investigate the magnetic excitations and charge order in Fe 4.75 GeTe 2 , a vdW ferromagnet with ≈ 315 K Curie temperature. Using Fe L 3 -edge resonant inelastic X-ray scattering, we observe a dual nature of magnetic excitations, comprising a coherent magnon and a broad non-dispersive continuum extending up to 150 meV, 50% higher than in Fe 2.72 GeTe 2 . The continuum intensity is sinusoidally modulated along the stacking direction L, with a period matching the inter-slab distance. Our results indicate that while the dual character of the magnetic excitations is generic to Fe-Ge-Te vdW magnets, Fe 4.75 GeTe 2 exhibits a longer out-of-plane magnetic correlation length, suggesting enhanced 3D magnetic character. Furthermore, resonant X-ray diffraction reveals that previously reported ±(1/3, 1/3, L) peaks originate from crystal structure rather than from charge order.

Ferromagnetism↗
DOE OSTI2022

Thermal expansion and phonon anharmonicity of cuprite studied by inelastic neutron scattering and ab initio calculations

Inelastic neutron scattering measurements were performed with a time-of-flight chopper spectrometer to observe phonons in all parts of the Brillouin zone of a single crystal of cuprite Cu 2 O. We reduced the experimental data to phonon dispersions in the high-symmetry directions, and changes between 10 and 300 K are reported. In this paper, we show ab initio quasiharmonic (QH) and anharmonic (AH) calculations of phonon dispersions. We performed all AH calculations with a temperature-dependent effective potential method. Both QH and AH calculations account for the small negative thermal expansion of cuprite at low temperatures. However, the measured temperature-dependent phonon behavior was predicted more accurately with the AH calculations than the QH ones. Nevertheless, at 300 K, the cubic AH used in this paper did not entirely account for the experimental phonon dispersions in cuprite.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Determination of the parton distribution functions of the proton using diverse ATLAS data from pp collisions at $$\sqrt{s} = 7$$, 8 and 13 TeV

This paper presents an analysis at next-to-next-to-leading order in the theory of quantum chromodynamics for the determination of a new set of proton parton distribution functions using diverse measurements in pp collisions at $\sqrt{s}$ = 7, 8 and 13 TeV, performed by the ATLAS experiment at the Large Hadron Collider, together with deep inelastic scattering data from ep collisions at the HERA collider. The ATLAS data sets considered are differential cross-section measurements of inclusive W ± and Z/γ* boson production, W ± and Z boson production in association with jets, $t\bar{t}$ production, inclusive jet production and direct photon production. In the analysis, particular attention is paid to the correlation of systematic uncertainties within and between the various ATLAS data sets and to the impact of model, theoretical and parameterisation uncertainties. The resulting set of parton distribution functions is called ATLASpdf21.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS2005

Simultaneous Conoscopic Holography and Raman Spectroscopy

A new instrument was developed for chemical characterization of surfaces that combines the analytical power of Raman spectroscopy with the three-dimensional topographic information provided by conoscopic holography. The figure schematically depicts the proposed hybrid instrument. The output of the conoscopic holographic portion of the instrument is a topographical map of the surface; the output of the Raman portion of the instrument is hyperspectral Raman data, from which the chemical and/or biological composition of the surface would be deduced. By virtue of the basic principles of design and operation of the instrument, the hyperspectral image data would be inherently spatially registered with the topographical data. In conoscopic holography, the object and reference beams of classical holography are replaced by the ordinary and extraordinary components generated by a single beam traveling through a birefringent, uniaxial crystal. In the basic conoscopic configuration, a laser light is projected onto a specimen and the resulting illuminated spot becomes a point source of diffuse light that propagates in every direction. The laser beam is rasterscanned in two dimensions (x and y) perpendicular to the beam axis (z), and at each x,y location, the pattern of interference between the ordinary and extraordinary rays is recorded. The recorded interferogram constitutes the conoscopic hologram. Of particular significance for the proposed instrument is that the conoscopic hologram contains information on the z coordinate (height) of the illuminated surface spot. Hence, a topographical map of the specimen is constructed point-by-point by rastering the laser beam in the x and y directions and correlating the x and y coordinates with the z information obtained from the interferograms. Conoscopic imaging is an established method, and conoscopic laboratory instruments for surface metrology are commercially available. In Raman spectroscopy of a surface, one measures the spectrum of laser light scattered inelastically from a laser-illuminated spot on the surface. The wavelengths of the inelastically scattered light differ from that of the incident laser beam by amounts that correspond to the energies of molecular vibrations. The resulting vibrational spectrum can be used to identify the molecules. Raman spectroscopy is a standard laboratory technique for identifying mineralogical, biological, and other specific chemical compositions.

Schramm, Harry F.↗
DOE OSTI2026

Determination of proton PDF uncertainties with Markov chain Monte Carlo

We present an analysis of parton distribution functions (PDFs) of the proton using Markov chain Monte Carlo (MCMC) methods. The MCMC approach naturally implements Bayes’ theorem and, thus, provides a means to directly sample the underlying probability distribution—in this case, the probability distribution of the PDF parameters. This allows for a straightforward propagation of the resulting uncertainties into any PDF-dependent observable, preserving their simple probabilistic interpretation. In our analysis we include a broad set of deep inelastic scattering data from HERA, BCDMS and NMC experiments along with the Drell-Yan, 𝑊 and 𝑍 boson data from LHC and Tevatron experiments, which combined with theoretical calculations at next-to-next-to-leading order in QCD allow for realistic determination of PDFs. The main focus of this analysis is to explore alternative methods for PDF uncertainty estimation that are more firmly grounded in statistical principles. We show that the flexibility of the Bayes framework, allowing one, e.g., to account for non-Gaussianity or inconsistencies of datasets, is crucial to extract realistic uncertainties when such assumptions are not fulfilled. We also demonstrate that MCMC allows one to determine the Δ⁢𝜒 2 value corresponding to a given confidence level in the sample, which can, in turn, be used as a statistically well-founded tolerance criterion used in the Hessian method, thus addressing one of its main long-standing drawbacks.

Risse, Peter Clemens [Universität Münster (Germany↗
DOE OSTI2023

Direct Conversion of Methane to Ethylene and Acetylene over an Iron-Based Metal–Organic Framework

Conversion of methane (CH 4 ) to ethylene (C 2 H 4 ) and/or acetylene (C 2 H 2 ) enables routes to a wide range of products directly from natural gas. However, high reaction temperatures and pressures are often required to activate and convert CH 4 controllably, and separating C 2+ products from unreacted CH 4 can be challenging. Here, we report the direct conversion of CH 4 to C 2 H 4 and C 2 H 2 driven by non-thermal plasma under ambient (25 °C and 1 atm) and flow conditions over a metal–organic framework material, MFM-300(Fe). The selectivity for the formation of C 2 H 4 and C 2 H 2 reaches 96% with a high time yield of 334 μmol g cat –1 h –1 . At a conversion of 10%, the selectivity to C 2+ hydrocarbons and time yield exceed 98% and 2056 μmol g cat –1 h –1 , respectively, representing a new benchmark for conversion of CH 4 . In situ neutron powder diffraction, inelastic neutron scattering and solid-state nuclear magnetic resonance, electron paramagnetic resonance (EPR), and diffuse reflectance infrared Fourier transform spectroscopies, coupled with modeling studies, reveal the crucial role of Fe–O(H)–Fe sites in activating CH 4 and stabilizing reaction intermediates via the formation of an Fe–O(CH 3 )–Fe adduct. In addition, a cascade fixed-bed system has been developed to achieve online separation of C 2 H 4 and C 2 H 2 from unreacted CH 4 for direct use. Integrating the processes of CH 4 activation, conversion, and product separation within one system opens a new avenue for natural gas utility, bridging the gap between fundamental studies and practical applications in this area.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Lattice dynamics of alpha-uranium measured on ARCS.

The combined inelastic neutron scattering data from ARCS for alpha-Uranium single crystal measured at 300K, 200K, 70K and 20K. The incident neutron energy was 30 meV. To obtain a significant four-dimensional Q-E volume, the crystal was measured in two scattering geometries, with the [001] and [011] directions oriented along the vertical rotations axis. For each geometry, the crystal was rotated in 1 degree steps with respect to the incident beam. All the individual angles data files at each temperature were merged and analyzed using SHIVER software package (https://neutrons.github.io/Shiver/). The files labeled a_U_HKL_20K_new.nxs, a_U_HKL_70K.nxs, a_U_HKL_200K.nxs and a_U_HKL_300K.nxs are for [011] oriented scattering at 20K, 70K, 200K, and 300K respectively. The files labeled a_U_HK0_70K.nxs, a_U_HK0_200K.nxs and a_U_HK0_300K.nxs are for [001] oriented scattering at temperatures 70K, 200K and 300K respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Physics-guided dual implicit neural representations for source separation

Significant challenges exist in efficient data analysis of most advanced experimental and observational techniques because the collected signals often include unwanted contributions, such as background and signal distortions, that can obscure the physically relevant information of interest. To address this, we have developed a self-supervised machine-learning approach for source separation using a dual implicit neural representation framework that jointly trains two neural networks: one for approximating distortions of the physical signal of interest and the other for learning the effective background contribution. Our method learns directly from the raw data by minimizing a reconstruction-based loss function without requiring labeled data or pre-defined dictionaries. We demonstrate the effectiveness of our framework by considering a challenging case study involving large-scale simulated, as well as experimental, momentum-energy-dependent inelastic neutron scattering data in a four-dimensional parameter space, characterized by heterogeneous background contributions and unknown distortions to the target signal. The method is found to successfully separate physically meaningful signals from a complex or structured background even when the signal characteristics vary across all four dimensions of the parameter space. An analytical approach that informs the choice of the regularization parameter is presented. Our method offers a versatile framework for addressing source separation problems across diverse domains, ranging from superimposed signals in astronomical measurements to structural features in biomedical image reconstructions.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Electromagnetic signals of inelastic dark matter scattering

Light dark sectors in thermal contact with the Standard Model can naturally produce the observed relic dark matter abundance and are the targets of a broad experimental search program. A key light dark sector model is the pseudo-Dirac fermion with a dark photon mediator. The dynamics of the fermionic excited states are often neglected. We consider scenarios in which a nontrivial abundance of excited states is produced and their subsequent de-excitation yields interesting electromagnetic signals in direct detection experiments. We study three mechanisms of populating the excited state: a primordial excited fraction, a component up-scattered in the Sun, and a component up-scattered in the Earth. We find that the fractional abundance of primordial excited states is generically depleted to exponentially small fractions in the early universe. Nonetheless, this abundance can produce observable signals in current dark matter searches. MeV-scale dark matter with thermal cross sections and higher can be probed by down-scattering following excitation in the Sun. Up-scatters of GeV-scale dark matter in the Earth can give rise to signals in current and upcoming terrestrial experiments and X-ray observations. We comment on the possible relevance of these scenarios to the recent excess in XENON1T.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Correlated $\ n-γ$ angular distributions from the $\ Q$ = 4.4398 MeV 12 C ($\ n, n' γ$) reaction for incident neutron energies from 6.5 MeV to 16.5 MeV

Neutron scattering cross sections and angular distributions represent one of the most glaring sources of uncertainty in calculations of nuclear systems. Even simple nuclei like 12 C show indications of errors in nuclear databases for scattering reactions. Measurements of inelastic neutron scattering have historically measured either the scattered neutrons or the nuclear deexcitation $\ γ$ emission. Only a very small number of experiments attempted correlated measurements of both the neutron and $\ γ$ data simultaneously, even though these $\ n-γ$ correlations could be essential for understanding particle transport in nuclear systems. In this work we describe a measurement of the $\ n, γ$, and correlated $\ n-γ$ angular distributions from the $\ Q$ = 4.4398 MeV 12 C ($\ n, n'γ$) reaction in a single experiment using an EJ-309 liquid scintillator detector array with wide angular coverage, and with a continuous incident neutron energy range from 6.5 to 16.5 MeV. We also provide a thorough covariance description of these results, including normalization of the probability distribution. While the measured n distributions agree well with the relatively large number of available literature measurements, there are comparatively very few measurements of the γ distributions from this reaction. However, our data support the presence of a nonzero α 4 Legendre polynomial component of the γ angular distribution suggested in past measurements, which is currently not incorporated in the ENDF/B-VIII.0 library despite the use of these same literature data for evaluation of the 12 C ($\ n, n'γ$) cross section. The correlated $\ n-γ$ distribution measurements are limited to three measurements at incident neutron energies near 14 MeV. Our results do not generally agree with any of these literature measurements. We observe clear indications of significant changes in the $\ n$ distribution for specific $\ γ$-detection angles and vice versa especially near thresholds for other reaction channels, which shows the potential for significant bias in experiments that, for example, tag on inelastic scattering using a single or small number of $\ γ$ -detection angles and could impact particle transport calculations.

6 ≤ A ≤ 19↗
NASA NTRS1991

The application of time-dependent wavepacket methods to reactive scattering

Several methods for performing numerically exact reactive scattering calculations using time-dependent wavepackets are reviewed. The basic idea is to take the multiarrangement reactive problem and reformulate it as one or more inelastic ones. In the simplest method, total reaction probabilities are extracted by calculating the flux of the wavepacket as it leaves the interaction region in the direction of the reactive arrangement. To make this practical, complex potentials that absorb the wavepacket before it reaches the numerical grid boundary are used. Methods that generate observables ranging from total, energy-averaged reaction probabilities up to energy- and state-resolved S-matrix elements are used. Techniques for efficiently performing the necessary inelastic wavepacket propagation are also reviewed.

Neuhauser, Daniel↗
DOE OSTI2026

Mapping resonant inelastic x-ray scattering onto electronic structure of iridium compounds

Resonant inelastic X-ray scattering (RIXS) is an indispensable tool that can selectively probe the electronic structure of active sites in energy conversion systems, for example, iridium oxides. However, decoding the RIXS spectra remains challenging due to its inherently complex many-body interactions. Here, we analyze Ir L 3 edge RIXS spectra of Ir, IrCl 3 , and IrO 2 , employing the joint density of states (JDOS) calculated directly from electronic band-structure calculations. The overall energy-loss features in the RIXS spectra are well reproduced by the JDOS, reaffirming the close correspondence between electron–hole excitations and RIXS spectra observed in prior studies. Intriguingly, the RIXS spectra above ∼4 eV in metallic Ir and IrO 2 follow a power-law behavior, 𝐼 RIXS ~Δ −𝑝 , where Δ is the energy loss and p is the associated power-law exponent. This is consistent with edge-singularity behavior commonly found in resonant X-ray scattering from metallic samples. Furthermore, orbital-projected JDOS enables a decomposition of the IrO 2 spectrum into specific dd transitions, providing a clear interpretation of orbital excitations and an efficient strategy for decoding RIXS spectra in iridium-based energy conversion systems.

5d↗
DOE OSTI2022

5f covalency from x-ray resonant Raman spectroscopy

Here, X-ray resonant Raman spectroscopy (XRRS), a variant of resonant inelastic x-ray scattering, has been used to investigate the two prototype systems, UF 4 and UO 2 . Both are U5f 2 and each is an example of 5f localized, ionic behavior and 5f localized, covalent behavior, respectively. From the M 5 XRRS measurements, the 5f band gap in each can be directly determined and, moreover, a clear and powerful sensitivity to 5f covalency emerges.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Electron Spin-Polarization Dependent Damage to Chiral Amino Acid l-Histidine

The damage of approximately monolayer films of L-histidine by low-energy spin-polarized electrons (SPE) ejected from a magnetized cobalt substrate has been probed using X-ray photoelectron spectroscopy (XPS). Average damage cross sections for N-containing motifs of L-histidine are 25-30 and 2-5 Mb for zwitterions and neutral molecules, respectively. Here, the magnetization direction of the substrate, which controls the ejected SPE helicity, was reversed in situ, and statistically significant differences in the damage cross sections of 10-30% were measured between positive and negative electron helicities. This is the first measurement of spin dichroism (SD) in an amino acid. The differential cross sections suggest that inelastic scattering of SPE with chiral molecules could contribute to the persistence of one enantiomer vs the other under certain irradiation conditions, particularly for the zwitterionic species.

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

Spin Wave Excitations in Honeycomb Antiferromagnet MnTiO 3

Using inelastic neutron scattering, we investigate the spin wave excitations on the antiferromagnetic MnTiO 3 (T N = 65 K), which has the stacked honeycomb structure. At T = 2 K, the spin wave energy extends up to 10.7 meV with the zone-center anisotropy gap of 0.8 meV. Whereas the dispersion is also strong along the direction perpendicular to the honeycomb planes, the exchange cancellation causes the magnetic interactions to be effectively anisotropic below TN. Furthermore, while the spin wave spectrum survives at temperatures above TN, we find that the magnetic interactions become more anisotropic within the paramagnetic phase. These results suggest that the magnetic interaction in MnTiO 3 crosses over from being three dimensional to two dimensional above T N .

36 MATERIALS SCIENCE↗
NASA NTRS1990

Magnetospheric interaction with Triton's ionosphere

The large electron densities measured by the Voyager radio occultation experiment are attributed to the precipitation of magnetospheric electrons with energy above 10 keV. Because the ionospheric electric Pedersen conductivity of Triton is about 10,000-20,000 mho and the Alfven conductance is about 3.5 mho, direct convective flow of plasma into the essentially infinitely conducting ionosphere is negligible. Magnetospheric electrons are transported to Triton's ionopause by curvature drift as a result of weak magnetic field line draping in a sub-Alfvenic plasma interaction with Triton. At the ionopause energetic electrons have a high probability of elastic and inelastic scattering and precipitate into the upper atmosphere. The average power dissipation is estimated to be about (2 - 3) x 10 to the 8th W.

Strobel, Darrell F.↗
DOE OSTI2022

Resolution of zigzag magnetic correlations in Na-deficient Na x IrO 3 without long-range ordering

The materials search for Kitaev quantum spin liquids led to the discovery of many honeycomb lattice materials. Much attention has been paid to materials without magnetic order down to the lowest temperatures. The newly synthesized Na-deficient Na x IrO 3 has been found to bear no sign of long-range magnetic order above 1 K from physical property measurements. In this paper, we report momentum-and energy-resolved excitation spectra in Na-deficient Na x IrO 3 measured using a resonant inelastic x-ray scattering spectrometer. Orbital excitation spectra show that the octahedral and trigonal crystal field splittings are larger in Na x IrO 3 than in Na 2 IrO 3 . On the other hand, the low-energy spectrum at low temperature shows a wave-vector dispersion and a spectral weight distribution that are similar to those of Na 2 IrO 3 , revealing that the two-dimensional zigzag magnetic correlations in Na x IrO 3 are similar to those in Na 2 IrO 3 in terms of the ordered magnetic moment direction and three dynamically fluctuating zigzag orders. The azimuth angle dependence of the low-energy spectrum corroborates these results. The two-dimensional zigzag magnetic correlations rapidly weaken until 50 K. At high temperatures, the spectral weight distribution of the low-energy excitation resembles that of the pure Kitaev model, indicating that the Kitaev interaction dominates the dynamic magnetic response at high temperature. Here we suggest that the larger crystal field and distortion and the weakened longer-range Heisenberg exchange interactions due to the Na deficiency in Na x IrO 3 contribute to bring Na x IrO 3 away from the zigzag long-range magnetic order phase.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗