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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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Magnetic pair distribution function data using polarized neutrons and ad hoc corrections

Here, we report the first example of magnetic pair distribution function (mPDF) data obtained through the use of neutron polarization analysis. Using the antiferromagnetic semiconductor MnTe as a test case, we present high-quality mPDF data collected on the HYSPEC instrument at the Spallation Neutron Source using longitudinal polarization analysis to isolate the magnetic scattering cross section. Clean mPDF patterns are obtained for MnTe in both the magnetically ordered state and the correlated paramagnet state, where only short-range magnetic order is present. We also demonstrate significant improvement in the quality of high-resolution mPDF data through the application of ad hoc corrections that require only minimal human input, minimizing potential sources of error in the data processing procedure. We briefly discuss the current limitations and future outlook of mPDF analysis using polarized neutrons. Overall, this work provides a useful benchmark for mPDF analysis using polarized neutrons and provides an encouraging picture of the potential for routine collection of high-quality mPDF data.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
NASA NTRS1986

Compton scattering in strong magnetic fields

The relativistic cross section for Compton scattering by electrons in strong magnetic fields is derived. The results confirm and extend earlier work which has treated only transitions to the lowest or first excited Landau levels. For the teragauss field strengths expected in neutron star magnetospheres, the relative rates for excited state transitions are found to be significant, especially for incident photon energies several times the cyclotron frequency. Since these transitions must result in the rapid emission of one or more cyclotron photons as well as the Compton-scattered photon, the scattering process actually becomes a photon 'splitting' mechanism which acts to soften hard photon spectra, and also provides a specific mechanism for populating higher Landau levels in the electron distribution function. The results should be significant for models of gamma-ray bursters and pulsating X-ray sources.

Daugherty, Joseph K.↗
DOE OSTI2021

Stripe antiferromagnetic ground state of the ideal triangular lattice compound KErSe 2

Rare-earth triangular lattice materials have been proposed as a good platform for the investigation of frustrated magnetic ground states. KErSe 2 , with the delafossite structure, contains perfect two-dimensional Er 3 + triangular layers separated by potassium ions, realizing this ideal configuration and inviting study. In this study, we investigate the magnetism of KErSe 2 at millikelvin temperatures by heat capacity and neutron powder diffraction. Heat capacity results reveal a magnetic transition at 0.2 K in zero applied field. This long-range order is suppressed by an applied magnetic field of 0.5 T below 0.08 K. Neutron powder diffraction suggests that the zero-field magnetic structure orders with k = ( 1 2 , 0 , 1 2 ) in a stripe spin structure. Unexpectedly, Er is found to have a reduced moment of 3.06(1) μ B /Er in the ordered state, and diffuse magnetic scattering, which originates at higher temperatures, is found to persist in the ordered state, potentially indicating magnetic fluctuations. Neutron diffraction collected under an applied field shows a metamagnetic transition at ~ 0.5 T to ferromagnetic order with k = ( 0 , 0 , 0 ) and two possible structures, which are likely dependent on the applied field direction. The zero-field stripe spin structure can be explained by the anisotropic interactions or the first-, second-, and third-neighbor couplings in the antiferromagnetic triangular lattice.

2-dimensional systems↗
DOE OSTI2022

Chiral structures of electric polarization vectors quantified by X-ray resonant scattering

Resonant elastic X-ray scattering (REXS) offers a unique tool to investigate solid-state systems providing spatial knowledge from diffraction combined with electronic information through the enhanced absorption process, allowing the probing of magnetic, charge, spin, and orbital degrees of spatial order together with electronic structure. A new promising application of REXS is to elucidate the chiral structure of electrical polarization emergent in a ferroelectric oxide superlattice in which the polarization vectors in the REXS amplitude are implicitly described through an anisotropic tensor corresponding to the quadrupole moment. Here, we present a detailed theoretical framework and analysis to quantitatively analyze the experimental results of Ti L-edge REXS of a polar vortex array formed in a PbTiO 3 /SrTiO 3 superlattice. Based on this theoretical framework, REXS for polar chiral structures can become a useful tool similar to x-ray resonant magnetic scattering (XRMS), enabling a comprehensive study of both electric and magnetic REXS on the chiral structures.

36 MATERIALS SCIENCE↗
NASA NTRS1986

One- and two-photon Compton scattering in strong magnetic fields

Calculations are made of the Compton scattering cross section in a very strong magnetic field, such as encountered in pulsars, for arbitrary photon and electron energies. The effect of the vacuum polarization in the weak-field limit is included as well as the plasma polarizability. Radiative transitions between any pair of Landau levels are included as well as two-photon scattering. The latter process, due to the presence of the cyclotron resonance, becomes comparable to nonresonant one-photon scattering when excited final states are allowed, and acts as a source of photons which is more important than bremsstrahlung at low plasma densities.

Bussard, R. W.↗
DOE OSTI2024

Time-resolved magneto-optical effects in the altermagnet candidate MnTe

α -MnTe is an antiferromagnetic semiconductor with above room temperature T N = 310 K, which is promising for spintronic applications. Recently, it was reported to be an altermagnet, containing bands with momentum-dependent spin splitting; time-resolved experimental probes of MnTe are, therefore, important both for understanding novel magnetic properties and potential device applications. Here, we investigate ultrafast spin dynamics in epitaxial MnTe(001)/InP(111) thin films using pump-probe magneto-optical measurements in the Kerr configuration. At room temperature, we observe an oscillation mode at 55 GHz that does not appear at zero magnetic field. Combining field and polarization dependence, we identify this mode as a magnon, likely originating from inverse stimulated Raman scattering. Magnetic field-dependent oscillations persist up to at least 335 K, which could reflect coupling to known short-range magnetic order in MnTe above T N . Additionally, we observe two optical phonons at 3.6 and 4.2 THz, which broaden and redshift with increasing temperature.

Gray, Isaiah↗
DOE OSTI2025

Hidden frustration in the triangular-lattice antiferromagnet NdCd 3 ⁢P 3

We report a study of the magnetic ground state and crystal electric field (CEF) scheme in the triangular-lattice antiferromagnet NdCd 3 ⁢P 3 . Combined neutron scattering, magnetization, and heat capacity measurements demonstrate that the Nd 3+ moments occupying the triangular lattice in this material harbor hidden signs of frustration not detected in typical Curie-Weiss-based parametrization of the frustration index (𝑓 = Θ 𝐶⁢𝑊 /𝑇 𝑁 ). This is evidenced by a zero-field splitting of the Kramers' ground state and first excited state doublets at temperatures far in excess of 𝑇 𝑁 as well as signatures of low-energy fluctuations for 𝑇 ≫ 𝑇 𝑁 . As a result, a suppression of the zero-field ordered moment relative to its field saturation value is observed, and the impacts of this magnetic frustration as well as the coexisting bond frustration in the CdP honeycomb network on the physical properties of NdCd 3 ⁢P3 are discussed.

Frustrated magnetism↗
DOE OSTI2024

In Situ Local Imaging of Ferromagnetism and Superconductivity in RbEuFe 4 As 4

The coexistence of superconductivity and ferromagnetism is an intrinsically interesting research focus in condensed matter physics but the study is limited by low superconducting (T c ) and magnetic (T m ) transition temperatures in related materials. Here, we used a scanning superconducting quantum interference device to image the in-situ diamagnetic and ferromagnetic responses of RbEuFe 4 As 4 with high T c and T m . We observed significant suppression of superfluid density in vicinity of the magnetic phase transition, signifying fluctuation-enhanced magnetic scatterings between Eu spins and Fe 3d conduction electrons. Intriguingly, we observed multiple ferromagnetic domains which should be absent in an ideal magnetic helical phase. The formation of these domains demonstrates a weak c-axis ferromagnetic component probably arising from Eu spin-canting effect, indicative of possible superconductivity-driven domain Meissner and domain vortex-antivortex phases as revealed in EuFe 2 (As 0.79 P 0.21 ) 2 . Finally, our observations highlight RbEuFe 4 As 4 is a unique system which includes multiple interplay channels between superconductivity and ferromagnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Mutual spin-phonon driving effects and phonon eigenvector renormalization in nickel (II) oxide

The physics of mutual interaction of phonon quasiparticles with electronic spin degrees of freedom, leading to unusual transport phenomena of spin and heat, has been a subject of continuing interests for decades. Despite its pivotal role in transport processes, the effect of spin-phonon coupling on the phonon system, especially acoustic phonon properties, has so far been elusive. By means of inelastic neutron scattering and first-principles calculations, anomalous scattering spectral intensity from acoustic phonons was identified in the exemplary collinear antiferromagnetic nickel (II) oxide, unveiling strong spin-lattice correlations that renormalize the polarization of acoustic phonon. In particular, a clear magnetic scattering signature of the measured neutron scattering intensity from acoustic phonons is demonstrated by its momentum transfer and temperature dependences. The anomalous scattering intensity is successfully modeled with a modified magneto-vibrational scattering cross-section, suggesting the presence of spin precession driven by phonon. The renormalization of phonon eigenvector is indicated by the observed “geometry-forbidden” neutron scattering intensity from transverse acoustic phonon. Importantly, the eigenvector renormalization cannot be explained by magnetostriction but instead, it could result from the coupling between phonon and local magnetization of ions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

CHESS: The future direct geometry spectrometer at the second target station

CHESS, chopper spectrometer examining small samples, is a planned direct geometry neutron chopper spectrometer designed to detect and analyze weak signals intrinsic to small cross sections (e.g., small mass, small magnetic moments, or neutron absorbing materials) in powders, liquids, and crystals. CHESS is optimized to enable transformative investigations of quantum materials, spin liquids, thermoelectrics, battery materials, and liquids. The broad dynamic range of the instrument is also well suited to study relaxation processes and excitations in soft and biological matter. The 15 Hz repetition rate of the Second Target Station at the Spallation Neutron Source enables the use of multiple incident energies within a single source pulse, greatly expanding the information gained in a single measurement. Furthermore, the high flux grants an enhanced capability for polarization analysis. This enables the separation of nuclear from magnetic scattering or coherent from incoherent scattering in hydrogenous materials over a large range of energy and momentum transfer. This paper presents optimizations and technical solutions to address the key requirements envisioned in the science case and the anticipated uses of this instrument.

47 OTHER INSTRUMENTATION↗
DOE OSTI2022

Intermediate scattering potential strength in electron-irradiated YBa 2 Cu 3 O 7-δ from London penetration depth measurements

Temperature-dependent London penetration depth, λ(T), of a high quality optimally-doped YBa 2 Cu 3 O 7-δ single crystal was measured using tunnel-diode-resonator technique. Controlled artificial disorder was induced by low-temperature 20 K irradiation by 2.5 MeV electrons at two large doses of 3.8 x 10 19 and 5.3 x 10 19 electrons per cm 2 . The irradiation caused significant suppression of the superconductor's critical temperature, T c , from 94.6 K to 90.0 K, and to 78.7 K, respectively. The low-temperature behavior of λ(T) evolves from a T-linear in pristine state to a T 2 behavior after irradiation, expected for a line-nodal d-wave superconductor. However, the original theory that explained such behavior assumed a unitary limit of the scattering potential, whereas usually in normal metals and semiconductors, Born scattering is sufficient to describe the experiment. To estimate the scattering potential strength, we calculated the superfluid density, ρ s (t = T/T c ) = λ 2 (0)/λ 2 (T), varying the amount and strength of non-magnetic scattering using a self-consistent t-matrix theory. Fitting the obtained curves to a power law, ρ s = 1 - Rt n , and to a polynomial, ρ s = 1 - At - Bt 2 , and comparing the coefficients n in one set and A and B in another with the experimental values, we estimate the phase shift to be around 70 and 65°, respectively. We correlate this result with the evolution of the density of states with nonmagnetic disorder.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Influence of the Cubic Sublattice on Magnetic Coupling between the Tetrahedral Sites of Garnet

Here, we present a study on the nuclear and magnetic structures of two iron-based garnets with magnetic cations isolated on tetrahedral sites. Ca 2 YZr 2 Fe 3 O 12 and Ca 2 LaZr 2 Fe 3 O 12 offer an interesting comparison for examining the effect of increasing cation size within the diamagnetic backbone of the garnet crystal structure, and how such changes affect the magnetic order. Despite both systems exhibiting well-pronounced magnetic transitions at low temperatures, we also find evidence for diffuse magnetic scattering due to a competition between the nearest-neighbor, next nearest-neighbor, and so on, within the tetrahedral sites. This competition results in a complex noncollinear magnetic structure on the tetrahedral sublattice creating a mixture of ferro- and antiferromagnetic interactions above the long-range ordering temperature near 20 K and suggests that the cubic site of the garnet plays a significant role in mediating the superexchange interactions between tetrahedral cations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1974

Mirroring within the Fokker-Planck formulation of cosmic ray pitch angle scattering in homogeneous magnetic turbulence

The Fokker-Planck coefficient for pitch angle scattering, appropriate for cosmic rays in homogeneous, stationary, magnetic turbulence, is computed from first principles. No assumptions are made concerning any special statistical symmetries the random field may have. This result can be used to compute the parallel diffusion coefficient for high energy cosmic rays moving in strong turbulence, or low energy cosmic rays moving in weak turbulence. Becuase of the generality of the magnetic turbulence which is allowed in this calculation, special interplanetary magnetic field features such as discontinuities, or particular wave modes, can be included rigorously. The reduction of this results to previously available expressions for the pitch angle scattering coefficient in random field models with special symmetries is discussed. The general existance of a Dirac delta function in the pitch angle scattering coefficient is demonstrated. It is proved that this delta function is the Fokker-Planck prediction for pitch angle scattering due to mirroring in the magnetic field.

Goldstein, M. L.↗
DOE OSTI2023

Second Target Station Project (CHESS Technical Report)

CHESS is a direct geometry neutron spectrometer designed to detect and analyze weak signals intrinsic to small cross-sections (e.g., small mass, small magnetic moments or neutron absorbing materials). This instrument is optimized to enable unprecedented characterization of quantum materials, spin liquids, thermoelectric and battery materials, liquids, and soft matter. The ability to simultaneously measure dynamic processes over a wide energy range for very small samples will make CHESS the spectrometer of choice for the initial exploration of new materials. The broad dynamic range will also be well matched to measurements of relaxation processes and excitations in soft and biological matter. The 15 Hz repetition rate of STS enables use of multiple incident energies within a single source pulse, greatly expanding the information gained in a single experiment. An essential feature of CHESS is the capability for polarization analysis to separate nuclear from magnetic scattering or coherent from incoherent scattering in hydrogenous materials, and better understanding spin-anisotropic correlations. This instrument will employ advanced sample environments such as high-pressure cells, dilution refrigerators, high field cryo-magnets and polarization devices, as well as combinations of these, to solve problems at the forefront of materials research. CHESS will be one of the flagship spectrometers of the Second Target Station (STS), providing world leading capabilities.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1981

A model for X-ray pulsar spectra

The transfer of X-rays through a hot, magnetized, scattering atmosphere is studied using a Monte Carlo technique. The process considered is Compton scattering, including cyclotron resonance scattering. Unlike previous work on the subject, the effects of frequency shifts, relativistic corrections, recoil, and polarization mode changes have been included. It is found that the emergent X-ray spectra harden toward the magnetic equator, are cut off at a frequency less than the fundamental cyclotron frequency, and depend on the magnetic field strength. The relevance of the results to observed pulsar spectra is indicated.

Pravdo, S. H.↗
DOE OSTI2021

Sudden Collapse of Magnetic Order in Oxygen-Deficient Nickelate Films

Antiferromagnetic order is a common and robust ground state in the parent (undoped) phase of several strongly correlated electron systems. The progressive weakening of antiferromagnetic correlations upon doping paves the way for a variety of emergent many-electron phenomena including unconventional superconductivity, colossal magnetoresistance, and collective charge-spin-orbital ordering. In this study, we explored the use of oxygen stoichiometry as an alternative pathway to modify the coupled magnetic and electronic ground state in the family of rare earth nickelates ( RENiO 3 - x ). Using a combination of x-ray spectroscopy and resonant soft x-ray magnetic scattering, we find that, while oxygen vacancies rapidly alter the electronic configuration within the Ni and O orbital manifolds, antiferromagnetic order is remarkably robust to substantial levels of carrier doping, only to suddenly collapse beyond 0.21 e - / Ni without an accompanying structural transition. Our work demonstrates that ordered magnetism in RENiO 3 - x is mostly insensitive to carrier doping up to significant levels unseen in other transition-metal oxides. The sudden collapse of ordered magnetism upon oxygen removal may provide a new mechanism for solid-state magnetoionic switching and new applications in antiferromagnetic spintronics.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Fermi surface reconstruction due to orthorhombic distortion in the Dirac semimetal YbMnSb 2

A Dirac semimetal with magnetic atoms as constituents delivers an interesting platform to investigate the interplay of Fermi surface (FS) topology, electron correlation, and magnetism. One such family of semimetals is YbMn⁢ 𝑃⁢𝑛 2 (𝑃⁢𝑛 = Sb,Bi), which is being actively studied due to the intertwined spin and charge degrees of freedom. Here, in this study, we investigate the relationship between the magnetic/crystal structures and FS topology of YbMnSb 2 using single crystal x-ray diffraction, neutron scattering, magnetic susceptibility, magnetotransport measurements, and complimentary density functional theory (DFT) calculations. Contrary to previous reports, the x-ray and neutron diffraction reveal that YbMnSb 2 crystallizes in an orthorhombic 𝑃⁢𝑛⁢𝑚⁢𝑎 structure with a notable antiphase displacement of the magnetic Mn ions that increases in magnitude upon cooling. As a result of this orthorhombicity, first-principles DFT calculation reveals a reduced Brillouin zone and more anisotropic FS of YbMnSb 2 compared to YbMnBi 2 . Moreover, the hole-type carrier density drops by two orders of magnitude as YbMnSb 2 orders antiferromagnetically, indicating band folding in a magnetic ordered state. In addition, the Landau level fan diagram yields a nontrivial nature of the Shubnikov–de Haas quantum oscillation frequency arising from the Dirac-like Fermi pocket. These results imply that YbMnSb 2 is an ideal platform to explore the interplay of subtle lattice distortion, magnetic order, and topological transport arising from relativistic quasiparticles.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗