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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 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Polaronic Conductivity in Cr 2 Ge 2 Te 6 Single Crystals

We report that intrinsic 2D ferromagnetic semiconductors are an important class of materials for spin-charge conversion applications. Cr 2 Ge 2 Te 6 retains long-range magnetic order in the bilayer at cryogenic temperatures and shows complex magnetic interactions with considerable magnetic anisotropy. Here, a series of structural, magnetic, X-ray scattering, electronic, thermal transport and first-principles calculation studies are performed, which reveal that localized electronic charge carriers in Cr 2 Ge 2 Te 6 are dressed by the surrounding lattice and are involved in polaronic transport via hopping that is observed via magnetocrystalline anisotropy. This opens the possibility for manipulation of charge transport in Cr 2 Ge 2 Te 6 —based devices by electron–phonon- and spin–orbit coupling-based tailoring of polaron properties.

2D materials↗
DOE OSTI2024

3D oxygen vacancy distribution and defect-property relations in an oxide heterostructure

Oxide heterostructures exhibit a vast variety of unique physical properties. Examples are unconventional superconductivity in layered nickelates and topological polar order in (PbTiO 3 ) n /(SrTiO 3 ) n superlattices. Although it is clear that variations in oxygen content are crucial for the electronic correlation phenomena in oxides, it remains a major challenge to quantify their impact. Here, we measure the chemical composition in multiferroic (LuFeO 3 ) 9 /(LuFe 2 O 4 ) 1 superlattices, mapping correlations between the distribution of oxygen vacancies and the electric and magnetic properties. Using atom probe tomography, we observe oxygen vacancies arranging in a layered three-dimensional structure with a local density on the order of 10 14 cm -2 , congruent with the formula-unit-thick ferrimagnetic LuFe 2 O 4 layers. The vacancy order is promoted by the locally reduced formation energy and plays a key role in stabilizing the ferroelectric domains and ferrimagnetism in the LuFeO 3 and LuFe 2 O 4 layers, respectively. The results demonstrate pronounced interactions between oxygen vacancies and the multiferroic order in this system and establish an approach for quantifying the oxygen defects with atomic-scale precision in 3D, giving new opportunities for deterministic defect-enabled property control in oxide heterostructures.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Imitation of spin density wave order in Cu 3 Nb 2 O 8

Spin density waves, based on modulated local moments, are usually associated with metallic materials, but have recently been reported in insulators which display coupled magnetic and structural order parameters. Here, we discuss one such example, the multiferroic Cu 3 Nb 2 O 8 , which is reported to undergo two magnetic phase transitions, first to an unknown antiferromagnetic phase at $T_N$ ≈ 26.5 K , and then to a helicoidal structure coupled to an electric polarization below $T_2$ ≈ 24 K which breaks the crystallographic inversion symmetry. By analogy with other complex oxides, one might naturally expect this intermediate phase to be a spin density wave phase. We apply spherical polarimetry to confirm the low-temperature magnetic structure, yet only observe a single magnetic phase transition to helicoidal order. We argue that the reported unknown phase actually supports an imitation spin density wave which originates from a decoupling of the components of the magnetic order parameter, as allowed by symmetry and driven by thermal fluctuations. This provides a mechanism for the magnetic, but not nuclear, structure to break inversion symmetry thereby creating an intermediate phase in the proximity of $T_N$ which imitates a spin density wave. As the temperature is reduced, this intermediate structure destabilizes the crystal such that a structural chirality is induced, as reflected by the emergence of the electric polarization, and the imitation spin density wave relaxes into a generic helicoid. This scenario in which critical fluctuations allow the magnetic structure to break inversion symmetry while the crystal structure remains centrosymmetric might be relevant to other complex multiferroics.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Three-dimensional ferromagnetism and magnetotransport in van der Waals Mn-intercalated tantalum disufide

Van der Waals (vdW) ferromagnets are an important class of materials for spintronics applications. The recent discovery of atomically vdW magnets CrI 3 and Cr 2 Ge 2 Te 6 has triggered a renaissance in the area of two-dimensional (2D) magnetism. Herein we systematically studied 2H-Mn 0.28 TaS 2 single crystal, a 2D vdW ferromagnet with T c ~ 82.3K, and a large in-plane magnetic anisotropy. Mn K-edge x-ray absorption spectroscopy was measured to provide information on its electronic state and local atomic environment. The detailed magnetic isotherms measured in the vicinity of T c indicates that the spin coupling inside 2H-Mn 0.28 TaS 2 is of a 3D Heisenberg type coupled with the attractive long-range interaction between spins that decay as J(r) ≈ r –4.85 . Both resistivity ρ(T) and thermopower S(T) exhibit anomalies near T c , confirming that the hole-type transport carriers strongly interact with local moments. Furthermore, an unusual angle-dependent magnetoresistance is further observed, suggesting a possible field-induced novel magnetic structure.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1980

The heating of a thermally conducting stratified medium. I - Self-gravitating gas threaded by magnetic fields

Two theoretical problems are presented to illustrate the equilibrium structure of a gravitating gas threaded by magnetic fields. The gas is heated everywhere at a rate proportional to the local density and cooled by thermal conduction channeled along the magnetic field. Emphasis is placed on the influence of anisotropic thermal conduction on the distribution of the gas. In the first problem, the magnetic field has straight field lines that are nearly parallel. The simple field line geometry allows us to calculate, in closed form, the equilibria configurations of a gas slab which is cooled nonuniformly by thermal conduction along field lines which leave the gas slab at varying angles. The problem involves the gas having a free boundary, which is one of the unknowns to be determined. A discussion is given of the set of all possible equilibria so constructed. In the second problem there is no electric current flowing in the gas but there is a potential magnetic field having curved field lines. Exact solutions of equilibriums are presented to illustrate the heating of stellar and galactic coronas.

Lerche, I.↗
NASA NTRS1982

Planetary radio astronomy observations during the Voyager 1 Titan flyby

During the Voyager 1 Titan flyby, unusual radio emissions were observed by the planetary radio astronomy experiment in the 20- to 97-kHz frequency range. It is shown that Titan itself is not the source of the observed radio emission. The emission features are attributed to modification of the normal Saturn kilometric radiation by propagation effects in enhanced density structures within the Titan wake. Furthermore, spiky emissions observed in the magnetic wake of Titan are interpreted in terms of local electrostatic instabilities at the electron plasma frequency. From these measurements a range of electron densities in the wake region is derived, and the consistency of the results is discussed.

Daigne, G.↗
NASA NTRS1992

Coronal density and temperature structure from coordinated observations associated with the total solar eclipse of 1988 March 18

This paper explores and compares diagnostics for temperature and density within large-scale structures of the inner corona based on cospatial and cotemporal spectrophotometric observations made at the time of the total solar eclipse of 1988 March 17/18. In the analysis a determination of plasma temperature T can be derived unambiguously from the intensity ratios Fe XIV/XUV or Fe XIV/Fe X since all the emission lines come from the ionized state of Fe and the ratios are only weakly dependent on density. These temperatures and the densities found in well-defined large-scale coronal structures are discussed. The emission-line temperature is found to be high (local maxima) in the coronal structures with enhanced white-light emission and associated with new cycle high-latitude magnetic fields separated from the old cycle polar field of opposite polarity. Also the average of the ratio of scale-height temperature/temperature over the entire range of position angle is roughly unity although the ratio is higher than unity (1.3-1.6) in the three most prominent streamers.

Guhathakurta, M.↗
NASA NTRS1993

Sunspot dynamics

This report describes recent results of our theoretical and observational work on dynamical phenomena in sunspots. The overall goal of this research has been a better understanding of the various oscillatory, transient, and steady motions in a sunspot and their relation to the basic structure of the sunspot. The principal topics of the research reported here are the following: (1) sunspot seismology, i.e., the study of the interaction of solar p-modes with a sunspot as a probe of the subsurface structure of a sunspot; (2) local sources of acoustic waves in the solar photosphere; and (3) siphon flows in isolated magnetic flux tubes and their relation to the photospheric Evershed flow and to intense magnetic elements outside of sunspots.

Thomas, John H.↗
DOE OSTI2021

Emergence and dynamics of unconfined self-organised vortices in active magnetic roller liquids

Actively driven colloids demonstrate complex out-of-equilibrium dynamics often rivaling self-organized patterns and collective behavior observed in living systems. Recent studies revealed the emergence of steady macroscopic states with multiple interacting vortices in an unconfined environment that emerge from the coupling between microscale particle rotation and translation. Yet, insights into the microscopic behavior during the vortex emergence, growth, and formation of a multi-vortical state remain lacking. Here, we investigate in experiments and simulations how the microscale magnetic roller behavior leads to the emergence of seed vortices, their aggregation or annihilation, and the formation of stable large-scale vortical structures. We reveal that the coupling of roller-induced hydrodynamic flows guides the local self-densifications and self-organization of the micro-rollers into seed vortices. Here, the resulting multi-vortical state is sensitive to the external magnetic field amplitude and allows tuning the rollers' number density in a vortex and its characteristic size.

36 MATERIALS SCIENCE↗
NASA NTRS1982

Coronal closed structures. IV - Hydrodynamical stability and response to heating perturbations

The response of magnetically confined atmospheres to perturbations in the temperature and density distribution, and the local heating rate by means of a one-dimensional time-dependent hydrodynamical code, which incorporates the full energy, momentum and mass conservation equations is studied. These studies extend the linear instability analysis of Habbal and Rosner (1979) into the finite-amplitude regime, and generalize the confined atmosphere models of Serio et al., to the time-dependent domain. The results show that closed coronal atmospheres are stable against finite-amplitude perturbations if the chromospheric response is taken into account; and observed correlated increases in coronal density and temperature can only be achieved under quiescent conditions by increasing the heat deposition rate relatively more in the chromosphere than in the corona.

Peres, G.↗
DOE OSTI2023

Electrochemical Control of Magnetism on the Breathing Kagome Network of Li x ScMo 3 O 8

Controlling properties within a given functional inorganic material structure type is often accomplished through tuning the electronic occupation, which is in turn dictated by the elemental composition determined at the time of material preparation. We employ electrochemical control of the lithium content, with associated electronic occupancy control, to vary the magnetic properties of a material where a kagome-derived network of Mo 3 triangles carry the spin. In this case, Li is electrochemically inserted into LiScMo 3 O 8 , a layered compound containing a breathing Mo kagome network. Up to two additional Li can be inserted into LiScMo 3 O 8 , transforming it into Li 3 ScMo 3 O 8 . Li 2 ScMo 3 O 8 prepared by electrochemical lithiation is compared to the quantum spin liquid candidate compound Li 2 ScMo 3 O 8 prepared through high-temperature solid-state methods, which has a slightly different structural stacking sequence but a similar kagome-derived network. Magnetic measurements are supported by first-principles calculations, showing that electrons remain localized on the Mo clusters throughout the doping series. As x is varied in Li x ScMo 3 O 8 , the measurements and calculations reveal the evolution from a diamagnetic band insulator at x = 1 to a geometrically frustrated magnet at x = 2, back to a diamagnetic insulator at x = 3. These results indicate a likelihood of strong coupling between the degree of Li disorder and charge/magnetic ordering over the Mo 3 clusters.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS2003

The Cytoskeleton and Force Response Mechanisms

The long term aim of this project was to define the mechanisms by which cells sense and respond to the physical forces experienced at 1g and missing in microgravity. Identification and characterization of the elements of the cells force response mechanism could provide pathways and molecules to serve as targets for pharmacological intervention to mitigate the pathologic effects of microgravity. Mechanical forces experienced by the organism can be transmitted to cells through molecules that allow cells to bind to the extracellular matrix and through other types of molecules which bind cells to each other. These molecules are coupled in large complexes of proteins to structural elements such as the actin cytoskeleton that give the cell the ability to sense, resist and respond to force. Application of small forces to tissue culture cells causes local elevation of intracellular calcium through stretch activated ion channels, increased tyrosine phosphorylation and a restructuring of the actin cytoskeleton. Using collagen coated iron oxide beads and strong magnets, we can apply different levels of force to cells in culture. We have found that force application causes the cells to polymerize actin at the site of mechanical deformation and unexpectedly, to depolymerize actin across the rest of the cell. Observations of GFP- actin expressing cells demonstrate that actin accumulates at the site of deformation within the first five minutes of force application and is maintained for many tens of minutes after force is removed. Consistent with the reinforcement of the cytoskeletal structures underlying the integrin-bead interaction, force also alters the motion of bound magnetic beads. This effect is seen following the removal of the magnetic field, and is only partially ablated by actin disruption with cytochalsin B. While actin is polymerizing locally at the site of force application, force also stimulates a global reduction in actin filament content within the cells. We have examined the roles of several actin filament disassembly factors in the global reduction of cellular actin filaments. The calcium regulated actin filament severing protein gelsolin is not necessary for the increased actin turnover, as cells derived from gelsolin null and wildtype mice still show a reduction in total actin filament content. Instead, our work suggests that the actin binding protein cofilin may be important for these changes in actin dynamics. Cofilin binds to and enhances the disassembly of actin filaments. Using immunological methods, we observe transient changes in the phosphorylation state of cofilin upon force application that suggests that cofilin may mediate actin filament turnover. Early after force application, cofilin is transiently dephosphorylated, activating its actin disassembly activity. Subsequently, we find a hyper-phosphorylation of cofilin, rendering it inactive. This reduction in cofilin activity may explain the stability of the force induced actin structuttes. In testing this hypothesis, we aimed to generate cells that express the constituitively active kinase (LIM-kinase) that phosphorylates cofilin. lnidial attempts in the cell lines used for the our previous studies proved unsuccessful. While we prepare this work for pubication, we are continuing to study other cell lines and tissue sources to determine whether they show a reduction in F-actin content after force application.

Allen, Philip Goodwin↗
DOE OSTI2024

Exploring the impact of Cr-doping on the crystallographic and magnetic structure of Mn 5 Si 3 antiferromagnetic alloy

Here, the role of Cr-doping on the structural and magnetic ground states of Mn 5 Si 3 alloy has been investigated through temperature-dependent neutron powder diffraction (NPD) and X-ray absorption fine structure (XAFS) techniques. All the Cr-doped alloys of nominal composition Mn 5-x Cr x Si 3 (for x = 0.05, 0.1, and 0.2) undergo two first-order magneto-structural phase transitions from hexagonal (space group: P6 3 /mcm) paramagnetic → orthorhombic (space group: Ccmm) collinear antiferromagnetic → orthorhombic (space group: Cc2m) non-collinear antiferromagnetic phase on cooling from room temperature. NPD studies at different constant temperatures indicate that both anti ferromagnetic phases are commensurate in nature and can be represented by q = (0,1,0) magnetic propagation vector for all the Cr-doped alloys. Such doping at the Mn-site results in a significant modification of the non-collinear antiferromagnetic structure (both moment size and orientation) and hence affects the unusual magnetic properties, like inverted hysteresis loop, thermomagnetic irreversibility, etc. The XAFS measurements were performed to interpret the local environment of doped Cr atoms in detail, which is critical for a microscopic understanding of the unusual properties of this class of Cr-doped Mn 5 Si 3 alloys. The analysis confirms the elemental state of Cr in the doped alloys and indicates a high degree of preservation of local crystallographic structure with varying Cr concentration and sample temperature. Doping induces intriguing changes in XAFS patterns, elucidated through different types of scattering mechanisms associated with the central absorbing Cr atom.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Local lattice distortions and electronic phases in perovskite manganite Pr 0.5 Sr 0.5 MnO 3

We use variable temperature and magnetic field total x-ray scattering to study the crystal structure of the strongly correlated Pr 0.5 Sr 0.5 MnO 3 perovskite, which is a paramagnetic insulator at room temperature, becomes a ferromagnetic metal at 272 K and, upon further decreasing the temperature, turns into an antiferromagnetic insulator at 105 K. We find that a model featuring a monoclinic symmetry captures the structure and its temperature and field evolution well, eliminating the need to evoke a phase segregation scenario as done in prior studies. It appears that coupled variations in Mn–oxygen bonding distances and angles from their values in an undistorted perovskite lattice, i.e., coupled local lattice distortions, assist the phase transitions in Pr 0.5 Sr 0.5 MnO 3 , contributing to its unique physical properties. Local structural distortions thus emerge as an important degree of freedom in strongly correlated systems, in particular perovskite manganates, and, therefore, they should be fully accounted for when their fascinating physics is considered.

36 MATERIALS SCIENCE↗
NASA NTRS2023

Heliocentric Distance and Solar Activity Dependence of Sustained Quasi-radial Interplanetary Magnetic Field Occurrence

Planets close to their stars experience an interplanetary magnetic field (IMF) that is dominantly quasi-radial. Our solar system serves as a laboratory to study how the occurrence of quasi-radial IMF varies away from the star and under different stellar activities. Furthermore, on time and spatial scales relevant to magnetospheric physics, solar wind variability prevails in the form of structures generated both at the Sun and locally in the interplanetary space. The stationary Parker spiral model only approximates the large-scale structure of the IMF. Deviations from the Parker spiral often result in strongly radial magnetic fields that give rise to kinetic foreshock turbulence, which in turn can impact planetary magnetospheres. The relative significance of this type of interaction can be estimated statistically based on the occurrence rate of cases where the IMF is directed along the radial direction, leading to the entire day-side magnetosphere being downstream of the ion foreshock. We use observations covering radial distances from 0.1 to 10 au and more than 2 solar cycles to quantify the prevalence of radial IMFs throughout the heliosphere. Near Earth's orbit, it is found that the occurrence rates of quasi-radial and southward IMF orientations are similar, and that the Pearson correlation coefficient is ${{ \mathcal R }}_{{xy}}\sim -0.7$ calculated between quasi-radial IMF occurrence rate and solar activity. A negative correlation is demonstrated for radial distances extending to at least Mars but not to Saturn.

Brandon L Burkholder↗
DOE OSTI2021

Stray-Capacitance As a Simple Tool for Monitoring and Locating Heat Generation Demonstrated in Three Superconducting Magnets

Real-time monitoring of heat loads in cryogenic systems is critical for many applications, particularly high field magnets. In this work, we demonstrated that by monitoring changes in the capacitance of local probes consisting of thin metal strips with a porous glass fiber dielectric, boiling helium from as little as 0.1 J of deposited heat can be located by an analysis of the response speed and amplitude in nearby probes. We further implemented stray-capacitance monitoring of a magnet's metal support structures, a more global probe of the magnet volumes, in three high temperature superconducting Bi-2212 magnet types. Global structural stray-capacitance monitoring was evaluated for a single racetrack coil, a common coil dipole, and a canted cosine theta winding, showing a quench response comparable to voltage signals without inductive effects. The response of the local and global probes was compared in the single racetrack coil with well-known quench properties. Monitoring the cool-down of the common coil dipole demonstrated the added benefit of the global stray-capacitance as a liquid level monitor. This simple method of monitoring the capacitance has proven to be a versatile and robust technique for monitoring and locating heat.

43 PARTICLE ACCELERATORS↗
DOE OSTI2022

Magnetic Ordering in a High‐Spin Donor–Acceptor Conjugated Polymer

Abstract The development of open‐shell organic molecules that magnetically order at room temperature,which can be practically applied, remains a grand challenge in chemistry, physics, and materials science. Despite the exploration of vast chemical space, design paradigms for organic paramagnetic centers generally result in unpaired electron spins that are unstable or isotropic. Here, a high‐spin conjugated polymer is demonstrated, which is composed of alternating cyclopentadithiophene and benzo[1,2‐ c ;4,5‐ c ′]bis[1,2,5]thiadiazole heterocycles, in which macromolecular structure and topology coalesce to promote the spin center generation and intermolecular exchange coupling. Electron paramagnetic resonance (EPR) spectroscopy is consistent with spatially localized spins, while magnetic susceptibility measurements show clear anisotropic spin ordering and exchange interactions that persist at room temperature. The application of long‐range π‐correlations for spin center generation promotes remarkable stability. This work offers a fundamentally new approach to the implementation of this long‐sought‐after physical phenomenon within organic materials and the integration of manifold properties within emerging technologies.

Steelman, Michael E.↗
DOE OSTI2024

Magnetochiral tunneling in paramagnetic Co 1/3 NbS 2

Electric currents have the intriguing ability to induce magnetization in nonmagnetic crystals with sufficiently low crystallographic symmetry. Some associated phenomena include the non-linear anomalous Hall effect in polar crystals and the nonreciprocal directional dichroism in chiral crystals when magnetic fields are applied. In this work, we demonstrate that the same underlying physics is also manifested in the electronic tunneling process between the surface of a nonmagnetic chiral material and a magnetized scanning probe. In the paramagnetic but chiral metallic compound Co 1/3 NbS 2 , the magnetization induced by the tunneling current is shown to become detectable by its coupling to the magnetization of the tip itself. This results in a contrast across different chiral domains, achieving atomic-scale spatial resolution of structural chirality. To support the proposed mechanism, we used first-principles theory to compute the chirality-dependent current-induced magnetization and Berry curvature in the bulk of the material. Our demonstration of this magnetochiral tunneling effect opens up an avenue for investigating atomic-scale variations in the local crystallographic symmetry and electronic structure across the structural domain boundaries of low-symmetry nonmagnetic crystals.

36 MATERIALS SCIENCE↗