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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 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Cadmium(II) terephthalates based on trinuclear units {Cd_3(bdc)_3}: control of coordination structure dimensionality and luminescence properties

Two metal-organic coordination polymers based on cadmium(II) ions and terephthalic (1,4-benzenedicarboxylic) acid were synthesized in dimethylformamide or a mixture of dimethylformamide and methanol: [Cd{sub 3}(bdc){sub 3}(dmf){sub 4}] (1, bdc is 1,4-benzenedicarboxylate, dmf is N,N´-dimethylformamide) and [Cd{sub 3}(bdc){sub 3}(dmf){sub 2}] (2), respectively. According to the single-crystal X-ray diffraction data, both compounds are built up by the trinuclear carboxylate units {Cd_3(O_2CR)_6}. Compound 1 has a layered structure, whereas compound 2 has a three-dimensional structure due to the presence of additional coordination interactions between adjacent trinuclear units and the formation of polymer chains {Cd_3(dmf)_2(μ_2-O_2CR-κ"1,κ"1)_2(μ_2-O_2CR-κ"2,κ"1)_4(O_2CR-κ"1)2}. The synthesized compounds were characterized by IR spectroscopy, thermogravimetric analysis, elemental analysis, and powder X-ray diffrac- tion. The luminescence properties of these compounds provide evidence for the existence of both intraligand and charge-transfer transitions. The luminescence quantum yields for both compounds are 10%.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Ferromagnetic MnBi 4 Te 7 obtained with low-concentration Sb doping: A promising platform for exploring topological quantum states

The tuning of the magnetic phase, chemical potential, and structure is crucial to observe diverse exotic topological quantum states in Mn Bi 2 Te 4 (Bi 2 Te 3 ) m (m = 0–3). Here we show a ferromagnetic (FM) phase with a chiral crystal structure in Mn (Bi 1–x Sb x ) 4 Te 7 , obtained via tuning the growth conditions and Sb concentration. Unlike previously reported Mn (Bi 1–x Sb x ) 4 Te 7 , which exhibits FM transitions only at high Sb doping levels, our samples show FM transitions (T C = 13.5 K) at 15%–27% doping levels. Furthermore, our single-crystal x-ray-diffraction structure refinements find Sb doping leads to a chiral structure with the space group of P3, contrasted with the centrosymmetric $P\bar{3}m1$ crystal structure of the parent compound MnBi 4 Te 7 . Through angle-resolved photoemission spectroscopy measurements, we also demonstrated that the nontrivial band topology is preserved in the Sb-doped FM samples. Given that the nontrivial band topology of this system remains robust for low Sb doping levels, our success in making FM Mn(Bi 1–x Sb x ) 4 Te 7 with x = 0.15, 0.175, 0.2, and 0.27 paves the way for realizing the predicted topological quantum states, such as the axion insulator and Weyl semimetals. Additionally, we also observed magnetic glassy behavior in both antiferromagnetic MnBi 4 Te 7 and FM Mn (Bi 1–x Sb x )4 Te 7 samples, which we believe originates from cluster spin-glass phases coexisting with long-range antiferromagnetic/FM orders. Further, we have also discussed how the antisite Mn ions impact the interlayer magnetic coupling and how FM interlayer coupling is stabilized in this system.

36 MATERIALS SCIENCE↗
DOE OSTI2026

An Atom-Precise Approach to Damp First-Order Phase Transitions and Its Implications for Neuromorphic Signal Processing

Neuromorphic computing inspired by mammalian intelligence aims to emulate the nonlinear dynamics of biological neurons and synapses to achieve fast, low-energy, and highly efficient information processing. Brain-inspired computing relies on the design and discovery of materials exhibiting nonlinear current–voltage profiles, frequently underpinned by electronic state transitions, to achieve spiking neurons and dynamically tunable synapses. A signature challenge in the design of artificial neurons is controlling the steepness of first-order transitions in active elements, as abrupt transitions are at risk of driving unstable voltage and temperature oscillations, which result in catastrophic device failure. A critical knowledge gap is the lack of structure–function correlations mapping the composition and atomistic structure of crystalline solids to nonlinear dynamical response characteristics. Here, we address the key question of how modification of atomistic structure correlates with alteration of neuron-like functionality. Constructing oscillator circuits from millimeter-scale single crystals enables high-resolution atomic structure solutions, which we use to demonstrate that the selective positioning of Pb cations modifies charge ordering along a one-dimensional CuxV2O5 framework even at low insertion stoichiometries, thereby providing an atom-precise design parameter for damping first-order transitions. We use temperature-variant X-ray diffraction and X-ray spectroscopy to elucidate the suppression of Cu-ion shuttling based on the precise positioning of Pb ions in seven-coordinated tunnel interstitial sites as the mechanistic basis for transition broadening, thus bridging a critical gap between statistical mechanics and quantum chemical descriptions of phase transitions. Such mechanistic understanding thus paves the way to site-selective modification strategies for modulating the sharpness of first-order transitions, with an exemplary demonstration here in tuning neuronal signal processing.

Crystal structure↗
DOE OSTI2019

Antiferromagnetic stacking of ferromagnetic layers and doping-controlled phase competition in Ca 1 - x Sr x Co 2 - y As 2

In search of a quantum phase transition between the two-dimensional (2D) ferromagnetism of Ca 1 - x Sr x Co 2 - y As 2 and stripe-type antiferromagnetism in SrCo2As 2 , we instead discover evidence for 1D magnetic frustration between magnetic square Co layers. We present neutron-diffraction data for Ca 1 - x Sr x Co 2 - y As 2 that reveal a sequence of $x$-dependent magnetic transitions which involve different stacking of 2D ferromagnetically aligned layers with different magnetic anisotropy. We report on the $x$-dependent changes to the magnetic order by utilizing classical analytical calculations of a 1D Heisenberg model where single-ion magnetic anisotropy and frustration of antiferromagnetic nearest- and next-nearest-layer exchange interactions are all composition dependent.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Ce-Doping and Reduction Annealing Effects on Magnetic Properties of Pr{sub 2-x}Ce{sub x}CuO{sub 4} Nanoparticles

Magnetic properties of reduced T’-Pr{sub 2-x}Ce{sub x}CuO{sub 4} (T’-PCCO) nanoparticles with x = 0, 0.10, and 0.15 have been investigated. The T’-PCCO samples were synthesized by the chemically dissolved method with HNO{sub 3} as dissolving agents and by the succeeding calcination at 1000 °C for 15 h in air. The reduced T’-PCCO samples were obtained by the reduction annealing at 700 °C for 10 h in Argon atmosphere and at 700 °C for 12 h in vacuum. Rietveld analyses of the X-ray diffraction (XRD) patterns for all the samples have revealed dependences of the c-axis length and the crystal size on the Ce doping and the reduction annealing. XRD analyses and transmission electron microscopy (TEM) images have confirmed T’-PCCO nanoparticles with the size of around 40–100 nm. Magnetic properties of all the T’-PCCO nanoparticles were studied by a superconducting quantum interference device (SQUID) showing a dominant paramagnetic feature with very weak ferromagnetism in the as-synthesized samples at room temperature. It has changed to a pure paramagnetic feature after the reduction annealing in argon atmosphere and an enhanced ferromagnetism through vacuum annealing. This is probably due to the change in the number of oxygen vacancies through the defect recovery and the removal of excess oxygen after both reduction annealing processes, respectively.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2021

Ultrafast all-optical diffraction switching using semiconductor metasurfaces

Ultrafast all-optical switching using Mie resonant metasurfaces requires both on-demand tunability of the wavefront of the light and ultrafast time response. However, devising a switching mechanism that has a high contrast between its “on” and “off” states without compromising speed is challenging. Here, we report the design of a tunable Mie resonant metasurface that achieves this behavior. Our approach utilizes a diffractive array of semiconductor resonators that support both dipolar and quadrupolar Mie resonances. By balancing the strengths of the dipole and quadrupole resonances, we can suppress radiation into the first diffraction order, thus creating a clearly delineated “off”-state at the operating wavelength. Then, we use optical injection of free- carriers to spectrally shift the multipoles and rebalance the multipole strengths, thereby enabling radiation into the diffraction order—all on an ultrafast timescale. We demonstrate ultrafast off-to-on switching with I on /I off ≈ 5 modulation of the diffracted intensity and ultrafast on-to-off switching with Ion/Ioff ≈ 9 modulation. Furthermore, both switches exhibit a fast τ tr ≈ 2.7 ps relaxation time at 215 μJ cm -2 pump fluence. Further, we show that for higher fluences, the temporal response of the metasurface is governed by thermo-optic effects. This combination of multipole engineering with lattice diffraction opens design pathways for tunable metasurface-based integrated devices.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Ultrafast disruptive probing: Simultaneously keeping track of tens of reaction pathways

Ultrafast science depends on different implementations of the well-known pump–probe method. Here, we provide a formal description of ultrafast disruptive probing, a method in which the probe pulse disrupts a transient species that may be a metastable ion or a transient state of matter. Disruptive probing has the advantage of allowing for simultaneous tracking of the yield of tens of different processes. Our presentation includes a numerical model and experimental data on multiple products resulting from the strong-field ionization of two different molecules, partially deuterated methanol and norbornene. The correlated enhancement and depletion signals between all the different fragmentation channels offer comprehensive information on photochemical reaction pathways. In combination with ion imaging and/or coincidence momentum imaging or as complementary to atom-specific probing or ultrafast diffraction methods, disruptive probing is a particularly powerful tool for the study of strong-field laser–matter interactions.

47 OTHER INSTRUMENTATION↗
DOE OSTI2018

Generation of nonuniformly polarised vortex Bessel beams by an interference polariser

A new optical system is proposed for the generation of azimuthally and radially polarised vortex Bessel beams. The system is based on the transformation of conical wavefronts passing through an interference polarisation plate. The polarisation characteristics during the beam formation are controlled by changing the curvature of the wavefront of the beam illuminating a diffractive axicon. Radially and azimuthally polarised vortex Bessel beams are experimentally obtained using a binary phase axicon. (laser beams)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2026

Pressure-Induced Chemical Bonding Effects on Lattice and Magnetic Instabilities in Antiferromagnetic Insulating CaMn 2 Sb 2

Exotic quantum phenomena often emerge near an electronic delocalization transition (EDT) from an antiferromagnetic insulating phase to a strongly correlated metallic state under pressure. We report the pressure-induced structural and magnetic evolution of the antiferromagnetic insulator CaMn 2 Sb 2 . Single-crystal X-ray diffraction reveals a first-order phase transition near 5.4 GPa from a trigonal P-3m1 structure to a monoclinic P2 1 /m phase accompanied by a ∼7% volume collapse. Residual electron density analysis at intermediate pressures reveals charge localization along Mn–Sb chains, signaling electronic instability preceding the structural transition. Bonding analysis indicates anisotropic Mn–Sb orbital reconfiguration under pressure, driving a distorted square-pyramidal geometry. Neutron scattering confirms the transition and identifies a pressure-induced incommensurate magnetic order, distinct from the ambient antiferromagnetic state. In the monoclinic phase, zigzag Mn chains exhibit antiferromagnetic coupling along the ac-plane, enabled by enhanced orbital overlap. These results establish CaMn 2 Sb 2 as a model system for studying the coupling of structural distortion, charge redistribution, and magnetic order in layered Mn pnictides under pressure.

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

Nitrogenated Graphene Quantum Dot-Derived Copper Single-Atom Catalyst for Oxygen Reduction Reaction

Despite the promise of fuel cells as sustainable energy conversion technologies, their widespread adoption is hindered by the high cost of platinum group metal (PGM) catalysts, particularly for catalyzing the oxygen reduction reaction (ORR) at the cathode. Here, we report copper-based single-atom catalysts (Cu-SACs) as cost-effective non-PGM alternatives for ORR. The catalysts were synthesized via simple pyrolysis of pyrene-derived amine-terminated graphene quantum dots (GQDs) in the presence of nitrogen and metal precursors. The abundant nitrogen functionality in GQDs effectively stabilized isolated Cu atoms during their conversion to porous carbon. A subsequent acid-washing step yielded a high density of atomically dispersed Cu active sites. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and high-resolution scanning transmission electron microscopy (HR-STEM) confirmed the effective incorporation of isolated Cu atoms into the carbon matrix through copper–nitrogen (Cu-N) coordination. Electrochemical measurements revealed excellent ORR activity with a dominant four-electron pathway. This synthetic strategy provides a practical route to developing economically viable, non-precious metal catalysts.

catalysts↗
DOE OSTI2020

Competing magnetic phases and fluctuation-driven scalar spin chirality in the kagome metal YMn 6 Sn 6

Identification, understanding, and manipulation of novel magnetic textures are essential for the discovery of new quantum materials for future spin-based electronic devices. In particular, materials that manifest a large response to external stimuli such as a magnetic field are subject to intense investigation. Here, we study the kagome-net magnet YMn 6 Sn 6 by magnetometry, transport, and neutron diffraction measurements combined with first-principles calculations. We identify a number of nontrivial magnetic phases, explain their microscopic nature, and demonstrate that one of them hosts a large topological Hall effect (THE). We propose a previously unidentified fluctuation-driven mechanism, which leads to the THE at elevated temperatures. This interesting physics comes from parametrically frustrated interplanar exchange interactions that trigger strong magnetic fluctuations. Our results pave a path to chiral spin textures, promising for novel spintronics.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Free-Standing Bialkali Photocathodes Using Atomically Thin Substrates

This study reports successful deposition of high quantum efficiency (QE) bialkali antimonide K2CsSb photocathodes on graphene films. The results pave the way for an ultimate goal of encapsulating technologically relevant photocathodes for accelerator technology with an atomically thin protecting layer to enhance lifetime while minimizing QE losses. A QE of 17% at approximate to 3.1 eV (405 nm) is the highest value reported so far on graphene substrates and is comparable to that obtained on stainless steel and nickel reference substrates. The spectral responses of the photocathodes on graphene exhibit signature features of K2CsSb including the characteristic absorption at approximate to 2.5 eV. Materials characterization based on X-ray fluorescence and X-ray diffraction reveals that the composition and crystal quality of these photocathodes deposited on graphene is comparable to those deposited on a reference substrate. Quantitative agreement between optical calculations and QE measurements for the K2CsSb on free suspended graphene and a graphene-coated metal substrate further confirms the high-quality interface between the photocathodes and graphene. Finally, a correlation between the QE and graphene quality as characterized by Raman spectroscopy suggests that a lower density of atomistic defects in the graphene films leads to higher QE of the deposited K2CsSb photocathodes.

accelerator technology↗
DOE OSTI2017

Magnetic phase transformations and magnetotransport phenomena in La{sub 0.7}Sr{sub 0.3}Mn{sub 1–} {sub x}Co{sub x}O{sub 3} perovskite compounds

The compositions La{sub 0.7}Sr{sub 0.3}Mn{sub 1–} {sub x}Co{sub x}O{sub 3} (0.13 ≤ x ≤ 1) are studied by neutron diffraction, magnetometry, and measuring the magnetotransport properties. The substitution of cobalt ions for manganese ions is shown to decrease the magnetization and the Curie temperature from 270 K (x = 0.13) to 140 K (x = 0.33). As the cobalt ion content increases to x = 0.5, the Curie temperature increases to 190 K, the magnetization decreases, and the electrical resistivity increases. At x > 0.5, the temperature of transition into a paramagnetic state decreases to 68 K (x = 0.8) and then again increases to 225 K for the La{sub 0.7}Sr{sub 0.3}CoO{sub 3} composition. The magnetoresistive effect in the range 0.3 ≤ x ≤ 0.4 reaches 97% and decreases gradually with increasing temperature without anomalies near the Curie point. At x ≤ 0.2, the magnetoresistive effect increases near the Curie temperature. The composition at x = 0.6 is stoichiometric, and no coherent magnetic contribution to neutron scattering is detected. The magnetic properties near x ∼ 0.5 are assumed to be caused by partial ordering of Co{sup 3+} and Mn{sup 4+} ions, and the Co{sup 3+} ions can be in both low- and high-spin states. The magnetic interaction between Co{sup 3+} ions in a high-spin state and Mn{sup 4+} is predominantly ferromagnetic, and the ferromagnetic part of the exchange interactions is close to the ferromagnetic part. These data are used to plot a magnetic La{sub 0.7}Sr{sub 0.3}Mn{sub 1–} {sub x}Co{sub x}O{sub 3} phase diagram.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Low-temperature magnetic crossover in the topological kagome magnet TbMn 6 Sn 6

Magnetic topological phases of quantum matter are an emerging frontier in physics and materials science, of which kagome magnets appear as a highly promising platform. Here, we explore magnetic correlations in the recently identified topological kagome system TbMn6 S n 6 using muon spin rotation, combined with local field analysis and neutron diffraction. Our studies identify an out-of-plane ferrimagnetic structure with slow magnetic fluctuations which exhibit a critical slowing down below $T^{*}_{C1}$ ≃ 120 K and finally freeze into static patches with ideal out-of-plane order below T C1 ≃ 20 K. We further show that hydrostatic pressure of 2.1 GPa stabilises the static out-of-plane topological ferrimagnetic ground state in the whole volume of the sample. Therefore the exciting perspective arises of a magnetically-induced topological system whose magnetism can be controlled through external parameters. The present results will stimulate theoretical investigations to obtain a microscopic understanding of the relation between the low-temperature volume-wise magnetic evolution of the static c-axis ferrimagnetic patches and the topological electronic properties in TbMn 6 Sn 6 .

36 MATERIALS SCIENCE↗
DOE OSTI2020

Bragg grating inscription in photosensitive materials by an optical replication process

This paper examines the feasibility of producing multiplexed Bragg gratings for use as diffractive optical elements (DOEs) by coherent optical replication of a surface-relief phase grating into thick-film photosensitive materials. The use of such DOEs in light guide plates (LGPs) for optical imaging systems ensures energy-efficient light input into and output from LGPs and allows one to minimise their weight and size. We present simulation results for the interference structure fabrication process and recording of a series of multiplexed diffraction gratings in photosensitive materials. (paper)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2024

Structure-Related Electronic and Magnetic Properties in Ultrathin Epitaxial Ni x Fe 3–x O 4 Films on MgO(001)

Off-stoichiometric Ni x Fe 3–x O 4 ultrathin films (x < 2.1) with varying Ni content x and thickness 16 (±2) nm were grown on MgO(001) by reactive molecular beam epitaxy. Synchrotron-based high-resolution X-ray diffraction measurements reveal vertical compressive strain for all films, resulting from a lateral pseudomorphic adaption of the film to the substrate lattice without any strain relaxation. Complete crystallinity with smooth interfaces and surfaces is obtained independent of the Ni content x. For x < 1 an expected successive conversion from Fe 3 O 4 to NiFe 2 O 4 is observed, whereas local transformation into NiO structures is observed for films with Ni content x > 1. However, angle-resolved hard X-ray photoelectron spectroscopy measurements indicate homogeneous cationic distributions without strictly separated phases independent of the Ni content, while X-ray absorption spectroscopy shows that also for x > 1, not all Fe 2+ cations are substituted by Ni 2+ cations. The ferrimagnetic behavior, as observed by superconducting quantum interference device magnetometry, is characterized by decreasing saturation magnetization due to the formation of antiferromagnetic NiO parts.

36 MATERIALS SCIENCE↗