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At least 37 records · Page 2

All sources NASA NTRS DOE OSTI

DOE OSTI2020

Direct observation of two-dimensional magnons in atomically thin CrI 3

Magnons are collective spin excitations in crystals with long-range magnetic order. The emergent van der Waals magnets provide a highly tunable platform to explore magnetic excitations in the two-dimensional limit with intriguing properties, manifesting from their honeycomb lattice structure and switchable magnetic configurations. Here, we report the direct observation of two-dimensional magnons through magneto-Raman spectroscopy with optical selection rules determined by the interplay between crystal symmetry, layer number and magnetic states in atomically thin CrI 3 . In monolayers, we observe an acoustic magnon mode at ~0.3 meV. It has strict cross-circularly polarized selection rules locked to the magnetization direction that originates from the conservation of angular momentum of photons and magnons dictated by three-fold rotational symmetry. Additionally, we reveal optical magnon modes at ~17 meV. This mode is Raman silent in monolayers, but optically active in bilayers and bulk due to a relaxation of the parity criterion resulting from the layer index. In the layered antiferromagnetic states, we directly resolve two degenerate optical magnon modes with opposite angular momentum and conjugate optical selection rules. From these measurements, we quantitatively extract the spin-wave gap, magnetic anisotropy and intralayer and interlayer exchange constants, and establish two-dimensional magnets as a new platform for exploring magnon physics.

36 MATERIALS SCIENCE↗

NASA NTRS2016

Dual-Telescope Multi-Channel Thermal-Infrared Radiometer for Outer Planet Fly-By Missions

The design of a versatile dual-telescope thermal-infrared radiometer spanning the spectral wavelength range 8-200 microns, in five spectral pass bands, for outer planet fly-by missions is described. The dual- telescope design switches between a narrow-field-of-view and a wide-field-of-view to provide optimal spatial resolution images within a range of spacecraft encounters to the target. The switchable dual-field- of-view system uses an optical configuration based on the axial rotation of a source-select mirror along the optical axis. The optical design, spectral performance, radiometric accuracy, and retrieval estimates of the instrument are discussed. This is followed by an assessment of the surface coverage performance at various spatial resolutions by using the planned NASA Europa Mission 13-F7 fly-by trajectories as a case study.

Aslam, Shahid↗

DOE OSTI2023

Power converter having a gate drive circuit and method of operating

A power converter apparatus comprises a set of switching elements communicatively coupled with a set of gate drive circuits. Each gate drive circuit is configured to provide a respective drive signal to a corresponding switching element, each switching element being switchably responsive to the respective drive signal. The apparatus includes a controller module configured to control an output state of the power converter, and selectively change one of a respective gate resistance and a respective gate current of a corresponding subset of the gate drive circuits based on the output state of the power converter.

24 POWER TRANSMISSION AND DISTRIBUTION↗

NASA NTRS1975

Dish antenna having switchable beamwidth

A switchable beamwidth antenna includes a concave parabolic main reflecting dish which has a central circular region and a surrounding coaxial annular region. A feed means selectively excites only the central region of the main dish via a truncated subreflector for wide beamwidth or substantially the entire main dish for narrow beamwidth. In one embodiment, the feed means comprises a truncated concave ellipsoid subreflector and separate feed terminations located at two foci of the ellipsoid. One feed termination directly views all of the main dish while the other feed termination, exciting the main dish via the subreflector, excites only the central region because of the subreflector truncation. In the another embodiment, the feed means comprises one feed termination and a convex hyperboloid subreflector via which the feed excites the main dish.

Schmidt, R. F.↗

DOE OSTI2026

Metallic d-wave altermagnetism in WFeB: a platform for electrically switchable perpendicular spin-splitter response

We report the synthesis and magnetic characterization of WFeB and identify it as a metallic d-wave altermagnet representative of a broader TiNiSi-type family. Neutron diffraction, Mössbauer spectroscopy, and magnetometry establish a collinear altermagnetic ordering confirmed by first-principles calculations. The electronic structure shows a nonrelativistic spin splitting of approximately 100 meV, but it also supports a strong spin-splitter transport response. This demonstrates that efficient spin-current generation can occur even with such modest band splitting. Symmetry analysis shows that selected film orientations permit deterministic switching of the Néel vector by current-induced staggered torques, enabling electrical control of a perpendicular spin-splitter response. These results establish WFeB and related TiNiSi-type antiferromagnets as a platform for electrically switchable charge-to-spin conversion driven by altermagnetic symmetry.

FOS: Physical sciences↗

DOE OSTI2024

Switchable planar chirality and spin texture in highly ordered ferroelectric hybrid perovskite domains

Chiral organic-inorganic hybrid perovskites offer a promising platform for developing non-linear chiro-optical applications and chiral-induced spin selectivity. Here, we show that achiral hybrid perovskites that have highly ordered ferroelectric domains with orthogonal polarization exhibit planar chirality, as manifested by second harmonic generation with strong circular dichroism. Interestingly, the handedness of the second harmonic generation circular dichroism response can be alternatingly switched between orthogonally polarized domains and domain walls. To correlate the origin of planar chirality in these domains, atom-resolved atomic force microscopy is used to reveal distinct arrangements of atomic rows in these ferroelectric crystals that indicate planar symmetry breaking. Remarkably, a large SHG anisotropy factor of 0.41 is measured at the domain wall. Additionally, spatially resolved two-photon photoluminescence excitation measurement provide evidence of larger Rashba spin-splitting in these chiral ferroelectric domains compared to domain-free areas. Our discoveries of domain-dependent planar chirality and spin texture hold great promise for advancement of domain-specific chiro-optical applications.

36 MATERIALS SCIENCE↗

DOE OSTI2023

Reversibly growing crosslinked polymers with programmable sizes and properties

Growth constitutes a powerful method to post-modulate materials’ structures and functions without compromising their mechanical performance for sustainable use, but the process is irreversible. To address this issue, we here report a growing-degrowing strategy that enables thermosetting materials to either absorb or release components for continuously changing their sizes, shapes, compositions, and a set of properties simultaneously. The strategy is based on the monomer-polymer equilibrium of networks in which supplying or removing small polymerizable components would drive the networks toward expansion or contraction. Using acid-catalyzed equilibration of siloxane as an example, we demonstrate that the size and mechanical properties of the resulting silicone materials can be significantly or finely tuned in both directions of growth and decomposition. The equilibration can be turned off to yield stable products or reactivated again. During the degrowing-growing circle, material structures are selectively varied either uniformly or heterogeneously, by the availability of fillers. Our strategy endows the materials with many appealing capabilities including environment adaptivity, self-healing, and switchability of surface morphologies, shapes, and optical properties. Since monomer-polymer equilibration exists in many polymers, we envision the expansion of the presented strategy to various systems for many applications.

36 MATERIALS SCIENCE↗

DOE OSTI2025

Coherent phonon flatband generated in GaAs/AlAs superlattices via layer-selective optical pumping

Flatbands, characterized by their dispersionless energy levels in electronic, magnetic, and phononic systems, hold substantial potential for advancements in electronics and quantum information processing. Most flatbands exist in thermal equilibrium and cannot be easily created or annihilated externally, limiting their flexibility as switchable knobs for use in microelectronics and quantum applications. In our work, we demonstrate the generation of a coherent phonon flatband in a GaAs/AlAs superlattice using 800 nm femtosecond laser pulses. This coherent phonon flatband does not correspond to a phonon eigenmode at equilibrium and exhibits strong coupling with two branches of coherently excited longitudinal phonon modes. With molecular dynamics simulations, we show more generally that the coherent phonon flatband can be induced by coherently and spatially modulated optical excitations of superlattice structures. Our results highlight a pathway for coherent phonon flatband creation in the time domain that can be generalized to various superlattice systems, potentially inspiring the realization of coherent flatband generation of other quasiparticles.

Condensed-matter physics↗

DOE OSTI2023

Rational design of a genetically encoded NMR zinc sensor

Elucidating the biochemical roles of the essential metal ion, Zn 2+ , motivates detection strategies that are sensitive, selective, quantitative, and minimally invasive in living systems. Fluorescent probes have identified Zn 2+ in cells but complementary approaches employing nuclear magnetic resonance (NMR) are lacking. Recent studies of maltose binding protein (MBP) using ultrasensitive 129 Xe NMR spectroscopy identified a switchable salt bridge which causes slow xenon exchange and elicits strong hyperpolarized 129 Xe chemical exchange saturation transfer (hyper-CEST) NMR contrast. To engineer the first genetically encoded, NMR-active sensor for Zn 2+ , we converted the MBP salt bridge into a Zn 2+ binding site, while preserving the specific xenon binding cavity. The zinc sensor (ZS) at only 1 μM achieved ‘turn-on’ detection of Zn 2+ with pronounced hyper-CEST contrast. This made it possible to determine different Zn 2+ levels in a biological fluid via hyper-CEST. ZS was responsive to low-micromolar Zn 2+ , only modestly responsive to Cu 2+ , and nonresponsive to other biologically important metal ions, according to hyper-CEST NMR spectroscopy and isothermal titration calorimetry (ITC). Protein X-ray crystallography confirmed the identity of the bound Zn 2+ ion using anomalous scattering: Zn 2+ was coordinated with two histidine side chains and three water molecules. Penta-coordinate Zn 2+ forms a hydrogen-bond-mediated gate that controls the Xe exchange rate. Metal ion binding affinity, 129 Xe NMR chemical shift, and exchange rate are tunable parameters via protein engineering, which highlights the potential to develop proteins as selective metal ion sensors for NMR spectroscopy and imaging.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2021

Selective acetate recognition and sensing using SWCNTs functionalized with croconamides

Croconamides are a new class of dual-hydrogen bond donors that have attracted interest in anion recognition and sensing applications due to their strong H-bond donation capability. In this work, we report chemiresistive sensors for the detection of anions based on poly(4-vinylpyridine) (P4VP)-wrapped single-walled carbon nanotubes (SWCNTs) functionalized with croconamide-based selectors. Model structures of asymmetric N,N-substituted croconamide selectors with N,N-substitutional groups of alkyl cationic pyridinium and electron-withdrawing moieties such as aniline (6) and 3,5-bis(trifluoromethyl)phenyl (7) groups were synthesized and their binding affinities for various anions were characterized using UV–vis and 1 H NMR titrations. Switchable binding of oxoanions was observed with selector 7, wherein HSO 4 – formed hydrogen bonding interactions and acetate (AcO – ) induced deprotonation of the NH- protons of the croconamide. Large sensitivity transitions were obtained toward AcO – for both P4VP-6-SWCNT (S = 95.68 ± 2.11 %) and P4VP-7-SWCNT (S = 140.91 ± 3.68 %) at 83.33 mM, which was attributed to deprotonation of the croconamide selectors upon addition of AcO – . In conclusion, three serial sensors were assembled in a sensor array that rapid and selective screening of AcO – was demonstrated against other oxoanions such as HSO 4 – and H 2 PO 4 – in real-time.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2026

Spin polarization engineering in 𝑑-wave altermagnets

Altermagnets host unconventional spin-polarized bands despite zero net magnetization, but controlling their spin structure remains challenging. Here, we theoretically propose a multifield approach to engineer spin polarization in 𝑑-wave altermagnets using gating, optical driving, and in-plane electric fields, which enable tunable and switchable polarizations along multiple directions. Optical driving induces out-of-plane (𝑧) polarization, while gating and in-plane fields generate 𝑥 and 𝑦 polarizations via the Edelstein effect, all of which are experimentally detectable. We further find that spin- and band-selective doping induces chiral optical activity, a feature unique to altermagnets. Our approach offers a versatile means to control the direction of spin polarization in altermagnets.

Floquet systems↗

NASA NTRS1993

SAW based systems for mobile communications satellites

Modern mobile communications satellites, such as INMARSAT 3, EMS, and ARTEMIS, use advanced onboard processing to make efficient use of the available L-band spectrum. In all of these cases, high performance surface acoustic wave (SAW) devices are used. SAW filters can provide high selectivity (100-200 kHz transition widths), combined with flat amplitude and linear phase characteristics; their simple construction and radiation hardness also makes them especially suitable for space applications. An overview of the architectures used in the above systems, describing the technologies employed, and the use of bandwidth switchable SAW filtering (BSSF) is given. The tradeoffs to be considered when specifying a SAW based system are analyzed, using both theoretical and experimental data. Empirical rules for estimating SAW filter performance are given. Achievable performance is illustrated using data from the INMARSAT 3 engineering model (EM) processors.

Peach, R. C.↗

NASA NTRS2008

Microwave Power Combiner/Switch Utilizing a Faraday Rotator

A proposed device for combining or switching electromagnetic beams would have three ports, would not contain any moving parts, and would be switchable among three operating states: Two of the ports would be for input; the remaining port would be for output. In one operating state, the signals at both input ports would be coupled through to the output port. In each of the other two operating states, the signal at only one input port would be coupled to the output port. The input port would be selected through choice of the operating state.

Perez, Raul↗

DOE OSTI2023

Switchable system for high-salinity brine desalination and fractional precipitation

A saline feed stream flows into a liquid-liquid extraction system; and a volatile organic solvent flows through a main compressor. The compressed volatile organic solvent then flows through a solvent regenerator, which can be a heat exchanger or a combination of a vaporization device and a condenser, to cool the volatile organic solvent. The cooled volatile organic solvent in liquid phase then flows into the liquid-liquid extraction system, where the saline feed stream contacts the volatile organic solvent to selectively extract water from the saline feed stream into the volatile organic solvent, producing a concentrated brine and an organic-rich mixture of water and the volatile organic solvent. The organic-rich mixture flows from the liquid-liquid extraction system into the solvent regenerator, where the organic-rich mixture is heated to produce an organic-rich vapor and desalinated water; and the organic-rich vapor is recycled as volatile organic solvent back into the liquid-liquid extraction system.

Wilson, Aaron D.↗

DOE OSTI2023

Asymmetric Electrode Work Function Customization via Top Electrode Replacement in Ferroelectric and Field–Induced Ferroelectric Hafnium Zirconium Oxide Thin Films

Non-volatile memory device structures such as ferroelectric random-access memory and ferroelectric tunnel junctions employ switchable spontaneous polarization to hold binary states. These devices can potentially benefit from the imposition of spontaneous internal biases and their resulting effect on the polarization properties of the ferroelectric (or field-induced ferroelectric/ antiferroelectric) layer. While HfO 2 -based thin films are ideal candidates for implementation into these devices due to their scalability and silicon compatibility, the phase purity of these oxides is sensitive to the selection of electrode material, preventing incorporation of asymmetric electrode layers into such structures. Within this work, electrode replacement following post-metallization anneal processing is introduced as a route to achieve ferroelectric and field-induced ferroelectric Hf x Zr 1–x O 2 (HZO) thin films with electrode-independent phase constitutions. The effects of this process and the corresponding internal biases imposed across the HZO layers due to asymmetric work functions are investigated. It is shown that internal biases vary in magnitude in accordance with prediction based on the work functions of the replaced electrode layers and affect remanent polarization magnitudes. Accordingly, electrode replacement presents a processing route that can readily produce HZO films with spontaneous internal biases and electrode- independent phase constitutions, facilitating implementation of these ferroelectrics into the next generation device structures.

36 MATERIALS SCIENCE↗

NASA NTRS1974

Unique properties of SBN and their use in a layered optical memory

Holographic storage experiments using crystal Sr(0.75)Ba(0.25)Nb2O6 (SBN 75/25) as a volume-phase holographic medium show that this material is the most sensitive crystalline storage medium yet discovered. An exposure level of 0.003 J/sq cm at 0.488 micron produces a 1% diffraction efficiency in a 5-mm length of crystal. In addition to high recording sensitivity, SBN 75/25 exhibits interesting electric-field induced effects that include electric-field enhanced recording sensitivity and voltage-switchable latent-to-active holographic reconstruction efficiency. These effects are explained in terms of drift and diffusion of photoionized carriers coupled with the nonlinear electrooptic behavior characteristic of this low Curie temperature ferroelectric crystal. A mechanism is proposed that correlates the observed electrically controlled holographic response with the dielectric behavior of the crystal. These interesting effects are used to implement a novel, layered optical memory having no moving parts and having electrical access for writing or reading selected layers.

Thaxter, J. B.↗

DOE OSTI2025

Exploiting Intersubband Plasmons in Vertically Aligned Carbon Nanotubes for Near-Infrared Electrochromic Windows

Optically transparent materials with switchable near-infrared (NIR) transmissivity are of significant interest for energy-saving smart window technologies. To this end, we demonstrate that semitransparent films of vertically aligned carbon nanotubes (CNTs) incorporated into electrochemically gated devices exhibit NIR transmittance changes up to 47% and bistable optical states that are appealing for low-power, large-area operation. The tunable NIR electrochromic response is driven by a doping-induced intersubband plasmon (ISBP) absorption, an optical feature in CNTs that is selective to light polarized perpendicular to the CNT axis. Vertically aligned CNT films (as opposed to more conventional planar CNT mats) thus allow us to isolate and study the ISBP resonance changes with applied voltage, electrode material, and film thickness.

absorption↗

DOE OSTI2020

Advances in Polymerizations Modulated by External Stimuli

Synthetic polymer chemistry endeavors to imitate the spatial and temporal control exhibited within biological systems to obtain well-defined polymeric materials with unique structures, properties, and applications. This is often approached through the development of dynamic catalyst (or initiator) systems that use external stimuli to elicit discrete, site specific transformations that impact the polymerization. In this work, we highlight developments in polymerizations that are modulated by external stimuli, with particular focus on those systems that enable notable changes in kinetics, monomer selectivity, polymer architecture, or tacticity. Examples of external stimuli include chemical oxidants or reductants, light, applied voltage, and mechanical force.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗