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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Magnetic coupling of vortices in a two-dimensional lattice

In this study, we investigated the magnetization reversal of magnetic vortex structures in a two-dimensional lattice. The structures were formed by permalloy (Py) film deposition onto large arrays of self-assembled spherical SiO 2 -particles with a diameter of 330 nm. We present the dependence of the nucleation and annihilation field of the vortex structures as a function of the Py layer thickness (aspect ratio) and temperature. By increasing the Py thickness up to 90 nm or alternatively by lowering the temperature the vortex structure becomes more stable as expected. However, the increase of the Py thickness results in the onset of strong exchange coupling between neighboring Py caps due to the emergence of Py bridges connecting them. In particular, we studied the influence of magnetic coupling locally by in-field scanning magneto-resistive microscopy and full-field magnetic soft x-ray microscopy, revealing a domain-like nucleation process of vortex states, which arises via domain wall propagation due to exchange coupling of the closely packed structures. By analyzing the rotation sense of the reversed areas, large connected domains are present with the same circulation sense. Furthermore, the lateral core displacements when an in-plane field is applied were investigated, revealing spatially enlarged vortex cores and a broader distribution with increasing Py layer thickness. Additionally, the presence of some mixed states, vortices and c-states, is indicated for the array with the thickest Py layer.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Coupled Hubbard ladders at weak coupling: Pairing and spin excitations

The Hubbard model provides a simple framework in which one can study how certain aspects of the electronic structure of strongly interacting systems can be tuned to optimize the superconducting pairing correlations and how these changes affect the mechanisms giving rise to them. Here we use a weak-coupling random phase approximation to study a two-dimensional Hubbard model with a unidirectional modulation of the hopping amplitudes as the system evolves from the uniform square lattice to an array of weakly coupled two-leg ladders. We find that the pairing correlations retain their dominant d x 2 - y 2- wavelike structure and that they are significantly enhanced for a slightly modulated lattice. This enhancement is traced backed to an increase in the strength of the spin-fluctuation pairing interaction due to favorable Fermi surface nesting in the modulated system. We then use a random-phase approximation BCS framework to examine the evolution of the neutron resonance in the superconducting state. We find that it changes only weakly for moderate modulations, but breaks up into two distinct resonances at incommensurate wave vectors in the limit of weakly coupled ladders.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Strong coupling critique of spin fluctuation driven charge order in underdoped cuprates

Charge order has emerged as a generic feature of doped cuprates, leading to important questions about its origin and its relation to superconductivity. Recent experiments on two classes of hole doped cuprates indicate a novel d -wave symmetry for the order. These were motivated by earlier spin fluctuation theoretical studies based on an expansion about hot spots in the Brillouin zone that indicated such an order would be competitive with d -wave superconductivity. In this work, we reexamine this problem by solving strong coupling equations in the full Brillouin zone for experimentally relevant parameters. We find that bond-oriented order, as seen experimentally, is strongly suppressed. We also include coupling to B 1g phonons and do not see any qualitative change. Our results argue against an itinerant model for the charge order, implying instead that such order is likely due to Coulombic phase separation of the doped holes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Thickness dependence of exchange coupling in (111)-oriented perovskite oxide superlattices

Epitaxial La 0.7 Sr 0.3 MnO 3 (LSMO)/La 0.7 Sr 0.3 FeO 3 (LSFO) superlattices on (111)-oriented SrTiO 3 substrates with sublayer thicknesses ranging from 3 to 60 unit cells (u.c.) were synthesized and characterized. Detailed analysis of their structural, electronic, and magnetic properties were performed to explore the effect of sublayer thickness on the magnetic structure and exchange coupling at (111)-oriented perovskite oxide interfaces. In the ultrathin limit (3-6 u.c.), we find that the antiferromagnetic (AF) properties of the LSFO sublayers are preserved with an out-of-plane canting of the AF spin axis, while the ferromagnetic (FM) properties of the LSMO sublayers are significantly depressed. For thicker LSFO layers (>9 u.c.), the out-of-plane canting of the AF spin axis is only present in superlattices with thick LSMO sublayers. In conclusion, exchange coupling in the form of spin-flop coupling exists only in superlattices which display both robust ferromagnetism and out-of-plane canting of the AF spin axis.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Electron-ion coupling in semiconductors beyond Fermi's Golden Rule [On the electron-ion coupling in semiconductors beyond Fermi's Golden Rule]

In the present study, a theoretical study of electron-phonon (electron-ion) coupling rates in semiconductors driven out of equilibrium is performed. Transient change of optical coefficients reflects the band gap shrinkage in covalently bonded materials, and thus, the heating of atomic lattice. Utilizing this dependence, we test various models of electron-ion coupling. The simulation technique is based on tight-binding molecular dynamics. Our simulations with the dedicated hybrid approach (XTANT) indicate that the widely used Fermi's golden rule can break down describing material excitation on femtosecond time scales. In contrast, dynamical coupling proposed in this work yields a reasonably good agreement of simulation results with available experimental data.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Direct Magnetization-Polarization Coupling in BaCuF 4

In this work, first-principles calculations based on density functional theory are used to describe the ideal magnetization reversal through polarization switching in BaCuF 4 which, according to our results, could be accomplished close to room temperature. Furthermore, we show that this ideal coupling is driven by a single soft mode that combines both polarization, and octahedral rotation. The later being directly coupled to the weak ferromagnetism of BaCuF 4 . This, added to its strong Jahn-Teller distortion and its orbital ordering, makes this material a very appealing prototype for crystals in the A B X 4 family for multifunctional applications. The described mechanism behaves ideally as it couples the ferroelectric and the magnetic properties naturally and it has not been reported previously.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Study of $WW\gamma $ and $WZ\gamma $ production in $pp$ collisions at $\sqrt{s} = {8} \,{\text {TeV}}$ and search for anomalous quartic gauge couplings with the ATLAS experiment

This paper presents a study of WWγ and WZγ triboson production using events from proton–proton collisions at a centre-of-mass energy of √s = 8 TeV recorded with the ATLAS detector at the LHC and corresponding to an integrated luminosity of 20.2 fb –1 . The WWγ production cross-section is determined using a final state containing an electron, a muon, a photon, and neutrinos (eνμνγ). Upper limits on the production cross-section of the eνμνγ final state and the WWγ and WZγ final states containing an electron or a muon, two jets, a photon, and a neutrino (eνjjγ or μνjjγ) are also derived. The results are compared to the cross-sections predicted by the Standard Model at next-to-leading order in the strong-coupling constant. In addition, upper limits on the production cross-sections are derived in a fiducial region optimised for a search for new physics beyond the Standard Model. The results are interpreted in the context of anomalous quartic gauge couplings using an effective field theory. Confidence intervals at 95% confidence level are derived for the 14 coupling coefficients to which WWγ and WZγ production are sensitive.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Structural color from a coupled nanowire pair beyond the bonding and antibonding model

Optical resonances in nanostructures can be harnessed to produce a wide range of structural colors. Conversely, the analysis of structural colors has been used to clarify the nature of optical resonances. Here, we show that silicon nanowire (NW) pairs can display a wide range of structural colors by controlling their radiative coupling. This is accomplished by exciting a series of Fabry–Pérot-like modes where light is repeatedly scattered between two NWs. These modes are beyond the expectation from the conventional chemical bonding model under a quasi-electrostatic approximation, in which only bonding and antibonding modes can be formed in a pair system through modal hybridization. The additional eigenmodes found in a two-resonator system originate from the nonlinear, frequency-dependent coupling strength derived from the radiative nature of low- Q resonators. The Fabry–Pérot modes can be tuned across the entire visible frequency range by varying the distance between two NWs, leading to what we believe is a new type of universal building blocks that can provide structural color within a subwavelength footprint. The presented results pave the way toward the design and usage of highly tunable resonances that exploit the radiative coupling of high-index nanostructures.

Li, Qitong↗
NASA NTRSDate not supplied

Application of Norton-Thevenin Receptance Coupling (NTRC) to Space Launch System (SLS) Payload Coupled Loads Analysis (CLA)

Norton-Thevenin Receptance Coupling (NTRC), as described in several NASA Engineering and Safety Center (NESC) papers shows promise in enabling loads development for payloads with less computational cost and analyst time as compared to a full integrated vehicle coupled loads analysis (CLA). NTRC allows the free vehicle responses and impedance (accelerance) at the payload to vehicle interface (derived from integrated vehicle CLA) to be used with a payload model in a payload CLA. Due to the NTRC damping differing from the full integrated vehicle damping, results are slightly different, so coverage factors were developed to ensure NTRC results enveloped results from a traditional CLA. NTRC results with coverage factors were very close to full CLA results. The Space Launch System (SLS) coupled loads team has developed an implementation of the NTRC method to enable support of co-manifested payloads with analysis needs that do not fit the primary SLS load cycle schedule. The NTRC method was successfully used to support the European System Providing Refueling, Infrastructure, and Communications (ESPRIT) module for Gateway planned to fly on Artemis V and has been adopted as the SLS approach for supporting payload CLA requests that do not align with vehicle load cycles.

SLS↗
DOE OSTI2026

A novel approach to RF power coupling in Radio-Frequency Quadrupole (RFQ) structures: built-in coaxial double-loop coupling port

Efficient and reliable RF power couplers in accelerating cavities require precise impedance matching and mechanical stability to ensure optimal beam energy transfer. In radio-frequency quadrupole (RFQ) accelerators, power is commonly delivered using waveguide iris or coaxial loop couplers. Iris couplers can handle high RF power but lack tunability, while coaxial loop couplers offer tuning flexibility but are limited in power handling and thermal performance. We propose a new RFQ power coupling concept utilizing a single input coaxial center-fed double-loop antenna built into a vane in an RFQ structure . The design integrates back-to-back loops into the RFQ vanes, fed by a TEM coaxial transmission line with standard 50-Ω characteristic impedance. The configuration can allow straightforward and easy ceramic window replacement without retuning, and coupling strength is adjusted with protruding tuning rods. Numerical simulations, performed with both a simplified RFQ model and the Spallation Neutron Source RFQ, demonstrate improved RF performance and reduced dipole mode excitation. The results establish the coaxial double-loop coupler as a practical alternative for high-power RFQ coupling applications.

Lee, Sung-Woo [ORNL] (ORCID:000000030915835X)↗
DOE OSTI2023

Comparison of state-interaction and spinor-representation calculations of spin-orbit coupling within exact two-component coupled-cluster theories

Here a benchmark study of state-interaction and spinor-representation calculations of spin-orbit coupling using the exact two-component Hamiltonians with atomic mean-field integrals (the X2CAMF schemes) at the equation-of-motion coupled-cluster singles and doubles level is reported. We adopt a version of the X2CAMF scheme with spin-orbit integrals correct to first order for the state-interaction calculations and a version correct to infinite order for the spinor-representation calculations. The differences between the state-interaction calculations with minimum active spaces to account for (quasi-)degeneracy and the spinor-representation calculations thus correspond to higher-order spin-orbit contributions. This state-interaction approach with scalar-relativistic effects accurately included in the reference functions and in the spin-orbit integrals is shown to exhibit robust performance for elements across the periodic table. On the other hand, the more rigorous spinor representation shows more rapid convergence with respect to the number of correlated electrons and is the preferred choice for accurate calculations for heavy elements.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2016

Constraining ALP-photon coupling using galaxy clusters

We study photon-ALP conversion by resonance effects in the magnetized plasma of galaxy clusters and compare the predicted distortion of the cosmic microwave background spectrum in the direction of such objects to measurements of the thermal Sunyaev-Zeldovich effect. Using galaxy cluster models based on current knowledge, we obtain upper limits on the photon-ALP coupling constant g of ≲O(10{sup −11} GeV{sup −1}). The constraints apply to the mass range of 2⋅10{sup −14} eV ≲m{sub ALP}≲3⋅10{sup −12} eV in which resonant photon-ALP conversions can occur. These limits are slightly stronger than current limits, and furthermore provide an independent constraint. We find that a next generation PRISM-like experiment would allow limits down to g≈O(10{sup −14} GeV{sup −1}), two orders of magnitude stronger than the currently strongest limits in this mass range.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Unveiling the Effects of Linker Substitution in Suzuki Coupling with Palladium Nanoparticles in Metal–Organic Frameworks [Unveiling the Effects of Linker Substitution in Suzuki Coupling Reaction with Palladium Nanoparticles in Metal–Organic Frameworks]

The establishment of structure–property relationships in heterogeneous catalysis is of prime importance but remains a formidable challenge. Metal–organic frameworks (MOFs) featuring excellent chemical tunability are emerging as an auspicious platform for the atomic-level control of heterogeneous catalysis. Herein, we encapsulate palladium nanoparticles (Pd NPs) in a series of isoreticular mixed-linker MOFs, and the obtained MOF-Pd NPs catalysts were used to unveil the electronic and steric effects of linker substitution on the activity of these catalysts in the Suzuki–Miyaura cross-coupling reactions. Significantly, m-6,6'-Me2bpy-MOF-Pd exhibits a remarkable enhancement in the activity compared to non-functionalized m-bpy-MOF-Pd and m-4,4'-Me 2 bpy-MOF-Pd. This study unambiguously demonstrates that the stereoelectronic properties of linker units are crucial to the catalytic activity of nanoparticles encapsulated in MOFs. More interestingly, the trend of activity change is consistent with our previous work on catalytic sites generated in situ from Pd(II) coordinated in MOFs bearing the same functional groups, which suggests that both MOF-Pd NPs and MOF-Pd(II) catalysts generate similar active centers during Suzuki–Miyaura coupling reactions. Lastly, this work paves a new avenue to the fabrication of advanced and tunable MOF-based catalysts through rational linker engineering.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Nonadiabatic Derivative Couplings through Multiple Franck–Condon Modes Dictate the Energy Gap Law for Near and Short-Wave Infrared Dye Molecules

Near infrared (NIR, 700–1000 nm) and short-wave infrared (SWIR, 1000–2000 nm) dye molecules exhibit significant nonradiative decay rates from the first singlet excited state to the ground state. While these trends can be empirically explained by a simple energy gap law, detailed mechanisms of nearly universal behavior have remained unsettled for many cases. Theoretical and experimental results for two representative NIR/SWIR dye molecules reported here clarify the key mechanism for the observed energy gap law behavior. It is shown that the first derivative nonadiabatic coupling terms serve as major coupling pathways for nonadiabatic decay processes from the first excited singlet state to the ground state for these NIR and SWIR dye molecules and that vibrational modes other than the highest frequency modes also make significant contributions to the rate. Finally, this assessment is corroborated by further theoretical comparison with possible alternative mechanisms of intersystem crossing to triplet states and also by comparison with experimental data for deuterated molecules.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Mechanistic Analysis of Metallaphotoredox C–N Coupling: Photocatalysis Initiates and Perpetuates Ni(I)/Ni(III) Coupling Activity

In this work, the combined use of reaction kinetic analysis, ultrafast spectroscopy, and stoichiometric organometallic studies has enabled the elucidation of the mechanistic underpinnings to a photocatalytic C–N cross-coupling reaction. Steady-state and ultrafast spectroscopic techniques were used to track the excited-state evolution of the employed iridium photocatalyst, determine the resting states of both iridium and nickel catalysts, and uncover the photochemical mechanism for reductive activation of the nickel cocatalyst. Stoichiometric organometallic studies along with a comprehensive kinetic study of the reaction, including rate–driving force analysis, unveiled the crucial role of photocatalysis in both initiating and sustaining a Ni(I)/Ni(III) cross-coupling mechanism. The insights gleaned from this study further enabled the discovery of a new photocatalyst providing a >30-fold rate increase.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗