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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 667 records · Page 37
All sourcesNASA NTRSDOE OSTI
DOE OSTI2019

Effective pairing interaction in a system with an incipient band

The nature and mechanism of superconductivity in the extremely electron-doped FeSe-based superconductors remains an outstanding problem. In these systems, the hole-like band has moved below the Fermi energy, and various spin-fluctuation theories involving pairing between states near the electron Fermi surface and states of this incipient band have been proposed. Using a nonperturbative dynamic cluster quantum Monte Carlo calculation for a bilayer Hubbard model, we show how spin-fluctuation scattering involving intermediate virtual states of the incipient band leads to an effective pairing interaction for the electrons on the remaining Fermi surface. The key feature of this interaction is that it is retarded due to the incipient nature of the hole band and the dynamics of the spin-fluctuation interaction. This allows the pairs to avoid the instantaneous repulsive Coulomb interaction and leads to a frequency dependence of the gap which should be observable in tunneling experiments. Our work provides a different perspective for the pairing mechanism in systems with an incipient band that can be tested in future experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Reminiscence of a Magnetization Plateau in a Magnetization Processes of Toy-Model Triangular and Tetrahedral Clusters

We discuss magnetization curves of a toy-model trigonal and tetrahedral clusters. Nonlinearity of magnetization with local minimum of differential susceptibility resembling known magnetization plateaus of triangular-lattice and pyrochlore lattice antiferromagnets is observed at intermediate temperature range J ≲ T ≲ Θ (here, J is the exchange coupling constant and Θ is a Curie–Weiss temperature). This behavior is due to increased statistical weight of the states with intermediate total spin of the cluster, which is related to the “order-by-disorder” mechanism of plateau stabilization of a macroscopic frustrated magnet.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Improving Charge Transport via Intermediate-Controlled Crystal Growth in 2D Perovskite Solar Cells

Reduced-dimensional hybrid perovskite semiconductors have recently attracted significant attention due to their promising stability and optoelectronic properties. However, the issue of poor charge transport in 2D perovskites limits its application. Here, studies on intermediate-controlled crystal growth are reported to improve charge carrier transport in 2D perovskite thin films. It is shown that the coordination strength of solvents with perovskite precursor affects the initial state of intermediate phase formation as well as the subsequent perovskite layer growth. Tuning the solvent composition with a mixture (5:5) of dimethyl formamide (DMF) and dimethyl sulfoxide (DMSO) leads to the growth of highly orientated 2D perovskite films with much-improved optoelectronic properties (faster transport by approx. ≈50x, longer carrier lifetime by approx. ≈4x, and lower defect density by approx. ≈30x) than the film prepared with pure DMF. Consequently, perovskite solar cells based on DMF/DMSO (5:5) show >80% efficiency improvement than the devices based on pure DMF.

14 SOLAR ENERGY↗
DOE OSTI2020

Mechanisms and dynamics of the NH + 2 + H + and NH + + H + + H fragmentation channels upon single-photon double ionization of NH 3

We present state-selective measurements on the NH$_2^{+}$ + H$^{+}$ and NH$^{+}$ + H$^{+}$ + H dissociation channels following single-photon double ionization at 61.5 eV of neutral NH$_{3}$, where the two photoelectrons and two cations are measured in coincidence using 3-D momentum imaging. Three dication electronic states are identified to contribute to the NH$_2^{+}$ + H$^{+}$ dissociation channel, where the excitation in one of the three states undergoes intersystem crossing prior to dissociation, producing a cold NH$_2^+$ fragment. In contrast, the other two states directly dissociate, producing a ro-vibrationally excited NH$_2^+$ fragment with roughly 1 eV of internal energy. The NH$^{+}$ + H$^{+}$ + H channel is fed by direct dissociation from three intermediate dication states, one of which is shared with the NH$_2^{+}$ + H$^{+}$ channel. We find evidence of autoionization contributing to each of the double ionization channels. Finally, the distributions of the relative emission angle between the two photoelectrons, as well as the relative angle between the recoil axis of the molecular breakup and the polarization vector of the ionizing field, are also presented to provide insight on the ionization mechanisms for the different dication states.

74 ATOMIC AND MOLECULAR PHYSICS↗
NASA NTRS1983

Vortices, stability, and turbulence

The transition process between laminar flow to turbulent flow is discussed in terms of a third state of fluid motion, vortical motion that is approximately periodic in space and/or time and displays symmetrical properties. The possibility that the transition from laminar to turbulent flow is accompanied by intermediate vortical states, with turbulence being an instability of the vortical states, is examined, with consideration given to the hypothesis that the turbulent state retains a memory of the vortex structures. Solutions are sought for the Navier-Stokes equations for a viscous liquid in a channel. Instances of inviscid vortical states and the presence of a three-dimensional instability are studied, demonstrating that transition phenomena can be explained qualitatively in terms of vortical state characteristics.

Saffman, P. G.↗
NASA NTRS1993

Solar flare model atmospheres

Solar flare model atmospheres computed under the assumption of energetic equilibrium in the chromosphere are presented. The models use a static, one-dimensional plane parallel geometry and are designed within a physically self-consistent coronal loop. Assumed flare heating mechanisms include collisions from a flux of non-thermal electrons and x-ray heating of the chromosphere by the corona. The heating by energetic electrons accounts explicitly for variations of the ionized fraction with depth in the atmosphere. X-ray heating of the chromosphere by the corona incorporates a flare loop geometry by approximating distant portions of the loop with a series of point sources, while treating the loop leg closest to the chromospheric footpoint in the plane-parallel approximation. Coronal flare heating leads to increased heat conduction, chromospheric evaporation and subsequent changes in coronal pressure; these effects are included self-consistently in the models. Cooling in the chromosphere is computed in detail for the important optically thick HI, CaII and MgII transitions using the non-LTE prescription in the program MULTI. Hydrogen ionization rates from x-ray photo-ionization and collisional ionization by non-thermal electrons are included explicitly in the rate equations. The models are computed in the 'impulsive' and 'equilibrium' limits, and in a set of intermediate 'evolving' states. The impulsive atmospheres have the density distribution frozen in pre-flare configuration, while the equilibrium models assume the entire atmosphere is in hydrostatic and energetic equilibrium. The evolving atmospheres represent intermediate stages where hydrostatic equilibrium has been established in the chromosphere and corona, but the corona is not yet in energetic equilibrium with the flare heating source. Thus, for example, chromospheric evaporation is still in the process of occurring.

Hawley, Suzanne L.↗
DOE OSTI2015

Quantum calculation of inelastic CO collisions with H. II. Pure rotational quenching of high rotational levels

Carbon monoxide is a simple molecule present in many astrophysical environments, and collisional excitation rate coefficients due to the dominant collision partners are necessary to accurately predict spectral line intensities and extract astrophysical parameters. We report new quantum scattering calculations for rotational deexcitation transitions of CO induced by H using the three-dimensional potential energy surface (PES) of Song et al. State-to-state cross sections for collision energies from 10{sup −5} to 15,000 cm{sup −1} and rate coefficients for temperatures ranging from 1 to 3000 K are obtained for CO (v = 0, j) deexcitation from j=1−45 to all lower j′ levels, where j is the rotational quantum number. Close-coupling and coupled-states calculations were performed in full-dimension for j=1−5, 10, 15, 20, 25, 30, 35, 40, and 45 while scaling approaches were used to estimate rate coefficients for all other intermediate rotational states. The current rate coefficients are compared with previous scattering results using earlier PESs. Astrophysical applications of the current results are briefly discussed.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1994

Solar flare model atmospheres

Solar flare model atmospheres computed under the assumption of energetic equilibrium in the chromosphere are presented. The models use a static, one-dimensional plane-parallel geometry and are designed within a physically self-consistent coronal loop. Assumed flare heating mechanisms include collisions from a flux of nonthermal electrons and X-ray heating of the chromosphere by the corona. The heating by energetic electrons accounts explicitly for variations of the ionized fraction with depth in the atmosphere. X-ray heating of the chromosphere by the corona incorporates a flare loop geometry by approximating distant portions of the loop with a series of point sources, while treating the loop leg closest to the chromospheric footpoint in the plane-parallel approximation. Coronal flare heating leads to increased heat conduction, chromospheric evaporation and subsequent changes in coronal pressure; these effects are included self-consistently in the models. Cooling in the chromosphere is computed in detail for the important optically thick H I, Ca II and Mg II transitions using the non-local thermodynamic equilibrium (non-LTE) prescription in the program MULTI. Hydrogen ionization rates from X-ray photoionization and collisional ionization by nonthermal electrons are included explicitly in the rate equations. The models are computed in the 'impulsive' and 'equilibrium' limits, and in a set of intermediate 'evolving' states. The impulsive atmospheres have the density distribution frozen in the pre-flare configuration, while the equilibrium models assume the entire atmosphere is in hydrostatic and energetic equilibrium. The evolving atmospheres represent intermediate stages where hydrostatic equilibrium has been established in the chromosphere and corona, but the corona is not yet in energetic equilibrium with the flare heating source. Thus, for example, chromospheric evaporation is still in the process of occurring. We have computed the chromospheric radiation that results from a range of coronal heating rates, with particular emphasis on the widely observed diagnostic H(alpha). Our conclusion is that the H(alpha) fluxes and profiles actually observed in flares can only be produced under conditions of a low-pressure corona with strong beam heating. Therefore we suggest that H(alpha) in flares is produced primarily at the footprints of newly heated loops where significant evaporation has not yet occurred. As a single loop evolves in time, no matter how strong the heating rate may become, the H(alpha) flux will diminish as the corona becomes denser and hence more effective at stopping the beam. This prediction leads to several observable consequences regarding the spatial and temporal signatures of the X-ray and H(alpha) radiation during flares.

Hawley, Suzanne L.↗
DOE OSTI2011

Catalytic Consequences of Acid Strength in the Conversion of Methanol to Dimethyl Ether

The effects of acid identity on CH{sub 3}OH dehydration are examined here using density functional theory (DFT) estimates of acid strength (as deprotonation energies, DPE) and reaction energies, combined with rate data on Keggin polyoxometalate (POM) clusters and zeolite H-BEA. Measured first-order (k{sub mono}) and zero-order (k{sub dimer}) CH3OH dehydration rate constants depend exponentially on DPE for POM clusters; the value of k{sub mono} depends more strongly on DPE than k{sub dimer} does. The chemical significance of these rate parameters and the basis for their dependences on acid strength were established by using DFT to estimate the energies of intermediates and transition states involved in elementary steps that are consistent with measured rate equations. We conclude from this treatment that CH{sub 3}OH dehydration proceeds via direct reactions of co-adsorbed CH{sub 3}OH molecules for relevant solid acids and reaction conditions. Methyl cations formed at ion-pair transition states in these direct routes are solvated by H{sub 2}O and CH{sub 3}OH more effectively than those in alternate sequential routes involving methoxide formation and subsequent reaction with CH{sub 3}OH. The stability of ion-pairs, prevalent as intermediates and transition states on solid acids, depends sensitively on DPE because of concomitant correlations between the stability of the conjugate anionic cluster and DPE. The chemical interpretation of k{sub mono} and k{sub dimer} from mechanism-based rate equations, together with thermochemical cycles of their respective transition state formations, show that similar charge distributions in the intermediate and transition state involved in k{sub dimer} cause its weaker dependence on DPE. Values of k{sub mono} involve uncharged reactants and the same ion-pair transition state as k{sub dimer}; these species sense acid strength differently and cause the larger effects of DPE on k{sub mono}. Confinement effects in H-BEA affect the value of k{sub mono} because the different sizes and number of molecules in reactants and transition states selectively stabilize the latter; however, they do not influence k{sub dimer}, for which reactants and transition states of similar size sense spatial constraints to the same extent. This combination of theory and experiment for solid acids of known structure sheds considerable light on the relative contributions from solvation, electrostatic and van der Waals interactions in stabilizing cationic transition states and provides predictive insights into the relative contributions of parallel routes based on the size and charge distributions of their relevant intermediates and transition states. These findings also demonstrate how the consequences of acid strength on measured turnover rates depend on reaction conditions and their concomitant changes in the chemical significance of the rate parameters measured. Moreover, the complementary use of experiment and theory in resolving mechanistic controversies has given predictive guidance about how rate and equilibrium constants, often inextricably combined as measured rate parameters, individually depend on acid strength based on the magnitude and spatial distributions of charges in reactants, products and transition states involved in relevant elementary steps. The unique relations between k{sub mono}, k{sub dimer} and DPE developed here for CH{sub 3}OH dehydration can be applied in practice to assess the acid strength of any solid acid, many of which have unknown structures, preventing reliable calculations of their DPE by theory.

10 SYNTHETIC FUELS↗
DOE OSTI2010

Multiphoton ionization and multiphoton resonances in the tunneling regime

The rate of ionization of an atom of helium, argon, or hydrogen exposed to an intense monochromatic laser field and the quasienergy spectrum of their dressed states are studied for values of the Keldysh parameter between 1 and 0.6 and wavelengths between 390 and 1300 nm. The calculations are carried out within the non-Hermitian Floquet theory. Resonances with intermediate excited states significantly affect ionization from the dressed ground state at all the intensities and all the wavelengths considered. The dressed excited states responsible for these structures are large-{alpha}{sub 0} states akin to the Kramers-Henneberger states of the high-frequency Floquet theory. Within the single-active-electron approximation, these large-{alpha}{sub 0} states become species independent at sufficiently high intensity or sufficiently long wavelength. Apart for the resonance structures arising from multiphoton coupling with excited states, the ab initio Floquet ionization rate is in excellent agreement with the predictions of two different calculations in the strong field approximation, one based on a length-gauge formulation of this approximation and one based on a velocity-gauge formulation. The calculations also confirm the validity of the {omega}{sup 2} expansion as an alternative to the strong field approximation for taking into account the nonadiabaticity of the ionization process in intense low-frequency laser fields.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Reduced Probabilities of E2-Transitions in {sup 174}Yb

This paper describes the ground (gr) and exited states of even-even deformed nuclei with a phenomenological model, which takes into account the mixing of gr states, 0{sub n}{sup +}({beta}{sub n})-, 2{sub n}{sup +}({gamma}{sub n})- and {Kappa}{sup {pi}} 1{sub n}{sup +}- rotational bands. The calculation has been done for the isotope Yb. The energy spectra are found to be consistent with the energies from experimental data. The reduced probabilities of the electric quadrupole E2-transitions from {beta}{sub n} and {gamma}{sub n} band states are calculated and agree quite well with the experimental values.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2014

Magnetic ordering in the frustrated J 1 - J 2 Ising chain candidate BaNd 2 O 4

The AR 2 O 4 family (R = rare earth) has recently been attracting interest as a new series of frustrated magnets, with the magnetic R atoms forming zigzag chains running along the c axis. In this paper, we have investigated polycrystalline BaNd 2 O 4 with a combination of magnetization, heat-capacity, and neutron powder diffraction measurements. Magnetic Bragg peaks are observed below T N = 1.7 K, and they can be indexed with a propagation vector of k = (0,1/2,1/2). The signal from magnetic diffraction is well described by long-range ordering of only one of the two types of Nd zigzag chains, with collinear up-up-down-down intrachain spin configurations (double Néel state). Furthermore, low-temperature magnetization and heat-capacity measurements reveal two magnetic-field-induced spin transitions at 2.75 and 4 T for T = 0.46 K. The high-field phase is paramagnetic, while the intermediate-field state may arise from a spin transition of the long-range ordered Nd chains. Finally, one possible candidate for the field-induced ordered state corresponds to an up-up-down intrachain spin configuration, as predicted for a classical J 1 -J 2 Ising chain with a double Néel ground state in zero field.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Predicting and verifying transition strengths from weakly bound molecules

We investigated transition strengths from ultracold weakly bound {sup 41}K{sup 87}Rb molecules produced via the photoassociation of laser-cooled atoms. An accurate potential energy curve of the excited state (3){sup 1}{Sigma}{sup +} was constructed by carrying out direct potential fit analysis of rotational spectra obtained via depletion spectroscopy. Vibrational energies and rotational constants extracted from the depletion spectra of v{sup '}=41-50 levels were combined with the results of the previous spectroscopic study and they were used for modifying an ab initio potential. An accuracy of 0.14% in vibrational level spacing and 0.3% in rotational constants was sufficient to predict the large observed variation in transition strengths among the vibrational levels. Our results show that transition strengths from weakly bound molecules are a good measure of the accuracy of an excited state potential.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2010

Electronic spin and valence states of Fe in CaIrO{sub 3} - type silicate post-perovskite in the earth's lowestmost mantle.

The electronic spin and valence states of Fe in post-perovskite ((Mg{sub 0.75}Fe{sub 0.25})SiO{sub 3}) have been investigated by synchrotron X-ray diffraction, Moessbauer and X-ray emission spectroscopy at 142 GPa and 300 K. Rietveld refinement of the X-ray diffraction patterns revealed that our sample was dominated by CaIrO{sub 3}-type post-perovskite. Combined Moessbauer and X-ray emission results show that Fe in post-perovskite is predominantly Fe{sup 2+} (70%) in the intermediate-spin state with extremely high quadrupole splitting of 3.77(25) mm/s. The remaining 30% Fe can be assigned to two sites. Compared with recent studies, our results indicate that the intermediate-spin Fe{sup 2+} is stabilized in CaIrO{sub 3}-type post-perovskite over a wide range of Fe content, whereas the low-spin Fe{sup 3+} is more dominant in the 2 x 1 kinked post-perovskite structure. The characterization of these structural and compositional effects on the spin and valence states of Fe in post-perovskite can help in understanding the geochemical and geophysical behavior of the core-mantle region.

58 GEOSCIENCES↗
DOE OSTI2020

Complex magnetic properties in the mixed 4 f - 5 d double perovskite iridates X 2 ZnIrO 6 ( X = Nd , Sm , Eu , Gd )

Here in this paper, we report on the synthesis and magnetic properties of a series of double perovskites Ln 2 ZnIrO 6 with Ln = Nd, Sm, Eu, Gd. These compounds present examples of the rare case of double perovskites (general formula A 2 BB'O6) with a magnetic 4ƒ-ion on the A site in combination with the strongly spin-orbit coupled 5d transition-metal ion Ir 4+ on the B sublattice. We discuss the impact of different rare earths on the macroscopic magnetic properties. Gd 2 ZnIrO 6 and Eu 2 ZnIrO 6 show ferrimagnetic or canted antiferromagnetic order below T N = 23 and 12 K, respectively. Sm 2 ZnIrO 6 orders antiferromagnetically at T N = 13 K. Nd 2 ZnIrO 6 exhibits more complex magnetic properties with a strong field dependence ranging from a two-step spin reorientation at μ 0 H = 0.01 T to an antiferromagnetic ground state at intermediate external fields to a spin-flop phase for 0H ≥ 4 T. This unique behavior suggests an interesting interplay between Nd 3+ and Ir 4+ . To further shed light on the relevant magnetic interaction, the zero-field magnetic ground state of Nd 2 ZnIrO 6 is examined via neutron powder diffraction. In general, the magnetic properties of Ln 2 ZnIrO 6 hint towards a substantial contribution of the rare-earth ions and a significant correlation between 5d and 4ƒ magnetism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Excitation of the 3p states in electron-sodium scattering at intermediate energies

A coupled-channel-optical method (CCOM), to investigate the excitation of the 3p states for e{sup −}-Na scattering at intermediate energies, is reported. Nine atomic states( Na(3s), Na(3p), Na(4s), Na(3d), Na(4p), Na(5s), Na(4d), Na(5p), Na(5d) ) together with three optical potentials are used in this work. The inelastic differential cross sections (DCS) as well as the reduced Stokes parameters are compared with latest theoretical data and experimental measurements.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2023

Projections of Hourly Meteorology by Balancing Authority Based on the IM3/HyperFACETS Thermodynamic Global Warming (TGW) Simulations

This dataset contains 40 years (1980-2019) of historical hourly meteorology and 80 years (2020-2099) of projected hourly meteorology for 54 Balancing Authorities (BAs) in the conterminous United States. Details about the scenarios and variables included in this dataset are in the readme.pdf file. This dataset is derived from the IM3/HyperFACETS Thermodynamic Global Warming (TGW) simulations (https://doi.org/10.57931/1885756). More details on the TGW approach can be found at: https://tgw-data.msdlive.org/. If you use this dataset please also cite the raw TGW dataset (Jones, A. D., Rastogi, D., Vahmani, P., Stansfield, A., Reed, K., Thurber, T., Ullrich, P., & Rice, J. S. (2022). IM3/HyperFACETS Thermodynamic Global Warming (TGW) Simulation Datasets (v1.0.0) [Data set]. MSD-LIVE Data Repository. https://doi.org/10.57931/1885756). To go from the TGW data to these BA-level aggregated data we first averaged the raw gridded data by county in the United States. That intermediate data step is also stored in MSD-LIVE (https://doi.org/10.57931/1960548). We then population-weight the county-level hourly data in order to create population-weighted meteorology time series for each BA. Historical populations are from the United States Census Bureau and the evolving future populations are based on Shared Socioeconomic Pathways (SSPs) 3 and 5. The four climate scenarios crossed with the two SSPs yield eight different future projections for each BA: rcp45cooler_ssp3, rcp45cooler_ssp5, rcp45hotter_ssp3, rcp45hotter_ssp5, rcp85cooler_ssp3, rcp85cooler_ssp5, rcp85hotterssp3, rcp85hotterssp5. For the historical period and each of the eight future scenarios the dataset has hourly estimates of the population-weighted average of five meteorological variables: Temperature, specific humidity, shortwave radiation, longwave radiation, and wind speed. All times are in Coordinated Universal Time (UTC). The code to go from the raw TGW data to county-level and then BA-level projections is available at: https://github.com/IMMM-SFA/im3components/tree/main/im3components/wrf_to_tell.

Balancing Authority↗