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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 1,765 records · Page 98
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Optical studies of the uniaxial stress-induced orbital alignment of the Cr{sup 2+} centers in KZnF{sub 3} single crystal

Observation of an intense optical linear dichroism arising in cubic KZnF{sub 3}:Cr crystal at low temperatures under uniaxial stress applied along the four-fold axis is reported. Dichroism occurs in the range of the wide vibronic absorption band corresponding to {sup 5}E{sub g} → {sup 5}T{sub 2g} transition of the Cr{sup 2+} ions. Strain dependences of the dichroism value were studied at the temperatures of 2.0 K, 4.2 K, and 77 K. We associate our observations with the Jahn-Teller effect in the {sup 5}E{sub g} ground state of the Cr{sup 2+} ion. The model is proposed based on a redistribution of the centers between the minima of the E⨂e problem adiabatic potential that become inequivalent under uniaxial stress applied along the four-fold axis of the crystal. It is shown that random strains in the sample have to be taken into account to achieve the quantitative agreement of the model predictions with experimental data. It is found that random strains in the studied sample originate predominantly from point defects. Obtained parameter values are inversion splitting δ = 9.2 ± 1.6 cm{sup −1}, electron-strain coupling constant qV{sub ES} = 16 500 ± 600 cm{sup −1}, width of the random strain distribution w = (6.9 ± 0.5) ⋅ 10{sup −5}. It is shown also that the minima of the [CrF{sub 6}]{sup 4−} cluster adiabatic potential correspond to the elongated along the four-fold axes configurations.

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

Three-dimensional nonlinear ideal MHD equilibria with field-aligned incompressible and compressible flows

The equilibrium properties of three-dimensional ideal magnetohydrodynamics (MHD) are investigated. Incompressible and compressible flows are considered. The governing equations are taken in a steady state such that the magnetic field is parallel to the plasma flow. Equations of stationary equilibrium for both of incompressible and compressible MHD flows are derived and described in a mathematical mode. For incompressible MHD flows, Alfvénic and non-Alfvénic flows with constant and variable magnetofluid density are investigated. For Alfvénic incompressible flows, the general three-dimensional solutions are determined with the aid of two potential functions of the velocity field. For non-Alfvénic incompressible flows, the stationary equilibrium equations are reduced to two differential constraints on the potential functions, flow velocity, magnetofluid density, and the static pressure. Some examples which may be of some relevance to axisymmetric confinement systems are presented. For compressible MHD flows, equations of the stationary equilibrium are derived with the aid of a single potential function of the velocity field. The existence of three-dimensional solutions for these MHD flows is investigated. Several classes of three-dimensional exact solutions for several cases of nonlinear equilibrium equations are presented.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2017

The effects of core-level broadening in determining band alignment at the epitaxial SrTiO 3 (001)/ p -Ge(001) heterojunction

Chemical effects at the surface and interface can broaden core-level spectra in X-ray photoemission for thin-film heterojunctions, as can electronic charge redistributions. We explore these effects and their influence on the measurement of valence and conduction band offsets at the epitaxial SrTiO 3 (001)/p-Ge(001) heterojunction. Here, we observe a clear broadening in Ge 3d and Sr 3d core-level X-ray photoelectron spectra relative to those of clean, bulk Ge(001), and homoepitaxial SrTiO 3 (001), respectively. Angle-resolved measurements indicate that this broadening is driven primarily by chemical shifts associated with surface hydroxylation, with built-in potentials playing only a minor role. The impact of these two interpretations on the valence band offset is significant on the scale of transport energetics, amounting to a difference of 0.2 eV.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Circular dichroism in photoelectron images from aligned nitric oxide molecules

We have used velocity map photoelectron imaging to study circular dichroism of the photoelectron angular distributions (PADs) of nitric oxide following two-color resonanceenhanced two-photon ionization via selected rotational levels of the A 2 Σ + , v' = 0 state. By using a circularly polarized pump beam and a counter-propagating, circularly polarized probe beam, cylindrical symmetry is preserved in the ionization process, and the images can be reconstructed using standard algorithms. The VMI set up enables individual ion rotational states to be resolved with excellent collection efficiency, rendering the measurements considerably simpler to perform than previous measurements conducted with a conventional photoelectron spectrometer. The results demonstrate that circular dichroism is observed even when cylindrical symmetry is maintained, and serve as a reminder that dichroism is a general feature of the multiphoton ionization of atoms and molecules. Furthermore, the observed PADs are in good agreement with calculations based on parameters extracted from previous experimental results obtained by using a time-offlight electron spectrometer.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Photodissociation of aligned CH3I and C6H3F2I molecules probed with time-resolved Coulomb explosion imaging by site-selective extreme ultraviolet ionization

We explore time-resolved Coulomb explosion induced by intense, extreme ultraviolet (XUV) femtosecond pulses from a free-electron laser as a method to image photo-induced molecular dynamics in two molecules, iodomethane and 2,6-difluoroiodobenzene. At an excitation wavelength of 267 nm, the dominant reaction pathway in both molecules is neutral dissociation via cleavage of the carbon–iodine bond. This allows investigating the influence of the molecular environment on the absorption of an intense, femtosecond XUV pulse and the subsequent Coulomb explosion process. We find that the XUV probe pulse induces local inner-shell ionization of atomic iodine in dissociating iodomethane, in contrast to non-selective ionization of all photofragments in difluoroiodobenzene. The results reveal evidence of electron transfer from methyl and phenyl moieties to a multiply charged iodine ion. In addition, indications for ultrafast charge rearrangement on the phenyl radical are found, suggesting that time-resolved Coulomb explosion imaging is sensitive to the localization of charge in extended molecules.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

Surface band bending and interface alignment of plasma-enhanced atomic layer deposited SiO2 on AlxGa1-xN

AlxGa1-xN is characterized by a significant spontaneous and piezoelectric polarization, which increases with the aluminum content. As a result, a surface bound charge is present, which favors compensation by surface states and influences the reliability of AlGaN/GaN devices. This work, therefore, focused on the effects of the polarization charge for GaN and AlGaN with three different aluminum concentrations 15%, 25%, and 35%. The band bending of AlxGa1-xN surfaces was measured after a N2/H2 plasma pretreatment, which reduced the carbon and oxygen contamination below the detection limit of x-ray photoelectron spectroscopy. Surface band bending was then related to surface states, where the band bending of oxygen-free surfaces—as obtained with a high-temperature, immersed hydrogen/nitrogen plasma clean—scales with the aluminum content. In addition, the band offsets at the plasma-enhanced atomic layer deposited SiO2/AlxGa1-xN interface were measured, giving 3.4 eV, 3.3 eV, 3.3 eV, and 3.0 eV for respective 0%, 15%, 25%, and 35% aluminum concentrations. These values are in accordance with the charge neutrality level model, which implies that SiO2 will confine carriers over nearly the full range of the aluminum content.

Eller, Brianna S.↗
DOE OSTI2015

THE WELL-ALIGNED ORBIT OF WASP-84b: EVIDENCE FOR DISK MIGRATION OF A HOT JUPITER

We report the sky-projected orbital obliquity (spin–orbit angle) of WASP-84 b, a 0.69M{sub Jup} planet in an 8.52 day orbit around a G9V/K0V star, to be λ = −0.3 ± 1.7°. We obtain a true obliquity of ψ = 17.3 ± 7.7° from a measurement of the inclination of the stellar spin axis with respect to the sky plane. Due to the young age and the weak tidal forcing of the system, we suggest that the orbit of WASP-84b is unlikely to have both realigned and circularized from the misaligned and/or eccentric orbit likely to have arisen from high-eccentricity migration. Therefore we conclude that the planet probably migrated via interaction with the protoplanetary disk. This would make it the first “hot Jupiter” (P<10 d) to have been shown to have migrated via this pathway. Further, we argue that the distribution of obliquities for planets orbiting cool stars (T{sub eff} < 6250 K) suggests that high-eccentricity migration is an important pathway for the formation of short-orbit, giant planets.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Enhanced light–matter interaction of aligned armchair graphene nanoribbons using arrays of plasmonic nanoantennas

We couple photoluminescent semiconducting 7-atom wide armchair edge graphene nanoribbons to plasmonic nanoantenna arrays and demonstrate an enhancement of the photoluminescence and Raman scattering intensity of the nanoribbons by more than an order of magnitude averaged over large areas, and by three orders of magnitude in the hot spots of plasmonic antennas. The increase in signal allows us to study Raman spectra with high signal-to-noise ratio. Using plasmonic enhancement we are able to detect the off-resonant Raman scattering from the modified radial breathing-like mode (RBLM) due to physisorbed molecules, the 3rd order RBLM, and C-H vibrations. We find excellent agreement between experimental data and simulations describing the spectral dependence of the enhancement and modifications of the polarization anisotropy. The strong field gradients in the optical near-field further allow us to probe the subwavelength coherence properties of the phonon modes in the nanoribbons. We theoretically model this considering a finite phonon correlation length along the GNR direction. Our results allow estimating the correlation length in graphene nanoribbons.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Probing star formation in the dense environments of z 1 lensing haloes aligned with dusty star-forming galaxies detected with the South Pole Telescope

We probe star formation in the environments of massive ~1013M⊙ dark matter halos at redshifts of z~1. This star formation is linked to a sub-millimetre clustering signal which we detect in maps of the Planck High Frequency Instrument that are stacked at the positions of a sample of high-redshift (z>2) strongly-lensed dusty star-forming galaxies (DSFGs) selected from the South Pole Telescope (SPT) 2500 deg2 survey. The clustering signal has sub-millimetre colours which are consistent with the mean redshift of the foreground lensing halos (z~1). We report a mean excess of star formation rate (SFR) compared to the field, of (2700±700)M⊙yr-1 from all galaxies contributing to this clustering signal within a radius of 3.5' from the SPT DSFGs. The magnitude of the Planck excess is in broad agreement with predictions of a current model of the cosmic infrared background. The model predicts that 80% of the excess emission measured by Planck originates from galaxies lying in the neighbouring halos of the lensing halo. Using Herschel maps of the same fields, we find a clear excess, relative to the field, of individual sources which contribute to the Planck excess. The mean excess SFR compared to the field is measured to be (370±40)M⊙yr-1 per resolved, clustered source. Our findings suggest that the environments around these massive z~1 lensing halos host intense star formation out to about 2Mpc. The flux enhancement due to clustering should also be considered when measuring flux densities of galaxies in Planck data.

79 ASTRONOMY AND ASTROPHYSICS↗