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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 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Influence of globally spin-aligned vector mesons to the measurements of the chiral magnetic effect in heavy-ion collisions

The chiral magnetic effect (CME) in high-energy heavy-ion collisions arises from the interplay between the chirality imbalance and the intense magnetic field and will cause a charge separation along the magnetic field direction. While the CME search is still ongoing in experiments, the non-CME contributions need to be excluded from the CME observables. In this work, we examine the influence of globally spin-aligned ρ mesons on the γ 112 correlator, the R Ψ 2 (ΔS) correlator, and the signed balance functions, via a toy model and a multiphase transport model (AMPT). We find that the CME observables are sensitive to the 00-component of the spin density matrix, ρ 00 : they receive positive (negative) contributions when ρ 00 is larger (smaller) than 1/3.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Effects of Ti incorporation on the interface properties and band alignment of HfTaO{sub x} thin films on sulfur passivated GaAs

Thin HfTaO{sub x} and HfTaTiO{sub x} gate dielectrics ({approx}7-8 nm) have been rf sputter-deposited on sulfur passivated GaAs. Our experimental results suggest that the formation of Ga-O at GaAs surface and As diffusion in dielectric may be effectively controlled by Ti incorporation. Possibility of tailoring of band alignment via Ti incorporation is shown. Valence band offsets of 2.6{+-}0.05 and 2.68{+-}0.05 eV and conduction-band offsets of 1.43{+-}0.05 and 1.05{+-}0.05 eV were found for HfTaO{sub x} (E{sub g{approx}}5.45 eV) and HfTaTiO{sub x} (E{sub g{approx}}5.15 eV), respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Field emission behavior of vertically aligned ZnO nanowire planar cathodes

A field emission (FE) study by scanning anode field emission microscopy was performed to evaluate the FE properties of vertically aligned zinc oxide (ZnO) nanowire arrays electrodeposited on a plane conductive surface. The specific FE behaviors of the cathode observed experimentally are (1) a turn-on macroscopic field of about 6 V/{mu}m for a FE current density J{sub FE} 5 x 10{sup -4} A/cm{sup 2}, (2) a stable FE characteristics for 5 x 10{sup -4} < J{sub FE} < 5 x 10{sup -2} A/cm{sup 2}, and (3) a brutal shut down of FE when J{sub FE} crossed a limiting value of {approx}0.05 A/cm{sup 2} due to a rapid evolution of the nanowires toward a bulbous tip geometry or a complete melting. A physical process of FE from ZnO nanostructures is proposed from the experimental data analyses. An effective surface barrier of about 1 eV was determined from the experimental Fowler-Nordheim plot and the presence of a Zn enriched surface was assumed in considering the possibility of important modifications of the crystallography and charge transfers at the surface of ZnO nanowires during the application of the strong electric field required for FE.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2011

Energy accommodation of gas molecules with free-standing films of vertically aligned single-walled carbon nanotubes

The scattering process of gas molecules on vertically aligned single-walled carbon nanotubes (VA-SWNTs) was investigated by the molecular beam technique. To investigate interactions between VA-SWNT films themselves and helium gas molecules without the presence of substrates, free-standing films were used. The scattered molecules are divided into three components; reflected molecules, diffusively transmitted molecules, and directly transmitted molecules without interaction with SWNTs. Even with the thin film, most molecules have interacted with the films, which suggests that most molecules have interacted at the randomly oriented layer at the topmost of the films. Because of low accommodation coefficients of helium gas, VA-SWNTs films are thought to be useful as a surface modification to enhance energy accommodation.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Surface orientation dependent band alignment for CuInSe{sub 2}-ZnSe-ZnO

The interface formation and band-lineup between ZnO and epitaxial CuInSe{sub 2} substrates is investigated with photoelectron spectroscopy. ZnO films prepared by a metal-organic molecular beam epitaxy process lead to the formation of an intrinsic ZnSe buffer layer. The effect of substrate orientation on the band alignment is studied for the orientations (112) and (001). We found a weak dependence of the interfacial dipole on orientation. Together with our findings for CuInS{sub 2} films, we suggest a ubiquitous CuInX{sub 2}-ZnX-ZnO (X=Se, S) structure with an ultra-thin ZnX buffer layer for ZnO growth on chalcopyrites at elevated temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Ultra-violet Sensing Characteristic and Field Emission Properties of Vertically Aligned Aluminum Doped Zinc Oxide Nanorod Arrays

Ultra-violet (UV) sensing behavior and field emission characteristic have been investigated on vertically aligned aluminum (Al) doped zinc oxide (ZnO) nanorod arrays prepared using sol-gel immersion method. Uniform and high coverage density of ZnO nanorod arrays have been successfully deposited on seeded-catalyst coated substrates. The synthesized nanorods have diameter sizes between 50 nm to 150 nm. The XRD spectra show Al doped ZnO nanorod array has high crystallinity properties with the dominancy of crystal growth along (002) plane or c-axis. UV photoresponse measurement indicates that Al doped ZnO nanorod array sensitively detects UV light as shown by conductance increment after UV illumination exposure. The nanorod array shows good field emission properties with low turn on field and threshold field at 2.1 V/{mu}m and 5.6 V/{mu}m, respectively. The result suggested that Al doped ZnO nanorod arrays prepared by low-cost sol-gel immersion method show promising result towards fabrication of multi applications especially in UV photoconductive sensor and field emission displays.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2011

Band alignment at the Cu{sub 2}ZnSn(S{sub x}Se{sub 1-x}){sub 4}/CdS interface

Energy band alignments between CdS and Cu{sub 2}ZnSn(S{sub x}Se{sub 1-x}){sub 4} (CZTSSe) grown via solution-based and vacuum-based deposition routes were studied as a function of the [S]/[S+Se] ratio with femtosecond laser ultraviolet photoelectron spectroscopy, photoluminescence, medium energy ion scattering, and secondary ion mass spectrometry. Band bending in the underlying CZTSSe layer was measured via pump/probe photovoltage shifts of the photoelectron spectra and offsets were determined with photoemission under flat band conditions. Increasing the S content of the CZTSSe films produces a valence edge shift to higher binding energy and increases the CZTSSe band gap. In all cases, the CdS conduction band offsets were spikes.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Tokamak equilibria with non field-aligned axisymmetric divergence-free rotational flows

Rotational ideal divergence-free magnetohydrodynamic (MHD) equations are expressed in terms of transformed variables w-vector{sub *}=({mu}{rho}){sup 1/2}v-vector and {mu}p{sub *} = ({mu}p + w{sub *}{sup 2}/2), where v-vector, p, and {rho} are plasma velocity, pressure, and mass density, respectively. With divergence-free flows, {nabla}{center_dot}v-vector=0, the plasma density {rho} does not appear in the MHD equations written in terms of w-vector{sub *} and {mu}p{sub *}. The non field-aligned rotational Grad-Shafranov equation is represented in spherical coordinates. Tokamak-like axisymmetric equilibria with v-vector perpendicular {nabla}{rho} are obtained analytically by solving for torus solutions under only three source functions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2011

Studies of Valence Band Alignment Between Nitrided GaPN/GaP (111) Interface Using X-ray Photoelectron Spectroscopy

The analysis of core levels positions of Ga{sub 3}d, N{sub 1s} and P{sub 2}p at different etching depth from the plasma nitrided GaP (111) surface shows that the nitrogen ions interact with both Gallium and Phosphorous ions with nearly equal probability. The analysis of valence band spectra shows the type-II band alignment between GaPN{sub 0.22}/GaP and the valence band offset is {approx}2.2{+-}0.1 eV.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Aligned and exchange-coupled FePt-based films

Two-phase nanostructures of hard L1{sub 0}-ordered FePt and soft iron-rich fcc Fe-Pt are investigated experimentally and by model calculations. The Fe-Pt thin films were produced by epitaxial co-sputtering onto MgO and have a thickness of about 10 nm. They form two-phase dots that cover a large fraction of the surface but are separated from each other. X-ray diffraction and TEM show that the c-axis of the phase FePt is aligned in the direction normal to the film plane. The experimental and theoretical hysteresis loops indicate archetypical exchange coupling, and excellent magnetic properties are obtained. The largest values of coercivity, saturation magnetization, and nominal energy product obtained in the samples studied are 51 kOe, 1287 emu/cc, and 54 MGOe, respectively.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Orbital alignment in photodissociation probed using strong field ionization

The photodissociation of molecules often produces atomic fragments with polarized electronic angular momentum, and the atomic alignment, for example, can provide valuable information on the dynamical pathways of chemical reactions unavailable by other means. In this work, we demonstrate for the first time that orbital polarization in chemical reactions can be measured with great sensitivity using strong field ionization by exploiting its extreme nonlinearity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2012

Coexistence of type-I and type-II band alignments in antimony-incorporated InAsSb quantum dot nanostructures

Antimony-incorporated InAsSb quantum dots (QDs) are grown by molecular beam epitaxy on GaAs(001) substrates. The QD density increases {approx}7 times while the QD height decreases {approx}50% due to the increase of QD nucleation sites after Sb incorporation into the GaAs buffer layer and into the InAs QDs. These Sb-incorporated InAsSb QDs show red-shift in the photoluminescence (PL) spectrum and large energy separation between confined energy levels. More interestingly, besides the typical type-I QD transition, an additional peak from the recombination at wetting layer interface develops as the excitation laser intensity increases. This peak clearly exhibits type-II characteristics from the measurement of a large blue-shift of the PL peak and a long PL decay time. Finally, the mechanism of the coexistence of type-I and type-II band alignments is discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Aligned and exchange-coupled L1{sub 0} (Fe,Co)Pt-based magnetic films

Films of aligned L1{sub 0}-structure (Fe,Co)Pt with fcc Fe(Co,Pt) are synthesized by co-sputtering Fe, Co, and Pt on an (001) MgO substrate with in situ heating at 830 deg. C. The nanostructures and magnetic properties of the films are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and superconducting quantum interference device (SQUID). The compositions of the samples (Fe,Co){sub x}Pt{sub 1-x} are designed to maintain an atomic Fe: Co ratio of 65: 35 while increasing the Fe,Co content in each successive sample. In samples with low Fe and Co concentration, the XRD patterns exhibit three strong peaks, namely L1{sub 0} (Fe,Co)Pt (001), L1{sub 0} (Fe,Co)Pt (002), and MgO (002). A fourth peak is observed in samples with high Fe and Co concentration and identified as fcc (002). The XRD patterns confirm the formation of L1{sub 0}-ordered (Fe,Co)Pt and its epitaxial growth on MgO. TEM shows that the (Fe,Co)Pt films form isolated magnetic grains of about 100 nm in diameter. Hysteresis-loop measurements show that the increase of the Fe,Co concentration from 57.3 to 68.3 at % enhances the saturation magnetization M{sub s} from 1245 emu/cm{sup 3} to 1416 emu/cm{sup 3}, and the coercivity decreases from 32 kOe to 8.9 kOe. The nominal maximum energy product per grain is 64 MGOe.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Modeling the propagation of whistler-mode waves in the presence of field-aligned density irregularities

We present a numerical study of propagation of VLF whistler-mode waves in a laboratory plasma. Our goal is to understand whistler propagation in magnetic field-aligned irregularities (also called channels or ducts). Two cases are examined, that of a high-frequency ({omega}>{Omega}{sub ce}/2) whistler in a density depletion duct and that of a low-frequency ({omega}<{Omega}{sub ce}/2) whistler in a density enhancement. Results from a numerical simulation of whistler wave propagation are compared to data from the UCLA Los Angeles Physics Teachers Alliance Group plasma device and whistler propagation in pre-existing density depletion and density enhancement ducts is demonstrated.

58 GEOSCIENCES↗
DOE OSTI2012

Field-aligned accelerations by plasma shocks propagating through interstellar magnetic fields

A kinetic model of particle acceleration by plasma shocks is analyzed theoretically and with numerical calculations. The shocks are propagating through weakly magnetized background plasmas, namely interstellar magnetic fields (IMFs). Particles located at the shock front are accelerated parallel to the magnetic field of the shock; this is defined as the field-aligned acceleration (FAA). The cross angle between IMF and the magnetic field of the shock plays an important role in creating the magnetic neutral sheet at the shock front. A test particle trapped by the neutral sheet obtains enormous energy due to the FAA. A reasonable formula for the highest energy gain is derived from theoretical analysis of the relativistic equations of motion. A possible configuration of the electric and magnetic fields in supernova remnants is also proposed by way of example.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Band alignment of InGaZnO{sub 4}/Si interface by hard x-ray photoelectron spectroscopy

Although amorphous InGaZnO{sub 4} has intensively been studied for a semiconductor channel material of thin-film transistors in next-generation flat-panel displays, its electronic structure parameters have not been reported. In this work, the electron affinities ({chi}) and the ionization potentials (I{sub p}) of crystalline and amorphous InGaZnO{sub 4} (c-IGZO and a-IGZO) were measured using bulk-sensitive hard x-ray photoelectron spectroscopy. First, the {chi} and I{sub p} values of c-IGZO and a-IGZO thin films were estimated by aligning the Zn 2p{sub 3/2} core level energies to a literature value for ZnO, which provided {chi} = 3.90 eV and I{sub p} = 7.58 eV for c-IGZO and 4.31 eV and 7.41 eV for a-IGZO. It was also confirmed that the escape depth of the photoelectrons excited by the photon energy of 5950.2 eV is 3.3 nm for a-IGZO and large enough for directly measuring the interface electronic structure using a-IGZO/c-Si heterojunctions. It provided the valence band offset of {approx}2.3 eV, which agrees well with the above data. The present results substantiate that the a-IGZO/c-Si interface follows well the Schottky-Mott rule.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Assembly of uniaxially aligned rare-earth-free nanomagnets

We report HfCo7 nanoparticles with appreciable permanent-magnet properties (magnetocrystalline anisotropy K-1 approximate to 10 Mergs/cm(3), coercivity H-c approximate to 4.4 kOe, and magnetic polarization J(s) approximate to 10.9 kG at 300 K) deposited by a single-step cluster-deposition method. The direct crystalline-ordering of nanoparticles during the gas-aggregation process, without the requirement of a high-temperature thermal annealing, provides an unique opportunity to align their easy axes uniaxially by applying a magnetic field of about 5 kOe prior to deposition, and subsequently to fabricate exchange-coupled nanocomposites having J(s) as high as 16.6 kG by co-depositing soft magnetic Fe-Co. This study suggests HfCo7 as a promising rare-earth-free permanent-magnet alloy, which is important for mitigating the critical-materials aspects of rare-earth elements. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4753950]

Balamurugan, B↗
DOE OSTI2012

Asymmetric chiral alignment in magnetized plasma turbulence

Multi species turbulence in inhomogeneous magnetised plasmas is found to exhibit symmetry breaking in the dynamical alignment of a third species with the fluctuating electron density and vorticity with respect to the magnetic field direction and the species' relative background gradients. The possibility of truly chiral aggregation of charged molecules in magnetized space plasma turbulence is discussed.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗