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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 1,693 records · Page 94
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Photon-triggered jets as probes of multi-stage jet modification

Prompt photons are created in the early stages of heavy ion collisions and traverse the QGP medium without any interaction. Therefore, photontriggered jets can be used to study the jet quenching in the QGP medium. In this work, photon-triggered jets are studied through different jet and jet substructure observables for different collision systems and energies using the JETSCAPE framework. Since the multistage evolution used in the JETSCAPE framework is adequate to describe a wide range of experimental observables simultaneously using the same parameter tune, we use the same parameters tuned for jet and leading hadron studies. The same isolation criteria used in the experimental analysis are used to identify prompt photons for better comparison. For the first time, high-accuracy JETSCAPE results are compared with multi-energy LHC and RHIC measurements to better understand the deviations observed in prior studies. This study highlights the importance of multistage evolution for the simultaneous description of experimental observables through different collision systems and energies using a single parameter tune.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Deciphering yield modification of hadron-triggered semi-inclusive recoil jets in heavy-ion collisions

Jet quenching is recognized as critical evidence for the existence of the quark-gluon plasma (QGP) and serves as an essential probe to study its transport properties. Measurements of hadron-triggered semi-inclusive recoil jets have gained popularity due to its capability to probe jets over an extended phase space at low transverse momenta (p T ) and large radii. Recent ALICE measurements showed that the I AA , yield ratio of recoil jets between heavy-ion and p+p collisions, rises with jet p T and exceeds unity at high p T , contradicting conventional expectations that jet quenching should result in I AA values less than one. In this contribution, we re-examine the surface bias and study the effects of energy losses for both trigger hadrons and recoil jets on I AA , employing the Linear Boltzmann Transport (LBT) model to simulate jet-medium interactions. Our findings suggest that a large portion of hadrons used for the triggers undergoes substantial energy loss, despite surface bias. In particular, the energy loss of the trigger hadrons elevates the I AA baseline, corresponding to the case of no energy loss for recoil jets, to be greatly larger than unity. This enhancement of the baseline implies that the measured I AA values being larger than unity could still signal jet quenching.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2012

Modification of anisotropic plasma diffusion via auxiliary electrons emitted by a carbon nanotubes-based electron gun in an electron cyclotron resonance ion source

The diffusion mechanism in magnetized plasmas is a largely debated issue. A short circuit model was proposed by Simon, assuming fluxes of lost particles along the axial (electrons) and radial (ions) directions which can be compensated, to preserve the quasi-neutrality, by currents flowing throughout the conducting plasma chamber walls. We hereby propose a new method to modify Simon's currents via electrons injected by a carbon nanotubes-based electron gun. We found this improves the source performances, increasing the output current for several charge states. The method is especially sensitive to the pumping frequency. Output currents for given charge states, at different auxiliary electron currents, will be reported in the paper and the influence of the frequency tuning on the compensation mechanism will be discussed.

43 PARTICLE ACCELERATORS↗
DOE OSTI2012

Molecular modification of coumarin dyes for more efficient dye sensitized solar cells

In this work, new coumarin based dyes for dye sensitized solar cells (DSSC) have been designed by introducing several substituent groups in different positions of the NKX-2311 structure. Two types of substitutions have been considered: the introduction of three electron-donating groups (-OH, -NH{sub 2}, and -OCH{sub 3}) and two different substituents with steric effect: -CH{sub 2}-CH{sub 2}-CH{sub 2}- and -CH{sub 2}-HC=CH-. The electronic absorption spectra (position and width of the first band and absorption threshold) and the position of the LUMO level related to the conduction band have been used as theoretical criteria to evaluate the efficiency of the new dyes. The introduction of a -NH{sub 2} group produces a redshift of the absorption maximum position and the absorption threshold, which could improve the cell efficiency. In contrast, the introduction of -CH{sub 2}-CH{sub 2}-CH{sub 2}- does not modify significantly the electronic structure of NKX-2311, but it might prevent aggregation. Finally, -CH{sub 2}-HC=CH- produces important changes both in the electronic spectrum and in the electronic structure of the dye, and it would be expected as an improvement of cell efficiency for these dyes.

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

Focused ion beam induced structural modifications in thin magnetic films

Focused ion beam techniques are one way to modify locally the properties of magnetic thin films. We report on structural investigations of 50 nm thick non-ordered nano-crystalline Permalloy (Ni{sub 81}Fe{sub 19}) films modified by 30 keV Ga{sup +} focused ion beam (FIB) irradiation. From the x-ray diffraction (XRD) measurements a considerable crystallite growth and a material texturing towards (111)-direction with a linearly increasing lattice constant was observed. In addition, cross-sectional transmission electron microscope (XTEM) images show that crystallites are growing through the entire film at high irradiation fluences. Extended x-ray absorption fine structure (EXAFS) analysis shows a perfect near-order coordination corresponding to a face-centered (fcc) unit cell for both Fe, Ni and Ga atom surrounding. The structural changes are accompanied by a decrease of saturation polarization with increasing ion fluence. Such a behavior is attributed to the incorporation of non-magnetic Ga atoms in the Permalloy film.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Irradiation induced modification in transport properties of LaNiO{sub 3} thin films: An x-ray absorption study

Highly c-axis oriented LaNiO{sub 3} thin films are grown on LaAlO{sub 3} (001) single crystal substrates using rf-magnetron sputtering. Swift heavy ion irradiation induced variations in structural and electrical transport properties of deposited films are studied. Pristine film shows unusual insulating character while the irradiated film exhibits metallic behaviour. X-ray diffraction study indicates that irradiation improves the crystalline character of films while retaining the oriented single phase growth. Electronic structure measurements performed using x-ray absorption spectroscopy at O K, La M{sub 5,4}, and Ni L{sub 3,2}-edges reveal that Ni-O hybridization-controlled localization of charge carriers is responsible for the observed transport behaviour.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Modification of {Delta}{sup Prime} by magnetic feedback and kinetic effects

Two possible ways of modifying the linear tearing mode index, by active magnetic feedback and by drift kinetic effects of deeply trapped particles, are analytically investigated. Magnetic feedback schemes, studied in this work, are found generally stabilizing for {Delta}{sup Prime }. The drift kinetic effects from both thermal particles and hot ions tend to reduce the power of the large solution from the outer region. This generally leads to a destabilization of {Delta} Prime for the toroidal analytic equilibria considered here.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Structural stability of BiFeO{sub 3} by chemical modification in Bi as well as Fe sites

Lanthanum and titanium doped BiFeO{sub 3} ceramics Bi{sub 1-x}La{sub x}Fe{sub 1-x}Ti{sub x}O{sub 3} (x and 0.0, 0.03 and 0.06) were synthesized by using the Sol-gel method. The phase purity has been studied by powder X-ray diffraction technique. No secondary phases were evident after lanthanum and titanium addition in certain proportion. The XRD patterns have been analyzed by employing Rietveld refinement technique with the help of Fullprof program. The XRD pattern could be refined by R3c space group in rhombohedral symmetry. The electrical characteristics for all samples have been studied by ferroelectric hysteresis loop at room temperature in the applied electric field of 60 kV/cm.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Density modification by two superposing TE{sub 10} modes in a plasma filled rectangular waveguide

Microwave and plasma interaction is examined via two fundamental TE{sub 10} modes propagating in a plasma filled rectangular waveguide after superposing at a smaller angle. The propagation of the resultant mode realized from these two modes is governed by a wave equation obtained using the Maxwell's equations. This equation is solved numerically using fourth order Runge-Kutta method for the field amplitude of the microwave in the waveguide considering the waveguide to be made up of a perfect conductor and filled with different types of initial plasma density distributions, viz. homogeneous density, linear density with gradient in the propagation direction, and the density with Gaussian profile along the waveguide width. A phenomenon similar to the duct formation by high power microwaves is found to take place, where the plasma density attains interesting profiles. These profiles can be controlled by the angle of superposition, phase difference between the fields of the two modes, microwave frequency and microwave field amplitude.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Modification of the formation of high-Mach number electrostatic shock-like structures by the ion acoustic instability

The formation of unmagnetized electrostatic shock-like structures with a high Mach number is examined with one- and two-dimensional particle-in-cell (PIC) simulations. The structures are generated through the collision of two identical plasma clouds, which consist of equally hot electrons and ions with a mass ratio of 250. The Mach number of the collision speed with respect to the initial ion acoustic speed of the plasma is set to 4.6. This high Mach number delays the formation of such structures by tens of inverse ion plasma frequencies. A pair of stable shock-like structures is observed after this time in the 1D simulation, which gradually evolves into electrostatic shocks. The ion acoustic instability, which can develop in the 2D simulation but not in the 1D one, competes with the nonlinear process that gives rise to these structures. The oblique ion acoustic waves fragment their electric field. The transition layer, across which the bulk of the ions change their speed, widens and their speed change is reduced. Double layer-shock hybrid structures develop.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Selective charge doping of chemical vapor deposition-grown graphene by interface modification

The doping and scattering effect of substrate on the electronic properties of chemical vapor deposition (CVD)-grown graphene are revealed. Wet etching the underlying SiO{sub 2} of graphene and depositing self-assembled monolayers (SAMs) of organosilane between graphene and SiO{sub 2} are used to modify various substrates for CVD graphene transistors. Comparing with the bare SiO{sub 2} substrate, the carrier mobility of CVD graphene on modified substrate is enhanced by almost 5-fold; consistently the residual carrier concentration is reduced down to 10{sup 11} cm{sup −2}. Moreover, scalable and reliable p- and n-type graphene and graphene p-n junction are achieved on various silane SAMs with different functional groups.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Electric field induced modification of magnetic properties in Co{sub 2}Si{sub 2} multilayers

Effects of an external electric field on the magnetization and magnetocrystalline anisotropy (MA) in Co{sub 2}Si{sub 2} multilayers are investigated by first-principles calculations. The results demonstrate that, due to the hybridization between the Si-2p orbitals and the Co-3d orbitals, the magnetic moment of the Co{sub 2}Si{sub 2} supercell changes non-linearly with external electric field and the behavior of band structures near the Fermi energy could be explained within near-rigid-band model. Moreover, spin-polarized charge redistribution leads to spin reorientation and the MA is not proportional to the orbital moment anisotropy under the external electric field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Steady-state modification of the electron density profile in the resonance region of moving plasma subjected to an alternating electric field

Self-consistent electron density profiles of the plasma stream interacting with the electric field of an ultrashort laser pulse are calculated in the framework of the one-dimensional steady-state model with nonlocal polarizability. These profiles are settled down against the background of a given (linear) ion density distribution as a result of the equilibrium between the averaged ponderomotive force, the static space-charge field, and the electron pressure. The average electron density and the longitudinal (parallel to the density gradient) ac electric field are found to be greatly space-modulated in the region downstream of the plasma resonance point.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Improved lateral photovoltaic effect of Ti and carbon films by interface modification with single-walled carbon nanotubes

An efficiently improved lateral photovoltaic effect (LPE) has been successfully observed in Ti/Si and amorphous carbon (a-C) film/Si structures by introducing single-walled carbon nanotubes (SWNTs) as modifying interface instead of native SiO{sub 2} layer grown on Si substrate. The largest lateral photovoltage (LPV) position sensitivity achieved is 67.02 mV/mm for the Ti/Si system and 2.23 mV/mm for the a-C/Si system. This corresponds to an improvement of 40% for the Ti/Si system and 2600% for the a-C/Si system. Besides, the SWNTs modified interface also induced a well-marked shift of optimal film thickness in both materials. An additional novel phenomenon is that the directly observed LPV is much larger in SWNTs/Si system compared to the improved a-C/SWNTs/Si structure. A mechanism based on the change of interface states is given to interpret these results, which not only suggests a new common modulation method for LPE, but also a new potential application of SWNTs for photo-electronic device.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Band gap modification and ferroelectric properties of Bi{sub 0.5}(Na,K){sub 0.5}TiO{sub 3}-based by Li substitution

We report on the reduction of band gap in Bi{sub 0.5}(Na{sub 0.82-x}Li{sub x}K{sub 0.18}){sub 0.5}(Ti{sub 0.95}Sn{sub 0.05})O{sub 3} from 2.99 eV to 2.84 eV due to the substitutions of Li{sup +} ions to Na{sup +} sites. In addition, the lithium substitution samples exhibit an increasing of the maximal polarizations from 21.8 to 25.7 μC/cm{sup 2}. The polarization enhancement of ferroelectric and reduction of the band gaps are strongly related to the Li substitution concentration as evaluated via the electronegative between A-site and oxygen and tolerance factor. The results are promising for photovoltaic and photocatalytic applications.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Thermal conductivity control by oxygen defect concentration modification in reducible oxides: The case of Pr{sub 0.1}Ce{sub 0.9}O{sub 2−δ} thin films

We demonstrate the impact on thermal conductivity of varying the concentration of oxygen vacancies and reduced cations in Pr{sub 0.1}Ce{sub 0.9}O{sub 2−δ} thin films prepared by pulsed laser deposition. The oxygen vacancy concentration is controlled by varying the oxygen partial pressure between 1 × 10{sup −4} and 1 atm at 650 °C. Corresponding changes in the oxygen non-stoichiometry (δ) are monitored by detecting the lattice parameters of the films with high-resolution X-ray diffraction, while the thermal properties are characterized by time-domain thermoreflectance measurements. The films are shown to exhibit a variation in oxygen vacancy content, and in the Pr{sup 3+}/Pr{sup 4+} ratio, corresponding to changes in δ from 0.0027 to 0.0364, leading to a reduction in the thermal conductivity from k = 6.62 ± 0.61 to 3.82 ± 0.51 W/m-K, respectively. These values agree well with those predicted by the Callaway and von Baeyer model for thermal conductivity in the presence of point imperfections. These results demonstrate the capability of controlling thermal conductivity via control of anion and cation defect concentrations in a given reducible oxide.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗