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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 703 records · Page 39
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Generation of broadband terahertz radiation using a backward wave oscillator and pseudospark-sourced electron beam

This paper presents for the generation of a small size high current density pseudospark (PS) electron beam for a high frequency (0.2 THz) Backward Wave Oscillator (BWO) through a Doppler up-shift of the plasma frequency. An electron beam ∼1 mm diameter carrying a current of up to 10 A and current density of 10{sup 8} A m{sup −2}, with a sweeping voltage of 42 to 25 kV and pulse duration of 25 ns, was generated from the PS discharge. This beam propagated through the rippled-wall slow wave structure of a BWO beam-wave interaction region in a plasma environment without the need for a guiding magnetic field. Plasma wave assisted beam-wave interaction resulted in broadband output over a frequency range of 186–202 GHz with a maximum power of 20 W.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

High temperature Raman investigation of few-layer MoTe{sub 2}

We present a Raman investigation of the temperature effect of single and few layer MoTe{sub 2} at an electronic device working temperature range from 300 K to 500 K. We observe linear frequency red-shifts with increasing temperature for the first order Raman active E{sup 1}{sub 2g}, A{sub 1g}, Raman inactive B{sup 1}{sub 2g} mode, and the second order ω{sub 2} mode, which can be attributed to the anharmonic effect of the interatomic potential energy. The temperature coefficients of the out-of-plane vibrational B{sup 1}{sub 2g} modes and inplane vibrational E{sup 1}{sub 2g} modes are similarly around −0.013 cm{sup −1}/K, while lower than that of out-of-plane vibration A{sub 1g} mode at −0.009 cm{sup −1}/K. The temperature coefficient of ω{sub 2} mode is −0.00521 cm{sup −1}/K, approximately half of those of the first order modes, and the temperature coefficient of transverse acoustic TA (M) mode is indirectly deduced as −0.0102 cm{sup −1}/K, which shows the corresponding Mo-Te stretching bonds of TA (M) mode behavior similarly to those of optical Raman vibrations. Our work thus provides temperature dependent lattice vibration information of MoTe{sub 2} and could be potentially useful in future optoelectronic devices based on MoTe{sub 2} related two dimensional materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

A plasma photonic crystal bandgap device

A fully tunable plasma photonic crystal is used to control the propagation of free space electromagnetic waves in the S to X bands of the microwave spectrum. An array of discharge plasma tubes forms a simple square crystal structure with the individual plasma dielectric constant tuned through variation in the plasma density. We show, through simulations and experiments, that transverse electric mode bandgaps exist, arising from the positive and negative dielectric constant regimes of the plasma, and that the respective bandgap frequencies can be shifted through changing the dielectric constant by varying discharge current density.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Raman spectra and structural properties of hexagonal Yb{sub 1-x}Dy{sub x}MnO{sub 3} (x = 0, 0.05 and 0.1)

Single phase Yb{sub 1-x}Dy{sub x}MnO{sub 3} (x= 0, 0.05 and 0.1) samples are prepared by a solid state reaction method. X-ray powder diffraction shows that all samples crystallize in the hexagonal phase with P6{sub 3}cm space group. The structural analysis shows there is increase in lattice parameter c and cell volume of the hexagonal unit cell with Dy substitution and the average bond length between Mn-O increases. Raman spectra show that the phonon peaks of Yb{sub 1-x}Dy{sub x}MnO{sub 3} slightly shift to lower frequencies with doping.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

High temperature magneto-electric effect in yittrium iron garnet (YIG)

We report a study on potential multiferroic characteristics of Yttrium Iron Garnet (YIG). The emergence of ferroelectricity in YIG is in debate but we provide evidence for strong magneto-electric coupling above room temperature from dielectric constant measurement with and without magnetic field. We find that the apparent pseudo-ferroelectric crossover temperature in YIG varies with frequency. For higher frequency the transition shifts towards higher temperature. This is indicative of relaxor behavior. We have also measured the dielectric constant in the presence of external magnetic field at high temperature that confirms interdependence of magnetic and dielectric properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Effect of oxygen vacancies and strain on the phonon spectrum of HfO 2 thin films

The effect of strain and oxygen deficiency on the Raman spectrum of monoclinic HfO 2 is investigated theoretically using first-principles calculations. 1% in-plane compressive strain applied to a and c axes is found to blue shift the phonon frequencies, while 1% tensile strain does the opposite. The simulations are compared, and good agreement is found with the experimental results of Raman frequencies greater than 110 cm -1 for 50 nm HfO 2 thin films. Several Raman modes measured below 110 cm -1 and previously assigned to HfO 2 are found to be rotational modes of gases present in air ambient (nitrogen and oxygen). However, localized vibrational modes introduced by threefold-coordinated oxygen (O 3 ) vacancies are identified at 96.4 cm -1 computationally. Finally, these results are important for a deeper understanding of vibrational modes in HfO 2 , which has technological applications in transistors and particularly in resistive random-access memory whose operation relies on oxygen-deficient HfO x .

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

Analysis of synthetic electron cyclotron emission from the high field side of HL-2M tokamak plasmas

A synthetic electron cyclotron emission (ECE) diagnostic is used to interpret ECE signals from preset plasma equilibrium profiles, including magnetic field, electron density, and electron temperature. According to the simulation results, the electron temperature (T e ) profile covering the harmonic overlap region can be obtained by receiving ECE signals at the high field side (HFS) of the HL-2M plasma. The third harmonic ECE at the low field side (LFS) cannot pass through the second harmonic resonance layer at the HFS unless the optical thickness (τ) of the second harmonic becomes gray (τ ≤ 2). In addition, the impact of the relativistic frequency down-shift has been evaluated and corrected. The measurable range of the HFS ECE has been calculated by scanning different parameters (electron density, temperature, and magnetic field). Higher plasma parameters allow a wider radial range of electron temperature measurements. The minimum inner measurable position can reach R = 120 cm (r/a = -0.89) when the product of core temperature (T e0 ) and density (n e0 ) is greater than 35 × 10 19 keV m -3 , which is extended by more than 30 cm inward compared with that of the LFS measurement. The HFS ECE will greatly improve the diagnostic ability of ECE systems on the HL-2M tokamak.

47 OTHER INSTRUMENTATION↗
DOE OSTI2018

Analysis of the influence of geometrical dimensions and external magnetic field on optical properties of InGaAs/GaAs quantum-dot slow light devices

The effect of the size of quantum dots (QDs) and an external magnetic field on InGaAs/GaAs QD slow light devices based on coherent population oscillations (CPOs) is simulated. Physical parameters of QDs, such as radius and height, are varied around their central mean values, which are assumed to be 8 and 5 nm, respectively. An attempt is made to obtain much better optical characteristics by applying a strong external magnetic field. The methods used to improve the optical properties of a QD slow light device, such as the slow down factor (SDF), bandwidth and exciton binding energy, are discussed. It is shown that a QD slow light device makes it possible to shift the centre frequency in a range of 5 THz and to achieve a slow down factor of 5000. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Mode loss spectra in THz quantum-cascade lasers with gold- and silver-based double metal waveguides

Spectra of the waveguide loss coefficient for THz radiation of a quantum-cascade laser with a gold- and silver-based double metal waveguide (DMW) are calculated based on the measurements of resistivity of the metals for different temperatures. It is shown that, taking into account the absorption of THz radiation by free carriers and optical phonons, the spectrum of total mode losses has a broad minimum in the range of 3 – 6 THz, which shifts to higher frequencies with increasing temperature. The minimum losses in the Au-based waveguide increase from 8 to 27 cm{sup –1} as temperature increases from 100 to 300 K. The use of an Ag-based DMW makes it possible to decrease the losses by 2 – 4 cm{sup –1} compared to the Au-based DMW. The bibliography includes 21 references. (terahertz radiation)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

A MULTIWAVELENGTH STUDY OF CYGNUS X-1: THE FIRST MID-INFRARED SPECTROSCOPIC DETECTION OF COMPACT JETS

We report on a Spitzer/InfraRed Spectrograph (mid-infrared), RXTE/PCA+HEXTE (X-ray), and Ryle (radio) simultaneous multiwavelength study of the microquasar Cygnus X-1, which aimed at an investigation of the origin of its mid-infrared emission. Compact jets were present in two out of three observations, and we show that they strongly contribute to the mid-infrared continuum. During the first observation, we detect the spectral break-where the transition from the optically thick to the optically thin regime takes place-at about 2.9 x 10{sup 13} Hz. We then show that the jet's optically thin synchrotron emission accounts for Cygnus X-1's emission beyond 400 keV, although it cannot alone explain its 3-200 keV continuum. A compact jet was also present during the second observation, but we do not detect the break, since it has likely shifted to higher frequencies. In contrast, the compact jet was absent during the last observation, and we show that the 5-30 {mu}m mid-infrared continuum of Cygnus X-1 stems from the blue supergiant companion star HD 226868. Indeed, the emission can then be understood as the combination of the photospheric Rayleigh-Jeans tail and the bremsstrahlung from the expanding stellar wind. Moreover, the stellar wind is found to be clumpy, with a filling factor f{sub {infinity}} {approx} 0.09-0.10. Its bremsstrahlung emission is likely anti-correlated to the soft X-ray emission, suggesting an anti-correlation between the mass-loss and mass-accretion rates. Nevertheless, we do not detect any mid-infrared spectroscopic evidence of interaction between the jets and Cygnus X-1's environment and/or the companion star's stellar wind.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Carbonic acid as a reserve of carbon dioxide on icy moons: The formation of carbon dioxide (CO{sub 2}) in a polar environment

Carbon dioxide (CO{sub 2}) has been detected on the surface of several icy moons of Jupiter and Saturn via observation of the ν{sub 3} band with the Near-Infrared Mapping Spectrometer on board the Galileo spacecraft and the Visible-Infrared Mapping Spectrometer on board the Cassini spacecraft. Interestingly, the CO{sub 2} band for several of these moons exhibits a blueshift along with a broader profile than that seen in laboratory studies and other astrophysical environments. As such, numerous attempts have been made in order to clarify this abnormal behavior; however, it currently lacks an acceptable physical or chemical explanation. We present a rather surprising result pertaining to the synthesis of carbon dioxide in a polar environment. Here, carbonic acid was synthesized in a water (H{sub 2}O)-carbon dioxide (CO{sub 2}) (1:5) ice mixture exposed to ionizing radiation in the form of 5 keV electrons. The irradiated ice mixture was then annealed, producing pure carbonic acid which was then subsequently irradiated, recycling water and carbon dioxide. However, the observed carbon dioxide ν{sub 3} band matches almost exactly with that observed on Callisto; subsequent temperature program desorption studies reveal that carbon dioxide synthesized under these conditions remains in solid form until 160 K, i.e., the sublimation temperature of water. Consequently, our results suggest that carbon dioxide on Callisto as well as other icy moons is indeed complexed with water rationalizing the shift in peak frequency, broad profile, and the solid state existence on these relatively warm moons.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

ANTI-CORRELATED TIME LAGS IN THE Z SOURCE GX 5-1: POSSIBLE EVIDENCE FOR A TRUNCATED ACCRETION DISK

We investigate the nature of the inner accretion disk in the neutron star source GX 5-1 by making a detailed study of time lags between X-rays of different energies. Using the cross-correlation analysis, we found anti-correlated hard and soft time lags of the order of a few tens to a few hundred seconds and the corresponding intensity states were mostly the horizontal branch (HB) and upper normal branch. The model independent and dependent spectral analysis showed that during these time lags the structure of the accretion disk significantly varied. Both eastern and western approaches were used to unfold the X-ray continuum and systematic changes were observed in soft and hard spectral components. These changes along with a systematic shift in the frequency of quasi-periodic oscillations (QPOs) made it substantially evident that the geometry of the accretion disk is truncated. Simultaneous energy spectral and power density spectral study shows that the production of the horizontal branch oscillations (HBOs) is closely related to the Comptonizing region rather than the disk component in the accretion disk. We found that as the HBO frequency decreases from the hard apex to upper HB, the disk temperature increases along with an increase in the coronal temperature, which is in sharp contrast with the changes found in black hole binaries where the decrease in the QPO frequency is accompanied by a decrease in the disk temperature and a simultaneous increase in the coronal temperature. We discuss the results in the context of re-condensation of coronal material in the inner region of the disk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Electron–phonon coupling enhancement and displacive magnetostructural transition in SrCr 2 As 2 under magneto-Raman spectroscopy

Here, a Raman spectroscopy study on high quality single crystals of SrCr 2 A s 2 (SCA) in the temperature T range 4 K < T < 300 K and high applied magnetic fields up to H = 9 T is presented. The chromium B 1g phonon analysis reveals two anomalous shifts in the frequency, the first below T = 250 K at H = 0 T in the saturated AFM G -type order likely due to an enhanced electron–phonon coupling by the magnetic order, whereas the second anomaly occurs above H = 4 T at T = 4 K likely as a consequence of a magnetostructural displacive transition. Renormalization of the electronic Raman spectra in both studies reveals a decrease in the electronic density of states with decreasing T and increasing H , respectively, with consequent changes in the Fermi surface, which are intrinsically related to the observed anomalies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Laser Stripping of Hydrogen Atoms by Direct Ionization

Direct ionization of hydrogen atoms by laser irradiation is investigated as a potential new scheme to generate proton beams without stripping foils. The time-dependent Schrödinger equation describing the atom-radiation interaction is numerically solved obtaining accurate ionization cross-sections for a broad range of laser wavelengths, durations and energies. Parameters are identified where the Doppler frequency up-shift of radiation colliding with relativistic particles can lead to efficient ionization over large volumes and broad bandwidths using currently available lasers.

43 PARTICLE ACCELERATORS↗
DOE OSTI2013

Cosmological parameter estimation: impact of CMB aberration

The peculiar motion of an observer with respect to the CMB rest frame induces an apparent deflection of the observed CMB photons, i.e. aberration, and a shift in their frequency, i.e. Doppler effect. Both effects distort the temperature multipoles a{sub lm}'s via a mixing matrix at any l. The common lore when performing a CMB based cosmological parameter estimation is to consider that Doppler affects only the l = 1 multipole, and neglect any other corrections. In this paper we reconsider the validity of this assumption, showing that it is actually not robust when sky cuts are included to model CMB foreground contaminations. Assuming a simple fiducial cosmological model with five parameters, we simulated CMB temperature maps of the sky in a WMAP-like and in a Planck-like experiment and added aberration and Doppler effects to the maps. We then analyzed with a MCMC in a Bayesian framework the maps with and without aberration and Doppler effects in order to assess the ability of reconstructing the parameters of the fiducial model. We find that, depending on the specific realization of the simulated data, the parameters can be biased up to one standard deviation for WMAP and almost two standard deviations for Planck. Therefore we conclude that in general it is not a solid assumption to neglect aberration in a CMB based cosmological parameter estimation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

The gravitational waves from the first-order phase transition with a dimension-six operator

We investigate in details the gravitational wave (GW) from the first-order phase transition (PT) in the extended standard model of particle physics with a dimension-six operator, which is capable of exhibiting the recently discovered slow first-order PT in addition to the usually studied fast first-order PT. To simplify the discussion, it is sufficient to work with an example of a toy model with the sextic term, and we propose an unified description for both slow and fast first-order PTs. We next study the full one-loop effective potential of the model with fixed/running renormalization-group (RG) scales. Compared to the prediction of GW energy density spectrum from the fixed RG scale, we find that the presence of running RG scale could amplify the peak amplitude by amount of one order of magnitude while shift the peak frequency to the lower frequency regime, and the promising regime of detection within the sensitivity ranges of various space-based GW detectors shrinks down to a lower cut-off value of the sextic term rather than the previous expectation.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Nonlinear Bose-Einstein-condensate dynamics induced by a harmonic modulation of the s-wave scattering length

In a recent experiment, a Bose-Einstein condensate of {sup 7}Li has been excited by a harmonic modulation of the atomic s-wave scattering length via Feshbach resonance. By combining an analytical perturbative approach with extensive numerical simulations, we analyze the emerging nonlinear dynamics of the system on the mean-field Gross-Pitaevskii level at zero temperature. Resulting excitation spectra are presented and prominent nonlinear features are found: mode coupling, higher harmonics generation, and significant shifts in the frequencies of collective modes. We indicate how nonlinear dynamical properties could be made clearly observable in future experiments and compared to our results.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

Blackbody radiation shift, multipole polarizabilities, oscillator strengths, lifetimes, hyperfine constants, and excitation energies in Hg{sup +}

Excitation energies of the [Xe]4f{sup 14}5d{sup 10}ns, [Xe]4f{sup 14}5d{sup 10}np{sub j}, [Xe]4f{sup 14}5d{sup 10}nd{sub j}, [Xe]4f{sup 14}5d{sup 10}n{sup '}f{sub j}, and [Xe]4f{sup 14}5d{sup 10}n{sup '}g{sub j} states in Hg{sup +} are evaluated (n{<=}10, n{sup '}{<=}9, and [Xe]=1s{sup 2}2s{sup 2}2p{sup 6}3s{sup 2}3p{sup 6}3d{sup 10}4s{sup 2}4p{sup 6}4d{sup 10}5s{sup 2}5p{sup 6}). First-, second-, third-, and all-order Coulomb energies and first- and second-order Coulomb-Breit energies are calculated. Reduced matrix elements, oscillator strengths, and transition rates are determined for electric-dipole transitions, including the ns (n=6-11), np (n=6-10), nd (n=6-10), nf (n=5-9), and ng (n=5-9) states. Lifetime values are determined for all above-mentioned states. The ground state E1, E2, and E3 polarizabilities are evaluated. The hyperfine structure in {sup 199}Hg{sup +} and {sup 201}Hg{sup +} ions is investigated. The hyperfine A and B values are determined for the first low-lying levels up to n = 7. The quadratic Stark effect on hyperfine structure levels of {sup 199}Hg{sup +} and {sup 201}Hg{sup +} ground states is investigated. The calculated shift for the {sup 199}Hg{sup +} (F = 1, M = 0) {r_reversible} (F = 0, M = 0) transition is -0.0597(2) Hz/(kV/cm){sup 2}, in agreement with previous theoretical result -0.060(3) Hz/(kV/cm){sup 2}. These calculations provide a theoretical benchmark for comparison with experiment and theory and provide values of blackbody radiation shifts for microwave frequency standards with {sup 199}Hg{sup +} and {sup 201}Hg{sup +} ions.

74 ATOMIC AND MOLECULAR PHYSICS↗