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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 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Non-uniform absorption of terahertz radiation on superconducting hot electron bolometer microbridges

We interpret the experimental observation of a frequency-dependence of superconducting hot electron bolometer (HEB) mixers by taking into account the non-uniform absorption of the terahertz radiation on the superconducting HEB microbridge. The radiation absorption is assumed to be proportional to the local surface resistance of the HEB microbridge, which is computed using the Mattis-Bardeen theory. With this assumption the dc and mixing characteristics of a superconducting niobium-nitride (NbN) HEB device have been modeled at frequencies below and above the equilibrium gap frequency of the NbN film.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Comments on the optical lineshape function: Application to transient hole-burned spectra of bacterial reaction centers

The vibrational spectral density is an important physical parameter needed to describe both linear and non-linear spectra of multi-chromophore systems such as photosynthetic complexes. Low-temperature techniques such as hole burning (HB) and fluorescence line narrowing are commonly used to extract the spectral density for a given electronic transition from experimental data. We report here that the lineshape function formula reported by Hayes et al. [J. Phys. Chem. 98, 7337 (1994)] in the mean-phonon approximation and frequently applied to analyzing HB data contains inconsistencies in notation, leading to essentially incorrect expressions in cases of moderate and strong electron-phonon (el-ph) coupling strengths. A corrected lineshape function L(ω) is given that retains the computational and intuitive advantages of the expression of Hayes et al. [J. Phys. Chem. 98, 7337 (1994)]. Although the corrected lineshape function could be used in modeling studies of various optical spectra, we suggest that it is better to calculate the lineshape function numerically, without introducing the mean-phonon approximation. New theoretical fits of the P870 and P960 absorption bands and frequency-dependent resonant HB spectra of Rb. sphaeroides and Rps. viridis reaction centers are provided as examples to demonstrate the importance of correct lineshape expressions. Comparison with the previously determined el-ph coupling parameters [Johnson et al., J. Phys. Chem. 94, 5849 (1990); Lyle et al., ibid. 97, 6924 (1993); Reddy et al., ibid. 97, 6934 (1993)] is also provided. The new fits lead to modified el-ph coupling strengths and different frequencies of the special pair marker mode, ω{sub sp}, for Rb. sphaeroides that could be used in the future for more advanced calculations of absorption and HB spectra obtained for various bacterial reaction centers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Impedance characteristics and PTCR effect in lead free BaTi1-xSnxO3 piezoceramics

Highlights: • The maximum d{sub 33} = 296 pC/N and P{sub r} = 6.92 μC/cm{sup 2} is observed for x = 0.08. • Nyquist plots reveal non-Debye type relaxation behavior of the samples. • Impedance studies indicate a possible correlation between d{sub 33} and microstructure. • Positive temperature coefficient of resistance (PTCR) is seen just above T{sub C}. - Abstract: BaTi{sub 1-x}Sn{sub x}O{sub 3} of compositions x = 0.07, 0.08, 0.09, 0.10 and 0.11 are synthesized to study their piezoelectric and impedance characteristics. The studies revealed that the composition x = 0.08 exhibits the maximum piezoelectric coefficient (d{sub 33} = 296 pC/N) and remnant polarization (P{sub r} = 6.92 μC/cm{sup 2}) values. The impedance analysis showed that the depressed semi-circular arc observed in the Nyquist’s plots gave evidence for the existence of non-Debye type relaxation behavior in the compounds. The impedance studies suggested a possible correlation between the d{sub 33} and microstructural properties. The activation energies extracted from ac conductivity plots strongly indicate the existence of defect dipole assisted conductivity mechanism in the samples. Notably, the samples displayed positive temperature coefficient of resistance (PTCR) just above their ferroelectric transition. The frequency dependent ac conductivity follows the Jonscher’s universal power law. The dominant conduction mechanism in the samples is determined to be the small polaron hopping mechanism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Determination of the Fermi level position in dilute magnetic Ga{sub 1-x}Mn{sub x}N films

We report on a combined theoretical and experimental determination of the Fermi level position in wurtzite Ga{sub 1−x}Mn{sub x}N films with x=4% and x=10% as grown by molecular beam epitaxy. By means of ellipsometric measurements, the real part of the frequency-dependent conductivity is determined. An electronic model in the framework of the effective bond-orbital model is parameterized in order to theoretically reproduce the measured transport properties. Predictions for the long-wavelength behaviour as a function of the Fermi level are made. The corresponding density of states obtained in this model is in qualitative agreement with first-principle calculations. The absence of a significant experimental peak in the AC conductivity for small frequencies indicates that the Fermi level lies in a gap between two Mn-related impurity bands in the host band gap.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Effect of Co doping on structural, optical, magnetic and dielectric properties of Bi{sub 2}Fe{sub 4}O{sub 9}

Polycrystalline Bi{sub 2}Fe{sub 4}O{sub 9} and 2% Co doped Bi{sub 2}Fe{sub 4}O{sub 9} were prepared by solid state reaction route. X-ray diffraction (XRD) result reveals that there is no change in the crystal structure due to Co doping and the compound has orthorhombic structure. UV-visible spectroscopy confirms the decrease in band gap due Co doping. Zero field cooled magnetization measurement at 100 Oe magnetic field shows substantial decrease in the magnetic transition temperature. Room temperature frequency dependent dielectric permittivity at 1V DC bias shows ∼10% increase in Co doped sample with respect to pure Bi{sub 2}Fe{sub 4}O{sub 9}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Synchrotron emission from dark matter in galactic subhalos. A look into the Smith cloud

One of the key predictions of the ''WIMP'' paradigm for Dark Matter (DM) is that DM particles can annihilate into charged particles. These annihilations will proceed in e.g. Galactic subhalos such as dwarf Galaxies or, as recently pointed out, high velocity clouds such as the ''Smith Cloud''. In this note, we focus on the radio emission associated with DM annihilations into electrons and positrons occurring in the Smith Cloud. The phenomenology of this emission is discussed in quite some detail. We argue that the uncertainties in the propagation can be captured by the typical diffusion-loss length parameter (Syrovatskii variable) but that the angle-integrated radio fluxes are independent of the propagation. We conclude that if the Smith Cloud is indeed dominated by DM, radio signals from DM annihilation stand out amongst other messengers. Furthermore, low frequencies such as the ones observed by e.g. the Low Frequency Array (LOFAR) and the next-generation Square Kilometre Array (SKA) are optimal for searches for DM in the Smith Cloud. As a practical application, we set conservative constraints on dark matter annihilation cross section using data of continuum radio emission from the Galaxy at 22 MHz and at 1.4 GHz. Stronger constraints could be reached by background subtraction, exploiting the profile and frequency dependence of the putative DM signal. We set stronger but tentative limits using the median noise in brightness temperature from the Green Bank Telescope and the LOFAR sensitivities.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

ALMA SCIENCE VERIFICATION DATA: MILLIMETER CONTINUUM POLARIMETRY OF THE BRIGHT RADIO QUASAR 3C 286

We present full-polarization observations of the compact, steep-spectrum radio quasar 3C 286 made with the Atacama Large Millimeter and Submillimeter Array (ALMA) at 1.3 mm. These are the first full-polarization ALMA observations, which were obtained in the framework of Science Verification. A bright core and a south–west component are detected in the total intensity image, similar to previous centimeter images. Polarized emission is also detected toward both components. The fractional polarization of the core is about 17%; this is higher than the fractional polarization at centimeter wavelengths, suggesting that the magnetic field is even more ordered in the millimeter radio core than it is further downstream in the jet. The observed polarization position angle (or electric vector position angle (EVPA)) in the core is ∼39{sup ◦}, which confirms the trend that the EVPA slowly increases from centimeter to millimeter wavelengths. With the aid of multi-frequency VLBI observations, we argue that this EVPA change is associated with the frequency-dependent core position. We also report a serendipitous detection of a sub-mJy source in the field of view, which is likely to be a submillimeter galaxy.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

DETECTION OF FLUX EMERGENCE, SPLITTING, MERGING, AND CANCELLATION OF NETWORK FIELD. I. SPLITTING AND MERGING

Frequencies of magnetic patch processes on the supergranule boundary, namely, flux emergence, splitting, merging, and cancellation, are investigated through automatic detection. We use a set of line-of-sight magnetograms taken by the Solar Optical Telescope (SOT) on board the Hinode satellite. We found 1636 positive patches and 1637 negative patches in the data set, whose time duration is 3.5 hr and field of view is 112'' Multiplication-Sign 112''. The total numbers of magnetic processes are as follows: 493 positive and 482 negative splittings, 536 positive and 535 negative mergings, 86 cancellations, and 3 emergences. The total numbers of emergence and cancellation are significantly smaller than those of splitting and merging. Further, the frequency dependence of the merging and splitting processes on the flux content are investigated. Merging has a weak dependence on the flux content with a power-law index of only 0.28. The timescale for splitting is found to be independent of the parent flux content before splitting, which corresponds to {approx}33 minutes. It is also found that patches split into any flux contents with the same probability. This splitting has a power-law distribution of the flux content with an index of -2 as a time-independent solution. These results support that the frequency distribution of the flux content in the analyzed flux range is rapidly maintained by merging and splitting, namely, surface processes. We suggest a model for frequency distributions of cancellation and emergence based on this idea.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Waveguide Modes in a Planar Graphene–Dielectric Thin-Layer Structure

Particular features of waveguide propagation of modes in a planar structure that consists of alternating layers of a dielectric and graphene are investigated. Within the effective-medium approximation, dispersion relations are obtained for symmetric and antisymmetric waveguide modes. Based on their numerical analysis, the frequency dependences of the propagation and decay constants, of the group and phase velocities, and of the energy flux carried by waveguide modes are constructed. The influence of the fraction of graphene in the structure on the behavior of waveguide modes is analyzed.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Hopping Conductivity in Poly(p-Xylylene)–Fe Nanocomposites

The structure and conductivity of hybrid nanocomposite poly(p-xylylene) films containing iron nanoparticles are studied. The films are obtained by the method of polymerization on a surface from the gas phase, and the concentration of the filler is 4 and 11 vol %. According to the analysis of microphotographs of nanostructures obtained by transmission electron microscopy, the particles are distributed evenly (with a size in the nanometer range and a rather narrow distribution). Studies of the frequency dependence of conductivity show that the transport of the charge carriers is realized by the hopping mechanism with a jump length of ~3.5 nm.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2015

Ab initio calculation of thermodynamic, transport, and optical properties of CH{sub 2} plastics

This work covers an ab initio calculation of thermodynamic, transport, and optical properties of plastics of the effective composition CH{sub 2} at density 0.954 g/cm{sup 3} in the temperature range from 5 kK up to 100 kK. The calculation is based on the quantum molecular dynamics, density functional theory, and the Kubo-Greenwood formula. The temperature dependence of the static electrical conductivity σ{sub 1{sub D{sub C}}}(T) has a step-like shape: σ{sub 1{sub D{sub C}}}(T) grows rapidly for 5 kK ≤ T ≤ 10 kK and is almost constant for 20 kK ≤ T ≤ 60 kK. The additional analysis based on the investigation of the electron density of states (DOS) is performed. The rapid growth of σ{sub 1{sub D{sub C}}}(T) at 5 kK ≤ T ≤ 10 kK is connected with the increase of DOS at the electron energy equal to the chemical potential ϵ = μ. The frequency dependence of the dynamic electrical conductivity σ{sub 1}(ω) at 5 kK has the distinct non-Drude shape with the peak at ω ≈ 10 eV. This behavior of σ{sub 1}(ω) was explained by the dip at the electron DOS.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2018

An Ellipsometric Investigation of the Optical Properties of Ru{sub 2}Ge{sub 3} and Ru{sub 2}Sn{sub 3} Compounds

Ellipsometric investigations of the optical properties of Ru{sub 2}Ge{sub 3} and Ru{sub 2}Sn{sub 3} intermetallic compounds are carried out in the wavelength range from 0.22 to 15 μm. The nature of interband light absorption is analyzed based on a comparative analysis of the experimental and theoretical frequency dependences of an optical conductivity. The obtained results confirm the existence of energy gaps at the Fermi level in the electronic spectra of these materials predicted earlier by the band-structure calculations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Charge carrier dynamics in nanocrystalline Dy substituted ceria based oxygen ion conductors

Nano-crystalline Ce{sub 1-x}Dy{sub x}O{sub 2-δ} (x = 0.1-0.5) materials were prepared using the low temperature citrate auto-ignition method. The Rietveld analysis of the XRD data confirmed the single phase cubic fluorite structure. The particle sizes of the sintered samples are in nano range and lattice parameter increases with Dy concentration. Polydispersed and agglomerated particles are observed by SEM. The EDAX spectra show good stoichiometry of the different atoms in the samples. The conductivity is found to have both grain and grain boundary contribution and shows highest value at x= 0.2. The frequency dependence of dielectric permittivity has been analyzed using Havrilliak-Negami formalism. The variation in different electrical properties has been explained by formation defect associates and their interaction with charge carriers.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Multiferroics properties in BiFeO{sub 3}/CoFe{sub 2}O{sub 4} heterostructures thin films deposited on (111) SrTiO{sub 3}

The multiferroic properties of BiFeO{sub 3}-CoFe{sub 2}O{sub 4} (BFO-CFO) heterostructures thin film grown on (111) SrTiO{sub 3} (STO) substrates by employing pulsed laser deposition (PLD) method, have been studied.X ray diffraction analysis reveals the presence of BFO and CFO in two separate phases. The heterostructures BFO/CFO/STO(111) thin films show well – shaped hysteresis loop with saturated magnetization and coercive field, which are much larger than pure BFO/STO thin film. The frequency dependent dielectric anomalies in BFO-CFO multilayer are attributed to the interfacial effect across the presence of CFO as a separate phase embedded in BFO matrix.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Generation of a Photoacoustic Signal in Two-Layer Transparent Samples on an Absorbing Substrate

A theory of generation of a photoacoustic signal in a two-layer transparent sample on an absorbing substrate to a buffer gas of a photoacoustic cell has been developed. We have shown that the frequency dependence of the photoacoustic signal amplitude excited in the absorbing substrate obeys the law ∝ ω{sup –1}, when a free path of photons is less than the thermal diffusion length, and is proportional to ω{sup –3/2} in the opposite case.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

SETI OBSERVATIONS OF EXOPLANETS WITH THE ALLEN TELESCOPE ARRAY

We report radio SETI observations on a large number of known exoplanets and other nearby star systems using the Allen Telescope Array (ATA). Observations were made over about 19000 hr from 2009 May to 2015 December. This search focused on narrowband radio signals from a set totaling 9293 stars, including 2015 exoplanet stars and Kepler objects of interest and an additional 65 whose planets may be close to their habitable zones. The ATA observations were made using multiple synthesized beams and an anticoincidence filter to help identify terrestrial radio interference. Stars were observed over frequencies from 1 to 9 GHz in multiple bands that avoid strong terrestrial communication frequencies. Data were processed in near-real time for narrowband (0.7–100 Hz) continuous and pulsed signals with transmitter/receiver relative accelerations from −0.3 to 0.3 m s{sup −2}. A total of 1.9 × 10{sup 8} unique signals requiring immediate follow-up were detected in observations covering more than 8 × 10{sup 6} star-MHz. We detected no persistent signals from extraterrestrial technology exceeding our frequency-dependent sensitivity threshold of 180–310 × 10{sup −26} W m{sup −2}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Observation of giant exchange bias in bulk Mn{sub 50}Ni{sub 42}Sn{sub 8} Heusler alloy

We report a giant exchange bias (EB) field of 3520 Oe in bulk Mn{sub 50}Ni{sub 42}Sn{sub 8} Heusler alloy. The low temperature magnetic state of the martensite phase has been studied by DC magnetization and AC susceptibility measurements. Frequency dependence of spin freezing temperature (T{sub f}) on critical slowing down relation and observation of memory effect in zero field cooling mode confirms the super spin glass (SSG) phase at low temperatures. Large EB is attributed to the strong exchange coupling between the SSG clusters formed by small regions of ferromagnetic order embedded in an antiferromagnetic (AFM) matrix. The temperature and cooling field dependence of EB have been studied and related to the change in unidirectional anisotropy at SSG/AFM interface. The training effect also corroborates with the presence of frozen (SSG) moments at the interface and their role in EB.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Dielectric spectroscopy at the nanoscale by atomic force microscopy: A simple model linking materials properties and experimental response

The use of an atomic force microscope for studying molecular dynamics through dielectric spectroscopy with spatial resolution in the nanometer scale is a recently developed approach. However, difficulties in the quantitative connection of the obtained data and the material dielectric properties, namely, frequency dependent dielectric permittivity, have limited its application. In this work, we develop a simple electrical model based on physically meaningful parameters to connect the atomic force microscopy (AFM) based dielectric spectroscopy experimental results with the material dielectric properties. We have tested the accuracy of the model and analyzed the relevance of the forces arising from the electrical interaction with the AFM probe cantilever. In this way, by using this model, it is now possible to obtain quantitative information of the local dielectric material properties in a broad frequency range. Furthermore, it is also possible to determine the experimental setup providing the best sensitivity in the detected signal.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗