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Young, A. T.

Publications and source records attributed to Young, A. T..

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Hartmann characterization of the PEEM-3 aberration-corrected X-ray photoemission electron microscope

Aberration correction by an electron mirror dramatically improves the spatial resolution and transmission of photoemission electron microscopes. We will review the performance of the recently installed aberration corrector of the X-ray Photoemission Electron Microscope PEEM-3 and show a large improvement in the efficiency of the electron optics. Hartmann testing is introduced as a quantitative method to measure the geometrical aberrations of a cathode lens electron microscope. Here, we find that aberration correction leads to an order of magnitude reduction of the spherical aberrations, suggesting that a spatial resolution of below 100 nm is possible at 100% transmission of the optics when using x-rays. We demonstrate this improved performance by imaging test patterns employing element and magnetic contrast.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Ni and CoO spin cantings induced by Fe layer in Ni/CoO/Fe/vicinal MgO(001)

Using element-resolved x-ray magnetic circular dichroism and x-ray magnetic linear dichroism measurements, we studied Ni/CoO/vicinal MgO(001) and Ni/CoO/Fe/vicinal MgO(001) systems at 350 and 78 K. Above the CoO Néel temperature, the Ni magnetization is fully in plane and parallel to the atomic steps in both systems due to step-induced magnetic anisotropy. Below the CoO Néel temperature, the CoO spins in Ni/CoO/vicinal MgO(001) are fully in plane and parallel to the atomic steps and the Ni magnetization is fully in plane and perpendicular to the atomic steps due to the 90° Ni/CoO magnetic coupling. The CoO spins in Ni/CoO/Fe/vicinal MgO(001), however, develop an out-of-plane canted spin component in addition to the in-plane component parallel to the atomic steps. Consequently, the Ni magnetization is canted towards the out-of-plane direction by an appreciable angle. Finally, photoemission electron microscopy imaging shows a 90° interfacial magnetic coupling at both the Ni/CoO and the CoO/Fe interfaces and an absence of a direct interlayer coupling, showing that the Ni spin canting is due to its coupling to the canted CoO spin components which is caused by the underlying ferromagnetic Fe layer in Ni/CoO/Fe/vicinal MgO(001).

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Electrical switching of the magnetic vortex circulation in artificial multiferroic structure of Co/Cu/PMN-PT(011)

For this research, Co films and micron sized disks were grown on top of piezoelectric PMN-PT(011) and Cu/PMN-PT(001) substrates and investigated by the Magneto-Optic Kerr Effect and Photoemission Electron Microscopy. By applying an electric field in the surface normal direction, we find that the strain of the ferroelectric PMN-PT(011) substrate induces an in-plane uniaxial magnetic anisotropy in the Co overlayer. Under specific conditions, the Co magnetic vortex could be switched between clockwise and counter-clockwise circulations. The variations of the Co vortex switching were attributed to the variations of the ferroelectric domains under the Co disks. Lastly, we speculate that the switching of the magnetic vortex circulation is a dynamical process which may involve pulses of appropriate magnitude and duration of the uniaxial magnetic anisotropy delivered to the magnetic vortex.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Correlation-driven insulator-metal transition in near-ideal vanadium dioxide films

We use polarization- and temperature-dependent x-ray absorption spectroscopy, in combination with photoelectron microscopy, x-ray diffraction, and electronic transport measurements, to study the driving force behind the insulator-metal transition in VO 2 . We show that both the collapse of the insulating gap and the concomitant change in crystal symmetry in homogeneously strained single-crystalline VO 2 films are preceded by the purely electronic softening of Coulomb correlations within V-V singlet dimers. Furthermore, this process starts 7 K (±0.3 K) below the transition temperature, as conventionally defined by electronic transport and x-ray diffraction measurements, and sets the energy scale for driving the near-room-temperature insulator-metal transition in this technologically promising material.

36 MATERIALS SCIENCE↗
NASA NTRS1989

Sulfur, ultraviolet radiation, and the early evolution of life

The present biosphere is shielded from harmful solar near ultraviolet (UV) radiation by atmospheric ozone. It is suggested that elemental sulfur vapor could have played a similar role in an anoxic, ozone-free, primitive atmosphere. Sulfur vapor would have been produced photochemically from volcanogenic SO2 and H2S. It is composed of ring molecules, primarily S8, that absorb strongly throughout the near UV, yet are expected to be relatively stable against photolysis and chemical attack. It is also insoluble in water and would thus have been immune to rainout or surface deposition over the oceans. Since the concentration of S8 in the primitive atmosphere would have been limited by its saturation vapor pressure, surface temperatures of 45 C or higher, corresponding to carbon dioxide partial pressures exceeding 2 bars, are required to sustain an effective UV screen. A warm, sulfur-rich, primitive atmosphere is consistent with inferences drawn from molecular phylogeny, which suggest that some of the earliest organisms were thermophilic bacteria that metabolized elemental sulfur.

Kasting, J. F.↗
NASA NTRS1988

A photometric approach to detecting earth-sized planets

The photometric method for searching for planets around stars depends on the observation of the decrease in light flux produced by the transit of a planet across the stellar disk. It is shown that a three-year observation period should permit meaningful statements to be made about the frequency of solar-type planetary systems. A state-of-the-art photometer is being developed to test components and concepts. The goal is the development of a photometer that can routinely measure the relative brightness of stars to a precision of 1 part in 100,000.

Borucki, W. J.↗
NASA NTRS1986

On the colors of Jupiter's satellite Io - Irradiation of solid sulfur at 77 K

The colors of seven sulfur allotropes at 77 K have been investigated both before and after irradiation by a high-pressure mercury lamp. S8 is white at 77 K but turns intense yellow within a few minutes on irradiation through quartz or DURAN. On warming, the yellow species (presumably chain-like sulfur diradicals characterized by an absorption at 430 nm) decomposes near 260 K and polymeric sulfur (S-infinity) is formed; the sample color changes gradually to the very similar color of an S8-S-infinity mixture, resulting in no visually detectable color change with temperatures from 77 K to 260 K. This is consistent with the lack of posteclipse brightening on Io. Other sulfur forms also turn intense yellow (S7, S10), grayish yellow (S12, S20, S-infinity), or brownish yellow (S6) on irradiation at 77 K, and these colors also change to the normal colors of the starting materials on warming to 298 K. It is suggested that the yellowish features on Io are caused by the effect of solar radiation on elemental sulfur. The yellow color thus produced may subsequently be modified by deposits of SO2 frosts and volcanic ash.

Steudel, R.↗
NASA NTRS1985

What color is the solar system?

Estimates are made of the true visual colors of various planets and moons in the solar system. Account is taken of the components of perceived color, i.e., hue, saturation and lightness. Earth is a blue planet while most of the others, including Mars, are yellow and differ only in their lightness. Widely disseminated Voyager images of Jupiter have been computer-enhanced to highlight details. A reflectance spectrum established by the International Commission on Illumination provides reference lines which are measured and compared with reference colors which would be seen on earth in normal daylight. Uranus is actually a light aqua color due to its absorption of 6190 A light and reflectance of 5000 A blue green light.

Young, A. T.↗
NASA NTRS1984

Proceedings of the Workshop on Improvements to Photometry

The purposes of the workshop were to determine what astronomical problems would benefit by increased photometric precision, determine the current level of precision, identify the processes limiting the precision, and recommend approaches to improving photometric precision. Twenty representatives of the university, industry, and government communities participated. Results and recommendations are discussed.

Borucki, W. J.↗
NASA NTRS1984

History of Photometric Precision

A history of photometric precision is given. The fixing of the scale of stellar magnitudes, progress toward precision and accuracy, and the use of photography are discussed.

Young, A. T.↗
NASA NTRS1984

Filter and Passband Problems

Problems associated with achieving precision in photometric measurements of stars are examined. The thermal stabilization of glass and interference filters and the determination of correct analytic representations of bandwidth effects in data reduction are particularly discussed. Spectral sampling requirements are also addressed.

Young, A. T.↗
NASA NTRS1984

A new interpretation of the Venera 11 spectra of Venus. I - The 0.94 micron water band

Detailed band models for CO2 and H2O absorption are used to reinterpret Venera 11 spectra of the transmitted sunlight at several levels in the Venus atmosphere. An effective path approximation is used to allow for scattering in the clouds. The atmospheric model has 10 layers and uses 211 CO2 and 15 H2O vibrational transitions at 5/cm resolution. The conclusion of Moroz et al. (1979) that the 0.94 micron feature of the spectra indicates a sharp maximum in the water-vapor profile near 50 km is confirmed. It is also confirmed that such a profile fails to account for the spectra in the 1.13 micron water bands.

Young, L. D. G.↗
NASA NTRS1984

No sulfur flows on Io

Physical and chemical properties of elemental sulfur are incompatible with the suggestion that the colored flows associated with volcanoes on Io are quenched unstable allotropes of sulfur. Either the volcanic flows are not sulfur, or some mechanism other than quenching is required to produce colored forms of sulfur in them. The properties of sulfur are unsuited to the production and survival of colored unstable allotropes on Io. The color of this object is probably due to some other material, possibly iron compounds.

Young, A. T.↗
NASA NTRS1983

Venus cloud microphysics

Because sulfuric acid does not wet sulfur, composite drops in the atmosphere of Venus cannot have sulfur 'cores', but must instead have sulfur coats. Both components then communicate with the vapor phase. Drops that are fully coated with sulfur are immune to coalescence; this sets a limit to growth that may explain 'Mode 3' particles. The sulfur coating is probably responsible for the anomalously low refractive indices derived from entry-probe nephelometer data. There appears to be about an order of magnitude less elemental sulfur than sulfuric acid in the clouds.

Young, A. T.↗
NASA NTRS1982

Generation of a strong core centering force in a submillimeter compound droplet system

By amplitude-modulating the driving voltage of an acoustic levitating apparatus, a strong core centering force was generated in a submillimeter compound droplet system suspended by the radiation pressure in a gaseous medium. Depending on the acoustic characteristics of the droplet system, it was found that the technique can be utilized advantageously in the multiple-layer coating of an inertial confinement fusion pellet.

Lee, M. C.↗
NASA NTRS1982

Polymer coating of glass microballoons levitated in a focused acoustic field

Inertial confinement fusion (ICF) glass microballoons (GMBs) levitated in a focusing radiator acoustic device can be coated with liquid materials by deploying the liquid into the levitation field with a stepped-horn atomizer. The GMB can be forced to the center of the coating liquid with a strong acoustically generated centering force. Water solutions of organic polymers, UV-curable liquid organic monomers, and paraffin waxes have been used to prepare solid coatings on the surface of GMBs using this technique.

Young, A. T.↗
NASA NTRS1981

On the Rayleigh-scattering optical depth of the atmosphere

It is pointed out that revised Rayleigh-scattering optical depths published by Hoyt (1977) are not correct, because King's (1923) formula, which was used by Hoyt, requires that the rotational lines be included in the polarization measurement. To exclude them is to exclude the contribution of the Raman-shifted photons to the total molecular extinction. Hoyt's error was repeated by Froehlich and Shaw (1980), who also confused the depolarization for natural light with that for (polarized) laser light. In an effort to clarify basic concepts, attention is given to the origin of the depolarization correction, depolarization ratios, vibrational Raman effects, and extinction measurements.

Young, A. T.↗