623 research outputs found
Superconductivity and Stoichiometry in the BSCCO-family Materials
We report on magnetization, c-axis and ab-plane resistivity, critical
current, electronic band structure and superconducting gap properties. Bulk
measurements and photoemission data were taken on similar samples.Comment: 4 pages, latex, to be published in Journal of Superconductivity. two
figures available from Jian Ma at [email protected]
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Direct retrieval of isoprene from satellite-based infrared measurements.
Isoprene is the atmosphere's most important non-methane organic compound, with key impacts on atmospheric oxidation, ozone, and organic aerosols. In-situ isoprene measurements are sparse, and satellite-based constraints have employed an indirect approach using its oxidation product formaldehyde, which is affected by non-isoprene sources plus uncertainty and spatial smearing in the isoprene-formaldehyde relationship. Direct global isoprene measurements are therefore needed to better understand its sources, sinks, and atmospheric impacts. Here we show that the isoprene spectral signatures are detectable from space using the satellite-borne Cross-track Infrared Sounder (CrIS), develop a full-physics retrieval methodology for quantifying isoprene abundances from these spectral features, and apply the algorithm to CrIS measurements over Amazonia. The results are consistent with model output and in-situ data, and establish the feasibility of direct global space-based isoprene measurements. Finally, we demonstrate the potential for combining space-based measurements of isoprene and formaldehyde to constrain atmospheric oxidation over isoprene source regions
Critical Reflectance Derived from MODIS: Application for the Retrieval of Aerosol Absorption over Desert Regions
Aerosols are tiny suspended particles in the atmosphere that scatter and absorb sunlight. Smoke particles are aerosols, as are sea salt, particulate pollution and airborne dust. When you look down at the earth from space sometimes you can see vast palls of whitish smoke or brownish dust being transported by winds. The reason that you can see these aerosols is because they are reflecting incoming sunlight back to the view in space. The reason for the difference in color between the different types of aerosol is that the particles arc also absorbing sunlight at different wavelengths. Dust appears brownish or reddish because it absorbs light in the blue wavelengths and scatters more reddish light to space, Knowing how much light is scattered versus how much is absorbed, and knowin~ that as a function of wavelength is essential to being able to quantify the role aerosols play in the energy balance of the earth and in climate change. It is not easy measuring the absorption properties of aerosols when they are suspended in the atmosphere. People have been doing this one substance at a time in the laboratory, but substances mix when they are in the atmosphere and the net absorption effect of all the particles in a column of air is a goal of remote sensing that has not yet been completely successful. In this paper we use a technique based on observing the point at which aerosols change from brightening the surface beneath to darkening it. If aerosols brighten a surface. they must scatter more light to space. If they darken the surface. they must be absorbing more. That cross over point is called the critical reflectance and in this paper we show that critical reflectance is a monotonic function of the intrinsic absorption properties of the particles. This parameter we call the single scattering albedo. We apply the technique to MODIS imagery over the Sahara and Sahel regions to retrieve the single scattering albedo in seven wavelengths, compare these retrievals to ground-based retrievals from AERONET instruments and compute error bars on each retrieval. The results show that we can retrieve single scattering albedo for pure dust to within +/-0.02 and mixtures of dust and smoke to within +/-0.03. No other space based instrument has achieved a retrieval of single scattering albedo that spans the spectrum from 0.47 microns to 2.13 microns and produces regional maps of aerosol absorption showing gradients and changes. Applied in a more operational fashion, such information will narrow uncertainties in estimating aerosol forcing on climate
and Oxygen Stoichiometry: Structure, Resistivity, Fermi Surface Topology and Normal State Properties
(2212) single crystal samples
were studied using transmission electron microscopy (TEM), plane
() and axis () resistivity, and high resolution
angle-resolved ultraviolet photoemission spectroscopy (ARUPS). TEM reveals that
the modulation in the axis for doped 2212 is dominantly
of type that is not sensitive to the oxygen content of the system, and the
system clearly shows a structure of orthorhombic symmetry. Oxygen annealed
samples exhibit a much lower axis resistivity and a resistivity minimum at
K. He-annealed samples exhibit a much higher axis resistivity and
behavior below 300K. The Fermi surface (FS) of oxygen annealed
2212 mapped out by ARUPS has a pocket in the FS around the
point and exhibits orthorhombic symmetry. There are flat, parallel sections of
the FS, about 60\% of the maximum possible along , and about 30\%
along . The wavevectors connecting the flat sections are about
along , and about along , rather than . The symmetry of the near-Fermi-energy dispersing
states in the normal state changes between oxygen-annealed and He-annealed
samples.Comment: APS_REVTEX 3.0, 49 pages, including 11 figures, available upon
request. Submitted to Phys. Rev. B
Critical Josephson Current in a Model Pb/YBa_2Cu_3O_7 Junction
In this article we consider a simple model for a c--axis
Pb/YBa_2Cu_3O_{7-\delta} Josephson junction. The observation of a nonzero
current in such a junction by Sun et al. [A. G. Sun, D. A. Gajewski, M. B.
Maple, R. C. Dynes, Phys. Rev. Lett. 72, 2267 (1994)] has been taken as
evidence against d--wave superconductivity in YBa_2Cu_3O_{7-\delta}. We
suggest, however, that the pairing interaction in the CuO_2 planes may well be
d--wave but that the CuO chains destroy the tetragonal symmetry of the system.
We examine two ways in which this happens. In a simple model of an incoherent
junction, the chains distort the superconducting condensate away from
d_{x^2-y^2} symmetry. In a specular junction the chains destroy the tetragonal
symmetry of the tunneling matrix element. In either case, the loss of
tetragonal symmetry results in a finite Josephson current. Our calculated
values of the critical current for specular junctions are in good agreement
with the results of Sun and co-workers.Comment: Latex File, 21 pages, 6 figures in uuencoded postscript, In Press
(Phys. Rev. B
Direct measurement of stellar angular diameters by the VERITAS Cherenkov Telescopes
The angular size of a star is a critical factor in determining its basic
properties. Direct measurement of stellar angular diameters is difficult: at
interstellar distances stars are generally too small to resolve by any
individual imaging telescope. This fundamental limitation can be overcome by
studying the diffraction pattern in the shadow cast when an asteroid occults a
star, but only when the photometric uncertainty is smaller than the noise added
by atmospheric scintillation. Atmospheric Cherenkov telescopes used for
particle astrophysics observations have not generally been exploited for
optical astronomy due to the modest optical quality of the mirror surface.
However, their large mirror area makes them well suited for such
high-time-resolution precision photometry measurements. Here we report two
occultations of stars observed by the VERITAS Cherenkov telescopes with
millisecond sampling, from which we are able to provide a direct measurement of
the occulted stars' angular diameter at the milliarcsecond scale.
This is a resolution never achieved before with optical measurements and
represents an order of magnitude improvement over the equivalent lunar
occultation method. We compare the resulting stellar radius with empirically
derived estimates from temperature and brightness measurements, confirming the
latter can be biased for stars with ambiguous stellar classifications.Comment: Accepted for publication in Nature Astronom
Probing Supergravity Models with Indirect Experimental Signatures
We explore the one-loop electroweak radiative corrections in the context of
the traditional minimal and the string-inspired
supergravity models by calculating explicitly vacuum-polarization and
vertex-correction contributions to the and
parameters. We also include in this analysis the constraint from whose inclusive branching ratio has been
actually measured very recently by CLEO. We find that by combining these three
most important indirect experimental signatures and using the most recent
experimental values for them, is excluded for
in both the minimal supergravity and the no-scale supergravity. We also find that is
excluded for any sign of in the minimal () supergravity
model.Comment: RevTeX 3.0, 16 Pages+4 figures(not included but available as a
uuencoded file from [email protected]), SNUTP-94-9
Measurement of Cosmic-ray Electrons at TeV Energies by VERITAS
Cosmic-ray electrons and positrons (CREs) at GeV-TeV energies are a unique
probe of our local Galactic neighborhood. CREs lose energy rapidly via
synchrotron radiation and inverse-Compton scattering processes while
propagating within the Galaxy and these losses limit their propagation
distance. For electrons with TeV energies, the limit is on the order of a
kiloparsec. Within that distance there are only a few known astrophysical
objects capable of accelerating electrons to such high energies. It is also
possible that the CREs are the products of the annihilation or decay of heavy
dark matter (DM) particles. VERITAS, an array of imaging air Cherenkov
telescopes in southern Arizona, USA, is primarily utilized for gamma-ray
astronomy, but also simultaneously collects CREs during all observations. We
describe our methods of identifying CREs in VERITAS data and present an energy
spectrum, extending from 300 GeV to 5 TeV, obtained from approximately 300
hours of observations. A single power-law fit is ruled out in VERITAS data. We
find that the spectrum of CREs is consistent with a broken power law, with a
break energy at 710 40 140 GeV.Comment: 17 pages, 2 figures, accepted for publication in PR
Neutralino relic density in a Universe with a non-vanishing cosmological constant
We discuss the relic density of the lightest of the supersymmetric particles
in view of new cosmological data, which favour the concept of an accelerating
Universe with a non-vanishing cosmological constant. Recent astrophysical
observations provide us with very precise values of the relevant cosmological
parameters. Certain of these parameters have direct implications on particle
physics, e.g., the value of matter density, which in conjunction with
electroweak precision data put severe constraints on the supersymmetry breaking
scale. In the context of the Constrained Minimal Supersymmetric Standard Model
(CMSSM) such limits read as: M_{1/2} \simeq 300 \GeV - 340 \GeV, m_0 \simeq
80 \GeV - 130 \GeV. Within the context of the CMSSM a way to avoid these
constraints is either to go to the large and region, or
make , the next to lightest supersymmetric particle (LSP), be
almost degenerate in mass with LSP.Comment: REVTeX, 50 pages, 35 eps figures; Minor changes, references and a
figure added; Better quality figures can be obtained upon request from
[email protected]
The Main Belt Comets and ice in the Solar System
We review the evidence for buried ice in the asteroid belt; specifically the questions around the so-called Main Belt Comets (MBCs). We summarise the evidence for water throughout the Solar System, and describe the various methods for detecting it, including remote sensing from ultraviolet to radio wavelengths. We review progress in the first decade of study of MBCs, including observations, modelling of ice survival, and discussion on their origins. We then look at which methods will likely be most effective for further progress, including the key challenge of direct detection of (escaping) water in these bodies
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