14,420 research outputs found
Chemical studies of the passivation of GaAs surface recombination using sulfides and thiols
Steady-state photoluminescence, time-resolved photoluminescence, and x-ray photoelectron spectroscopy have been used to study the electrical and chemical properties of GaAs surfaces exposed to inorganic and organic sulfur donors. Despite a wide variation in S2â(aq) concentration, variation of the pH of aqueous HSâsolutions had a small effect on the steady-state n-type GaAs photoluminescence intensity, with surfaces exposed to pH=8, 0.1-M HSâ(aq) solutions displaying comparable luminescence intensity relative to those treated with pH=14, 1.0-M Na2S·9H2O(aq). Organic thiols (R-SH, where R=âCH2CH2SH or âC6H4Cl) dissolved in nonaqueous solvents were found to effect increases in steady-state luminescence yields and in time-resolved luminescence decay lifetimes of (100)-oriented GaAs. X-ray photoelectron spectroscopy showed that exposure of GaAs surfaces to these organic systems yielded thiols bound to the GaAs surface, but such exposure did not remove excess elemental As and did not form a detectable As2S3 overlayer on the GaAs. These results imply that complete removal of As0 or formation of monolayers of As2S3 is not necessary to effect a reduction in the recombination rate at etched GaAs surfaces. Other compounds that do not contain sulfur but that are strong Lewis bases, such as methoxide ion, also improved the GaAs steady-state photoluminescence intensity. These results demonstrate that a general class of electron-donating reagents can be used to reduce nonradiative recombination at GaAs surfaces, and also imply that prior models focusing on the formation of monolayer coverages of As2S3 and Ga2S3 are not adequate to describe the passivating behavior of this class of reagents. The time-resolved, high level injection experiments clearly demonstrate that a shift in the equilibrium surface Fermi-level energy is not sufficient to explain the luminescence intensity changes, and confirm that HSâ and thiol-based reagents induce substantial reductions in the surface recombination velocity through a change in the GaAs surface state recombination rate
Weak-Field Thermal Hall Conductivity in the Mixed State of d-Wave Superconductors
Thermal transport in the mixed state of a d-wave superconductor is considered
within the weak-field regime. We express the thermal conductivity,
, and the thermal Hall conductivity, , in terms of
the cross section for quasiparticle scattering from a single vortex. Solving
for the cross section (neglecting the Berry phase contribution and the
anisotropy of the gap nodes), we obtain and
in surprisingly good agreement with the qualitative features
of the experimental results for YBaCuO. In particular, we
show that the simple, yet previously unexpected, weak-field behavior,
, is that of thermally-excited nodal
quasiparticles, scattering primarily from impurities, with a small skew
component provided by vortex scattering.Comment: 5 pages, 2 figures; final version as published in Phys Rev Let
A Scattered Light Echo around SN 1993J in M81
A light echo around SN 1993J was observed 8.2 years after explosion by a HST
WFPC2 observation, adding to the small family of supernovae with light echoes.
The light echo was formed by supernova light scattered from a dust sheet, which
lies 220 parsecs away from the supernova, 50 parsecs thick along the line of
sight, as inferred from radius and width of the light echo. The dust inferred
from the light echo surface brightness is 1000 times denser than the intercloud
dust. The graphite to silicate fraction can not be determined by our BVI
photometric measurements, however, a pure graphite model can be excluded based
on comparison with the data. With future observations, it will be possible to
measure the expansion rate of the light echo, from which an independent
distance to M81 can be obtained.Comment: 10 pages, 6 figures, in AASTeX format, submitted to ApJ Part
Constraining the Atmospheric Composition of the Day-Night Terminators of HD 189733b : Atmospheric Retrieval with Aerosols
A number of observations have shown that Rayleigh scattering by aerosols
dominates the transmission spectrum of HD 189733b at wavelengths shortward of 1
m. In this study, we retrieve a range of aerosol distributions consistent
with transmission spectroscopy between 0.3-24 m that were recently
re-analyzed by Pont et al. (2013). To constrain the particle size and the
optical depth of the aerosol layer, we investigate the degeneracies between
aerosol composition, temperature, planetary radius, and molecular abundances
that prevent unique solutions for transit spectroscopy. Assuming that the
aerosol is composed of MgSiO, we suggest that a vertically uniform aerosol
layer over all pressures with a monodisperse particle size smaller than about
0.1 m and an optical depth in the range 0.002-0.02 at 1 m provides
statistically meaningful solutions for the day/night terminator regions of HD
189733b. Generally, we find that a uniform aerosol layer provide adequate fits
to the data if the optical depth is less than 0.1 and the particle size is
smaller than 0.1 m, irrespective of the atmospheric temperature, planetary
radius, aerosol composition, and gaseous molecules. Strong constraints on the
aerosol properties are provided by spectra at wavelengths shortward of 1 m
as well as longward of 8 m, if the aerosol material has absorption
features in this region. We show that these are the optimal wavelengths for
quantifying the effects of aerosols, which may guide the design of future space
observations. The present investigation indicates that the current data offer
sufficient information to constrain some of the aerosol properties of
HD189733b, but the chemistry in the terminator regions remains uncertain.Comment: Transferred to ApJ and accepted. 11 pages, 10 figures, 1 tabl
Seasonal Variability of Saturn's Tropospheric Temperatures, Winds and Para-H from Cassini Far-IR Spectroscopy
Far-IR 16-1000 m spectra of Saturn's hydrogen-helium continuum measured
by Cassini's Composite Infrared Spectrometer (CIRS) are inverted to construct a
near-continuous record of upper tropospheric (70-700 mbar) temperatures and
para-H fraction as a function of latitude, pressure and time for a third of
a Saturnian year (2004-2014, from northern winter to northern spring). The
thermal field reveals evidence of reversing summertime asymmetries superimposed
onto the belt/zone structure. The temperature structure that is almost
symmetric about the equator by 2014, with seasonal lag times that increase with
depth and are qualitatively consistent with radiative climate models. Localised
heating of the tropospheric hazes (100-250 mbar) create a distinct perturbation
to the temperature profile that shifts in magnitude and location, declining in
the autumn hemisphere and growing in the spring. Changes in the para-H
() distribution are subtle, with a 0.02-0.03 rise over the spring
hemisphere (200-500 mbar) perturbed by (i) low- air advected by both the
springtime storm of 2010 and equatorial upwelling; and (ii) subsidence of
high- air at northern high latitudes, responsible for a developing
north-south asymmetry in . Conversely, the shifting asymmetry in the
para-H disequilibrium primarily reflects the changing temperature structure
(and the equilibrium distribution of ), rather than actual changes in
induced by chemical conversion or transport. CIRS results interpolated to
the same point in the seasonal cycle as re-analysed Voyager-1 observations show
qualitative consistency, with the exception of the tropical tropopause near the
equatorial zones and belts, where downward propagation of a cool temperature
anomaly associated with Saturn's stratospheric oscillation could potentially
perturb tropopause temperatures, para-H and winds. [ABRIDGED]Comment: Preprint accepted for publication in Icarus, 29 pages, 18 figure
Sugar beets
Only two varieties of sugar beets were grown on the station grounds last year (1890), namely, the White Improved Vilmorin and the Klein-Wanzleben. Data on these are given in tabular form below.
In November last, when about to commence the analysis of the station beets, one of us caused to be published in various papers an offer on the part of the station, authorized by the director, to analyze all samples of beets sent in by residents of the state, accompanied by a record of the kind of soil on which they were grown, the manuring, cultivation, and variety name of seed
On the potential of the EChO mission to characterise gas giant atmospheres
Space telescopes such as EChO (Exoplanet Characterisation Observatory) and
JWST (James Webb Space Telescope) will be important for the future study of
extrasolar planet atmospheres. Both of these missions are capable of performing
high sensitivity spectroscopic measurements at moderate resolutions in the
visible and infrared, which will allow the characterisation of atmospheric
properties using primary and secondary transit spectroscopy. We use the NEMESIS
radiative transfer and retrieval tool (Irwin et al. 2008, Lee et al. 2012) to
explore the potential of the proposed EChO mission to solve the retrieval
problem for a range of H2-He planets orbiting different stars. We find that
EChO should be capable of retrieving temperature structure to ~200 K precision
and detecting H2O, CO2 and CH4 from a single eclipse measurement for a hot
Jupiter orbiting a Sun-like star and a hot Neptune orbiting an M star, also
providing upper limits on CO and NH3. We provide a table of retrieval
precisions for these quantities in each test case. We expect around 30
Jupiter-sized planets to be observable by EChO; hot Neptunes orbiting M dwarfs
are rarer, but we anticipate observations of at least one similar planet.Comment: 22 pages, 30 figures, 4 tables. Accepted for publication in MNRA
Dark energy and curvature from a future baryonic acoustic oscillation survey using the Lyman-alpha forest
We explore the requirements for a Lyman-alpha forest (LyaF) survey designed
to measure the angular diameter distance and Hubble parameter at 2~<z~<4 using
the standard ruler provided by baryonic acoustic oscillations (BAO). The goal
would be to obtain a high enough density of sources to probe the
three-dimensional density field on the scale of the BAO feature. A
percent-level measurement in this redshift range can almost double the Dark
Energy Task Force Figure of Merit, relative to the case with only a similar
precision measurement at z~1, if the Universe is not assumed to be flat. This
improvement is greater than the one obtained by doubling the size of the z~1
survey, with Planck and a weak SDSS-like z=0.3 BAO measurement assumed in each
case. Galaxy BAO surveys at z~1 may be able to make an effective LyaF
measurement simultaneously at minimal added cost, because the required number
density of quasars is relatively small. We discuss the constraining power as a
function of area, magnitude limit (density of quasars), resolution, and
signal-to-noise of the spectra. For example, a survey covering 2000 sq. deg.
and achieving S/N=1.8 per Ang. at g=23 (~40 quasars per sq. deg.) with an
R~>250 spectrograph is sufficient to measure both the radial and transverse
oscillation scales to 1.4% from the LyaF (or better, if fainter magnitudes and
possibly Lyman-break galaxies can be used). At fixed integration time and in
the sky-noise-dominated limit, a wider, noisier survey is generally more
efficient; the only fundamental upper limit on noise being the need to identify
a quasar and find a redshift. Because the LyaF is much closer to linear and
generally better understood than galaxies, systematic errors are even less
likely to be a problem.Comment: 18 pages including 6 figures, submitted to PR
Diffusion of a granular pulse in a rotating drum
The diffusion of a pulse of small grains in an horizontal rotating drum is
studied through discrete elements methods simulations. We present a theoretical
analysis of the diffusion process in a one-dimensional confined space in order
to elucidate the effect of the confining end-plate of the drum. We then show
that the diffusion is neither subdiffusive nor superdiffusive but normal. This
is demonstrated by rescaling the concentration profiles obtained at various
stages and by studying the time evolution of the mean squared deviation.
Finally we study the self-diffusion of both large and small grains and we show
that it is normal and that the diffusion coefficient is independent of the
grain size
RISK PERCEPTIONS AND MANAGEMENT RESPONSES: PRODUCER-GENERATED HYPOTHESES FOR RISK MODELING
Farm level risk analyses have used price and yield variability almost exclusively to represent risk. Results from a survey of 149 agricultural producers in 12 states indicate that producers consider a broader range of sources of variability in their operations. Significant differences exist among categories with respect to the importance of the sources of variability in crop and livestock production. Producers also used a variety of management responses to variability. There were significant difference among categories in the importance given to particular responses and their use of them. These results have implications for research, extension, and policy programs.Risk and Uncertainty,
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