597 research outputs found
GaN directional couplers for integrated quantum photonics
Large cross-section GaN waveguides are proposed as a suitable architecture to
achieve integrated quantum photonic circuits. Directional couplers with this
geometry have been designed with aid of the beam propagation method and
fabricated using inductively coupled plasma etching. Scanning electron
microscopy inspection shows high quality facets for end coupling and a well
defined gap between rib pairs in the coupling region. Optical characterization
at 800 nm shows single-mode operation and coupling-length-dependent splitting
ratios. Two photon interference of degenerate photon pairs has been observed in
the directional coupler by measurement of the Hong-Ou-Mandel dip with 96%
visibility.Comment: 4 pages, 5 figure
Alternative Designs of Molten Salt Storage Shells for Use in Solar Energy Storage
Molten salt (MS) storage systems in the 565°C range can store green solar energy from thermal solar power station, such as the Crescent Dunes solar plant in Nevada. Large containers can be used to store energy and generate electricity for eight hours or more to be used at night or during peak demand hours, depending on the container size. Energy storage can reduce the fluctuation due to weather conditions experienced at thermal solar power stations because stable diurnal energy supply is made available by MS energy storage. Supported by the Office of Naval Research (ONR), the research presented discusses the considerations for designing molten salt storage tanks. An alternate molten salt storage cylindrical tank design layout is presented, including an improved roof design concept. A preliminary heat transfer analysis is presented and discussed for the alternate cylindrical tank design. This preliminary analysis was used to determine the thickness of insulating material in and around the cylindrical tank to reduce heat flux. These insulating materials include the use of firebrick and ceramic insulation to complement the structural carbon steel and the stainless steel that is used for corrosion resistance. This paper also introduces the alternate designs of a semi-buried spherical tank and drop shell tank that can be used storing molten salts
Forestry Bulletin No. 18: Geography of the Southern Forest Region
The Southern Forest Region, as here discussed, comprises the area east of the Texas and Oklahoma prairies and south of the Missouri, Ohio, and Potomac rivers, plus an extension along the Atlantic coast to central New Jersey.https://scholarworks.sfasu.edu/forestrybulletins/1014/thumbnail.jp
Extending Optical Flare Models to the UV: Results from Comparing of TESS and GALEX Flare Observations For M Dwarfs
The ultraviolet (UV) emission of stellar flares may have a pivotal role in
the habitability of rocky exoplanets around low-mass stars. Previous studies
have used white-light observations to calibrate empirical models which describe
the optical and UV flare emission. However, the accuracy of the UV predictions
of models have previously not been tested. We combined TESS optical and GALEX
UV observations to test the UV predictions of empirical flare models calibrated
using optical flare rates of M stars. We find that the canonical 9000 K
blackbody model used by flare studies underestimates the GALEX NUV energies of
field age M stars by up to a factor of 6.50.7 and the GALEX FUV energies
of fully convective field age M stars by 30.610.0. We calculated energy
correction factors that can be used to bring the UV predictions of flare models
closer in line with observations. We calculated pseudo-continuum flare
temperatures that describe both the white-light and GALEX NUV emission. We
measured a temperature of 10,700 K for flares from fully convective M stars
after accounting for the contribution from UV line emission. We also applied
our correction factors to the results of previous studies of the role of flares
in abiogenesis. Our results show that M stars do not need to be as active as
previously thought in order to provide the NUV flux required for prebiotic
chemistry, however we note that flares will also provide more FUV flux than
previously modelled.Comment: 20 pages, 9 figures, 4 tables. Accepted for publication in the
Monthly Notices of the Royal Astronomical Societ
Patterns of Undergraduates' Use of Scholarly Databases in a Large Research University
Authentication data was utilized to explore undergraduate usage of subscription electronic databases. These usage patterns were linked to the information literacy curriculum of the library. The data showed that out of the 26,208 enrolled undergraduate students, 42% of them accessed a scholarly database at least once in the course of the entire semester. Despite their higher levels of learning and expected sophistication in informa- tion seeking skills, juniors and seniors used databases proportionately less than freshmen and sophomores. The University Library conducts a variety of introductory seminars that introduce freshmen to databases in the Fall semester. There was no evidence that this momentum is sustained in the subsequent years when higher-level more sophisticated skills are needed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95700/1/Patterns-of-Undergraduates-Use-of-Scholarly-Databases-in-a-Large-Research-University.pd
Strong Continuum-continuum Couplings In The Direct Ionization Of Ar And He Atoms By 6-MeV/u U38+ And Th38+ Projectiles
Doubly differential cross sections have been measured as a function of the electron energy and observation angle for electron emission following collisions of 6-MeV/u U38+ and Th38+ on He and Ar. The electron-emission data show an enhancement at forward angles and a decrease at backward angles with respect to scaled-cross-section results based on the Born approximation. Comparison with classical-trajectory Monte Carlo calculations suggests that the deviation from the Born approximation can be explained by continuum-continuum couplings. By comparing with previously published data, we found that the forward enhancement as well as the backward decrease follow a q/vp (q,vp are the projectile charge and velocity) scaling. © 1989 The American Physical Society
Holomorphic Quantization on the Torus and Finite Quantum Mechanics
We construct explicitly the quantization of classical linear maps of on toroidal phase space, of arbitrary modulus, using the holomorphic
(chiral) version of the metaplectic representation. We show that Finite Quantum
Mechanics (FQM) on tori of arbitrary integer discretization, is a consistent
restriction of the holomorphic quantization of to the subgroup
, being the principal congruent subgroup mod l,
on a finite dimensional Hilbert space. The generators of the ``rotation group''
mod l, , for arbitrary values of l are determined as
well as their quantum mechanical eigenvalues and eigenstates.Comment: 12 pages LaTeX (needs amssymb.sty). Version as will appear in J.
Phys.
Temporal evolution of shallow marine diagenetic environments: Insights from carbonate concretions
Early diagenesis of marine organic matter dramatically impacts Earth's surface chemistry by changing the burial potential of carbon and promoting the formation of authigenic mineral phases including carbonate concretions. Marine sediment-hosted carbonate concretions tend to form as a result of microbial anaerobic diagenetic reactions that degrade organic matter and methane, some of which require an external oxidant. Thus, temporal changes in the oxidation state of Earth's oceans may impart a first-order control on concretion authigenesis mechanisms through time. Statistically significant variability in concretion carbonate carbon isotope compositions indicates changes in shallow marine sediment diagenesis associated with Earth's evolving redox landscape. This variability manifests itself as an expansion in carbon isotope composition range broadly characterized by an increase in maximum and decrease in minimum isotope values through time. Reaction transport modelling helps to constrain the potential impacts of shifting redox chemistry and highlights the importance of organic carbon delivery to the seafloor, marine sulfate concentrations, methane production and external methane influx. The first appearance of conclusively anaerobic oxidation of methane-derived concretions occurs in the Carboniferous and coincides with a Paleozoic rise in marine sulfate. The muted variability recognized in older concretions (and in particular for Precambrian concretions) likely reflects impacts of a smaller marine sulfate reservoir and perhaps elevated marine dissolved inorganic carbon concentrations. Causes of the increase in carbon isotope maximum values through time are more confounding, but may be related to isotopic equilibration of dissolved inorganic carbon with externally derived methane. Ultimately the concretion isotope record in part reflects changes in organic matter availability and marine oxidation state, highlighting connections with the subsurface biosphere and diagenesis throughout geologic time
Flares, Rotation, Activity Cycles and a Magnetic Star-Planet Interaction Hypothesis for the Far Ultraviolet Emission of GJ 436
Variability in the far ultraviolet (FUV) emission produced by stellar
activity affects photochemistry and heating in orbiting planetary atmospheres.
We present a comprehensive analysis of the FUV variability of GJ 436, a
field-age, M2.5V star ( d) orbited by a warm,
Neptune-size planet (, ,
d). Observations at three epochs from 2012 to 2018
span nearly a full activity cycle, sample two rotations of the star and two
orbital periods of the planet, and reveal a multitude of brief flares. Over
2012-2018, the star's yr activity cycle produced the largest
observed variations, % in the summed flux of major FUV emission lines.
In 2018, variability due to rotation was %. An additional %
scatter at 10 min cadence, treated as white noise in fits, likely has both
instrumental and astrophysical origins. Flares increased time-averaged emission
by 15% over the 0.88 d of cumulative exposure, peaking as high as 25
quiescence. We interpret these flare values as lower limits given that flares
too weak or too infrequent to have been observed likely exist. GJ 436's flare
frequency distribution (FFD) at FUV wavelengths is unusual compared to other
field-age M dwarfs, exhibiting a statistically-significant dearth of high
energy ( erg) events that we hypothesize to be the result of
a magnetic star-planet interaction (SPI) triggering premature flares. If an SPI
is present, GJ 436 b's magnetic field strength must be 100 G to
explain the statistically insignificant increase in orbit-phased FUV emission.Comment: 18 pages, 9 figures, accepted to the Astronomical Journa
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