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Report of Investigations No. 135 Oligocene Volcanism and Multiple Caldera Formation in the Chinati Mountains, Presidio County, Texas
UT Librarie
Nitrogen uptake and utilization by intact plants
The results of experiments support the proposed conceptual model that relates nitrogen uptake activity by plants as a balanced interdependence between the carbon-supplying function of the shoot and the nitrogen-supplying function of the roots. The data are being used to modify a dynamic simulation of plant growth, which presently describes carbon flows through the plant, to describe nitrogen uptake and assimilation within the plant system. Although several models have been proposed to predict nitrogen uptake and partitioning, they emphasize root characteristics affecting nutrient uptake and relay on empirical methods to describe the relationship between nitrogen and carbon flows within the plant. Researchers, on the other hand, propose to continue to attempt a mechanistic solution in which the effects of environment on nitrogen (as well as carbon) assimilation are incorporated through their direct effects on photosynthesis, respiration, and aging processes
Regulation of nitrogen uptake and assimilation: Effects of nitrogen source, root-zone pH, and aerial CO2 concentration on growth and productivity of soybeans
An important feature of controlled-environment crop production systems such as those to be used for life support of crews during space exploration is the efficient utilization of nitrogen supplies. Making decisions about the best sources of these supplies requires research into the relationship between nitrogen source and the physiological processes which regulate vegetative and reproductive plant growth. Work done in four areas within this research objective is reported: (1) experiments on the effects of root-zone pH on preferential utilization of NO3(-) versus NH4(+) nitrogen; (2) investigation of processes at the whole-plant level that regulate nitrogen uptake; (3) studies of the effects of atmospheric CO2 and NO3(-) supply on the growth of soybeans; and (4) examination of the role of NO3(-) uptake in enhancement of root respiration
Observations of the diffuse UV radiation field
Spectra are presented for the diffuse UV radiation field between 1250 to 3100 A from eight different regions of the sky, which were obtained with the Johns Hopkins UVX experiment. UVX flew aboard the Space Shuttle Columbia (STS-61C) in January 1986 as part of the Get-Away Special project. The experiment consisted of two 1/4 m Ebert-Fastie spectrometers, covering the spectral range 1250 to 1700 A at 17 A resolution and 1600 to 3100 A at 27 A resolution, respectively, with a field of view of 4 x .25 deg, sufficiently small to pick out regions of the sky with no stars in the line of sight. Values were found for the diffuse cosmic background ranging in intensity from 300 to 900 photons/sq cm/sec/sr/A. The cosmic background is spectrally flat from 1250 to 3100 A, within the uncertainties of each spectrometer. The zodiacal light begins to play a significant role in the diffuse radiation field above 2000 A, and its brightness was determined relative to the solar emission. Observed brightnesses of the zodiacal light in the UV remain almost constant with ecliptic latitude, unlike the declining visible brightnesses, possibly indicating that those (smaller) grains responsible for the UV scattering have a much more uniform distribution with distance from the ecliptic plane than do those grains responsible for the visible scattering
Phonons and d-wave pairing in the two-dimensional Hubbard model
We analyze the influence of phonons on the d-wave pairing instability in the
Hubbard model on the two-dimensional square lattice at weak to moderate
interaction U, using a functional renormalization group scheme with
frequency-dependent interaction vertices. As measured by the pairing scale, the
B1g buckling mode enhances the pairing, while other phonon modes decrease the
pairing. When various phonon modes are included together, the net effect on the
scale is small. However, in situations where d-wave superconductivity and other
tendencies, e.g. antiferromagnetism, are closely competing, the combined effect
of different phonons may be able to tip the balance towards pairing.Comment: 4 pages, 3 figure
Surface Encapsulation for Low-Loss Silicon Photonics
Encapsulation layers are explored for passivating the surfaces of silicon to
reduce optical absorption in the 1500-nm wavelength band. Surface-sensitive
test structures consisting of microdisk resonators are fabricated for this
purpose. Based on previous work in silicon photovoltaics, coatings of SiNx and
SiO2 are applied under varying deposition and annealing conditions. A short dry
thermal oxidation followed by a long high-temperature N2 anneal is found to be
most effective at long-term encapsulation and reduction of interface
absorption. Minimization of the optical loss is attributed to simultaneous
reduction in sub-bandgap silicon surface states and hydrogen in the capping
material.Comment: 4 pages, 3 figure
ELSA: An Integrated, Semi-Automated Nebular Abundance Package
We present ELSA, a new modular software package, written in C, to analyze and
manage spectroscopic data from emission-line objects. In addition to
calculating plasma diagnostics and abundances from nebular emission lines, the
software provides a number of convenient features including the ability to
ingest logs produced by IRAF's splot task, to semi-automatically merge spectra
in different wavelength ranges, and to automatically generate various data
tables in machine-readable or LaTeX format. ELSA features a highly
sophisticated interstellar reddening correction scheme that takes into account
temperature and density effects as well as He II contamination of the hydrogen
Balmer lines. Abundance calculations are performed using a 5-level atom
approximation with recent atomic data, based on R. Henry's ABUN program.
Improvements planned in the near future include use of a three-region
ionization model, similar to IRAF's nebular package, error propagation, and the
addition of ultraviolet and infrared line analysis capability. Detailed
documentation for all aspects of ELSA are available at
http://www.williams.edu/Astronomy/research/PN .Comment: 2 pages, contributed paper, IAU Symp. 234, Planetary Nebulae in Our
Galaxy and Beyon
Static and Dynamic Spectroscopy of (Al,Ga)As/GaAs Microdisk Lasers with Interface Fluctuation Quantum Dots
We have studied the steady state and dynamic optical properties of
semiconductor microdisk lasers whose active region contains interface
fluctuation quantum dots in GaAs/(Ga,Al)As quantum wells. Steady-state
measurements of the stimulated emission via whispering gallery modes yield a
quality factor and a coupling constant . The
broad gain spectrum produces mode hopping between spectrally adjacent
whispering gallery modes as a function of temperature and excitation power.
Time- and energy-resolved photoluminescence measurements show that the emission
rise and decay rates increase significantly with excitation power. Marked
differences are observed between the radiative decay rates in processed and
unprocessed samples.Comment: To appear in Phys. Rev.
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