4,017 research outputs found
Band-edge diagrams for strained III-V semiconductor quantum wells, wires, and dots
We have calculated band-edge energies for most combinations of zincblende
AlN, GaN, InN, GaP, GaAs, InP, InAs, GaSb and InSb in which one material is
strained to the other. Calculations were done for three different geometries,
quantum wells, wires, and dots, and mean effective masses were computed in
order to estimate confinement energies. For quantum wells, we have also
calculated band-edges for ternary alloys. Energy gaps, including confinement,
may be easily and accurately estimated using band energies and a simple
effective mass approximation, yielding excellent agreement with experimental
results. By calculating all material combinations we have identified novel and
interesting material combinations, such as artificial donors, that have not
been experimentally realized. The calculations were perfomed using
strain-dependent k-dot-p theory and provide a comprehensive overview of band
structures for strained heterostructures.Comment: 9 pages, 17 figure
The Mass-to-Light Ratios of the Draco and Ursa Minor Dwarf Spheroidal Galaxies. II. The Binary Population and Its Effect in the Measured Velocity Dispersions of Dwarf Spheroidal Galaxies
We use a large set of radial velocities in the Ursa Minor and Draco dwarf
spheroidal galaxies to search for binary stars and to infer the binary
frequency. Of the 118 stars in our sample with multiple observations, six are
velocity variables with probabilities below 0.001. We use Monte Carlo
simulations that mimic our observations to determine the efficiency with which
our observations find binary stars. Our best, though significantly uncertain,
estimate of the binary frequency for stars near the turnoff in Draco and UMi is
0.2--0.3 per decade of period in the vicinity of periods of one year, which is
3--5 that found for the solar neighborhood. This frequency is high
enough that binary stars might significantly affect the measured velocity
dispersions of some dwarf spheroidal galaxies according to some previous
numerical experiments. However, in the course of performing our own
experiments, we discovered that this previous work had inadvertently
overestimated binary orbital velocities. Our first set of simulations of the
effects of binaries is based on the observed scatter in the individual velocity
measurements for the multiply-observed Draco and Ursa Minor stars. This scatter
is small compared to measured velocity dispersions and, so, the effect of
binaries on the dispersions is slight. This result is supported by our second
set of experiments, which are based on a model binary population normalized by
the observed binary frequency in Draco and Ursa Minor. We conclude that binary
stars have had no significant effect on the measured velocity dispersion and
inferred mass-to-light ratio of any dwarf spheroidal galaxy.Comment: 33 pages, 95kb uuencoded, gzipped postscript; Accepted by
Astronomical Journal; gzipped, tarred postscript of text, tables, figures
available at ftp://as.arizona.edu/pub/edo (binaries_in_dsph.tar.gz
Fabry-Perot Measurements of the Dynamics of Globular Cluster Cores: M15 (NGC~7078)
We report the first use of the Rutgers Imaging Fabry-Perot Spectrophotometer
to study the dynamics of the cores of globular clusters. We have obtained
velocities for cluster stars by tuning the Fabry-Perot to take a series of
narrow-band images at different wavelengths across one of the Na D (5890 AA)
absorption lines. Measuring the flux in every frame yields a short portion of
the spectrum for each star simultaneously. This proves to be a very efficient
method for obtaining accurate stellar velocities; in crowded regions we are
able to measure hundreds of velocities in 3-4 hours of observing time. We have
measured velocities with uncertainties of less than 5 km/s for 216 stars within
1.5' of the center of the globular cluster M15 (NGC 7078). The paper is a
uuencoded compressed postscript file
Principled Pluralism and Contract Remedies
Critiques leading accounts for judicial vindication of the expectation interest in contracts and proffers an account drawn from Christian doctrines of the dominion mandate, creation of humanity in the image of God, and sin
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