2,469 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
Accuracy of circular polarization as a measure of spin polarization in quantum dot qubits
A quantum dot spin LED provides a test of carrier spin injection into a
qubit, as well as a means of analyzing carrier spin injection in general and
local spin polarization. The polarization of the observed light is, however,
significantly influenced by the dot geometry so the spin may be more polarized
than the emitted light would naively suggest. We have calculated carrier
polarization-dependent optical matrix elements using 8-band strain-dependent
k.p theory for InAs/GaAs self-assembled quantum dots (SAQDs) for electron and
hole spin injection into a range of quantum dot sizes and shapes, and for
arbitrary emission directions. The observed circular polarization does not
depend on whether the injected spin-polarized carriers are electrons or holes,
but is strongly influenced by the SAQD geometry and emission direction.
Calculations for typical SAQD geometries with emission along [110] show light
that is only ~5% circularly polarized for spin states that are 100% polarized
along [110]. Therefore observed polarizations [Chye et al. PRB 66, 201301(R)]
of ~1% imply a spin polarization within the dot of ~20%. We also find that
measuring along the growth direction gives near unity conversion of spin to
photon polarization, and is the least sensitive to uncertainties in SAQD
geometry.Comment: 4 pages, 6 figure
Electrical manipulation of an electronic two-state system in Ge/Si quantum dots
We calculate that the electron states of strained self-assembled Ge/Si
quantum dots provide a convenient two-state system for electrical control. An
electronic state localized at the apex of the quantum dot is nearly degenerate
with a state localized at the base of the quantum dot. Small electric fields
shift the electronic ground state from apex-localized to base-localized, which
permits sensitive tuning of the electronic, optical and magnetic properties of
the dot. As one example, we describe how spin-spin coupling between two Ge/Si
dots can be controlled very sensitively by shifting the individual dot's
electronic ground state between apex and base
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
g-Factors and diamagnetic coefficients of electrons, holes and excitons in InAs/InP quantum dots
The electron, hole, and exciton g-factors and diamagnetic coefficients have
been calculated using envelope-function theory for cylindrical InAs/InP quantum
dots in the presence of a magnetic field parallel to the dot symmetry axis. A
clear connection is established between the electron g-factor and the amplitude
of the those valence-state envelope functions which possess non-zero orbital
momentum associated with the envelope function. The dependence of the exciton
diamagnetic coefficients on the quantum dot height is found to correlate with
the energy dependence of the effective mass. Calculated exciton g-factor and
diamagnetic coefficients, constructed from the values associated with the
electron and hole constituents of the exciton, match experimental data well,
however including the Coulomb interaction between the electron and hole states
improves the agreement. Remote-band contributions to the valence-band
electronic structure, included perturbatively, reduce the agreement between
theory and experiment.Comment: 12 pages, 7 figure
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