810 research outputs found
Research and development of superconducting thin films Final report, 15 Dec. 1965 - 15 Dec. 1966
Superconducting niobium thin film productio
The use of mixed methods as reflected in two eminent South African educational research journals
Published ArticleThe epistemological and ontological orientations relevant for this research are positivism, interpretivism and pragmatism. These paradigms of inquiry are associated with quantitatively oriented research traditions, qualitatively oriented research traditions and mixed methods research respectively. Researchers who use mixed methods build on the strengths of quantitative and qualitative methods and minimize their weaknesses. Since educational research is primarily evidence-based, the aim of the study was to explore the extent to which mixed methods research was reflected in two eminent South African educational research journals during the 11 year period, 2000 to 2010. To this end 1392 articles were analysed. Of the research articles published in the two journals, 17.8% and 15.1% respectively reported on the use of mixed methods. Quantitative methods dominated between 2000 and 2002, followed by a paradigm war in 2003 to 2007, and mainly qualitative methods from 2008 onwards. Mixed methods research was mostly used in the educational domains of didactics (inclusive of curriculum studies), management and social studies. The most dominant themes investigated in these fields were related to curricula and the NQF/OBE, transformation, staff diversity, e-learning and other teaching methods. The need to develop mixed methods research in all branches of social research in South Africa is indicated
Indirect band gap semiconductors for thin-film photovoltaics: High-throughput calculation of phonon-assisted absorption
Discovery of high-performance materials remains one of the most active areas
in photovoltaics (PV) research. Indirect band gap materials form the largest
part of the semiconductor chemical space, but predicting their suitability for
PV applications from first principles calculations remains challenging. Here we
propose a computationally efficient method to account for phonon assisted
absorption across the indirect band gap and use it to screen 127 experimentally
known binary semiconductors for their potential as thin film PV absorbers.
Using screening descriptors for absorption, carrier transport, and nonradiative
recombination, we identify 28 potential candidate materials. The list, which
contains 20 indirect band gap semiconductors, comprises both well established
(3), emerging (16), and previously unexplored (9) absorber materials. Most of
the new compounds are anion rich chalcogenides (TiS, GaTe) and
phosphides (PdP, CdP, MgP, BaP) containing homoelemental bonds,
and represent a new frontier in PV materials research. Our work highlights the
previously underexplored potential of indirect band gap materials for
optoelectronic thin-film technologies
High Mass Triple Systems: The Classical Cepheid Y Car
We have obtained an HST STIS ultraviolet high dispersion Echelle mode
spectrum the binary companion of the double mode classical Cepheid Y Car. The
velocity measured for the hot companion from this spectrum is very different
from reasonable predictions for binary motion, implying that the companion is
itself a short period binary. The measured velocity changed by 7 km/ s during
the 4 days between two segments of the observation confirming this
interpretation. We summarize "binary" Cepheids which are in fact members of
triple system and find at least 44% are triples. The summary of information on
Cepheids with orbits makes it likely that the fraction is under-estimated.Comment: accepted by A
Accurate Dielectric Response of Solids: Combining the Bethe-Salpeter Equation with the Random Phase Approximation
The Bethe-Salpeter equation (BSE) can provide an accurate description of
low-energy optical spectra of insulating crystals - even when excitonic effects
are important. However, due to high computational costs it is only possible to
include a few bands in the BSE Hamiltonian. As a consequence, the dielectric
screening given by the real part of the dielectric function can be
significantly underestimated by the BSE. Here we show that universally accurate
optical response functions can be obtained by combining a four-point BSE-like
equation for the irreducible polarisability with a two-point Dyson equation
which includes the higher-lying transitions within the random phase
approximation (RPA). The new method is referred to as BSE+. It has a
computational cost comparable to the BSE but a much faster convergence with
respect to the size of the electron-hole basis. We use the method to calculate
refractive indices and electron energy loss spectra for a test set of
semiconductors and insulators. In all cases the BSE+ yields excellent agreement
with experimental data across a wide frequency range and outperforms both BSE
and RPA.Comment: 6 pages, 4 figure
Hubble Space Telescope WFPC-2 Imaging of Cassiopeia A
The young SNR Cassiopeia A was imaged with WFPC-2 through four filters
selected to capture the complete velocity range of the remnant's main shell in
several important emission lines. Primary lines detected were [O III]
4959,5007, [N II] 6583, [S II] 6716,6731 + [O II] 7319,7330 + [O I] 6300,6364,
and [S III] 9069,9532. About 3/4th of the remnant's main shell was imaged in
all four filters. Considerable detail is observed in the reverse-shocked ejecta
with typical knot scale lengths of 0.2"-0.4" (1 - 2 x 10^16 cm). Both bright
and faint emission features appear highly clumped. Large differences in [S III]
and [O III] line intensities indicating chemical abundance differences are also
seen, particularly in knots located along the bright northern limb and near the
base of the northeast jet. A line of curved overlapping filament in the
remnant's northwestern rim appears to mark the location of the remnant's
reverse shock front in this region. Finger-like ejecta structures elsewhere
suggest cases where the reverse shock front is encountering the remnant's
clumped ejecta. Narrow-band [N II] images of the remnant's circumstellar knots
("QSFs") reveal them to be 0.1"-0.6" thick knots and filaments, often with
diffuse edges facing away from the center of expansion. Three color composite
images of the whole remnant and certain sections along with individual filter
enlargements of selected regions of the bright optical shell are presented and
discussed.Comment: 26 pages, 12 figures Accepted to the Astronomical Journa
Analysis of alpha Centauri AB including seismic constraints
Detailed models of alpha Cen A and B based on new seismological data for
alpha Cen B by Carrier & Bourban (2003) have been computed using the Geneva
evolution code including atomic diffusion. Taking into account the numerous
observational constraints now available for the alpha Cen system, we find a
stellar model which is in good agreement with the astrometric, photometric,
spectroscopic and asteroseismic data. The global parameters of the alpha Cen
system are now firmly constrained to an age of t=6.52+-0.30 Gyr, an initial
helium mass fraction Y_i=0.275+-0.010 and an initial metallicity
(Z/X)_i=0.0434+-0.0020. Thanks to these numerous observational constraints, we
confirm that the mixing-length parameter alpha of the B component is larger
than the one of the A component, as already suggested by many authors (Noels et
al. 1991, Fernandes & Neuforge 1995 and Guenther & Demarque 2000): alpha_B is
about 8% larger than alpha_A (alpha_A=1.83+-0.10 and alpha_B=1.97+-0.10).
Moreover, we show that asteroseismic measurements enable to determine the radii
of both stars with a very high precision (errors smaller than 0.3%). The radii
deduced from seismological data are compatible with the new interferometric
results of Kervella et al. (2003) even if they are slightly larger than the
interferometric radii (differences smaller than 1%).Comment: 13 pages, 9 figures, accepted for publication in A&
Polaris the Cepheid returns: 4.5 years of monitoring from ground and space
We present the analysis of 4.5 years of nearly continuous observations of the
classical Cepheid Polaris, which comprise the most precise data available for
this star. We have made spectroscopic measurements from ground and photometric
measurements from the WIRE star tracker and the SMEI instrument on the Coriolis
satellite. Measurements of the amplitude of the dominant oscillation (P = 4
days), that go back more than a century, show a decrease from 120 mmag to 30
mmag (V magnitude) around the turn of the millennium. It has been speculated
that the reason for the decrease in amplitude is the evolution of Polaris
towards the edge of the instability strip. However, our new data reveal an
increase in the amplitude by about 30% from 2003-2006. It now appears that the
amplitude change is cyclic rather than monotonic, and most likely the result of
a pulsation phenomenon. In addition, previous radial velocity campaigns have
claimed the detection of long-period variation in Polaris (P > 40 days). Our
radial velocity data are more precise than previous datasets, and we find no
evidence for additional variation for periods in the range 3-50 days with an
upper limit of 100 m/s. However, in the WIRE data we find evidence of variation
on time-scales of 2-6 days, which we interpret as being due to granulation.Comment: Accepted by ApJ. 8 pages. Quality of figures 1+3+7 degrade
Vortex Structure Around a Magnetic Dot in Planar Superconductors
The problem of the giant vortex state around a magnetic dot which is embedded
in a superconducting film is investigated. The full non-linear, self-consistent
Ginzburg-Landau equations are solved numerically in order to calculate the free
energy, the order parameter of the host superconductor, the internal magnetic
field due to the supercurrents, the corresponding current density, the
magnetization probed in the vicinity of the dot, and the normal electron
density as a function of the various parameters of the system. We find that, as
we increase the magnetic moment of the dot, higher flux quanta vortex states
become energetically more favorable, as they can better compete with the
external magnetic field via the Meissner effect. In addition to that, they
progressively become closer to each other in energy with direct experimental
consequences, i.e. physical quantities like magnetization may fluctuate when
measured, for example, as a function of a uniform external magnetic field.Comment: text 21 pages (REVTEX), 8 figures available upon reques
- âŠ