32,932 research outputs found
Luminescent solar concentrators. 2: Experimental and theoretical analysis of their possible efficiencies
Experimental techniques are developed to determine the applicability of a particular luminescing center
for use in a luminescent solar concentrator (LSC). The relevant steady-state characteristics of eighteen
common organic laser dyes are given. The relative spectral homogeneity of such dyes are shown to depend upon the surrounding material using narrowband laser excitation. We developed three independent techniques for measuring self-absorption rates; these are time-resolved emission, steady-state polarization anisotropy, and spectral convolution. Preliminary dye degradation and prototype efficiency measurements are included. Finally, we give simple relationships relating the efficiency and gain of an LSC to key spectroscopic parameters of its constituents
A webometric analysis of Australian Universities using staff and size dependent web impact factors (WIF)
This study describes how search engines (SE) can be employed for automated, efficient data gathering for Webometric studies using predictable URLs. It then compares the usage of staffrelated Web Impact Factors (WIFs) to sizerelated impact factors for a ranking of Australian universities, showing that rankings based on staffrelated WIFs correlate much better with an established ranking from the Melbourne Institute than commonly used sizedependent WIFs. In fact sizedependent WIFs do not correlate with the Melbourne ranking at all. It also compares WIF data for Australian Universities provided by Smith (1999) for a longitudinal comparison of the WIF of Australian Universities over the last decade. It shows that sizedependent WIF values declined for most Australian universities over the last ten years, while staffdependent WIFs rose
Parity effect and single-electron injection for Josephson-junction chains deep in the insulating state
We have made a systematic investigation of charge transport in 1D chains of
Josephson junctions where the characteristic Josephson energy is much less than
the single-island Cooper-pair charging energy, . Such
chains are deep in the insulating state, where superconducting phase coherence
across the chain is absent, and a voltage threshold for conduction is observed
at the lowest temperatures. We find that Cooper-pair tunneling in such chains
is completely suppressed. Instead, charge transport is dominated by tunneling
of single electrons, which is very sensitive to the presence of BCS
quasiparticles on the superconducting islands of the chain. Consequently we
observe a strong parity effect, where the threshold voltage vanishes sharply at
a characteristic parity temperature , which is significantly lower than
the the critical temperature, . A measurable and thermally-activated
zero-bias conductance appears above , with an activation energy equal to
the superconducting gap, confirming the role of thermally-excited
quasiparticles. Conduction below and above the voltage threshold occurs
via injection of single electrons/holes into the Cooper-pair insulator, forming
a non-equilibrium steady state with a significantly enhanced effective
temperature. Our results explicitly show that single-electron transport
dominates deep in the insulating state of Josephson-junction arrays. This
conduction process has mostly been ignored in previous studies of both
superconducting junction arrays and granular superconducting films below the
superconductor-insulator quantum phase transition.Comment: 8 pages, 6 figure
The Effects of Age on Red Giant Metallicities Derived from the Near-Infrared Ca II Triplet
We have obtained spectra with resolution 2.5 Angstroms in the region
7500-9500 Angstroms for 116 red giants in 5 Galactic globular clusters and 6
old open clusters (5 with published metallicities, and one previously
unmeasured). The signal-to-noise ranges from 20 to 85. We measure the
equivalent widths of the infrared Ca II triplet absorption lines in each stars
and compare to cluster metallicities taken from the literature. With globular
cluster abundances on the Carretta & Gratton scale, and open cluster abundances
taken from the compilation of Friel and collaborators, we find a linear
relation between [Fe/H] and Ca II line strength spanning the range -2 < [Fe/H]
< -0.2 and ages from 2.5 - 13 Gyr. No evidence for an age effect on the
metallicity calibration is observed. Using this calibration, we find the
metallicity of the old open cluster Trumpler 5 to be [Fe/H] = -0.56 +/-0.11.
Considering the 10 clusters of known metallicity shifted to a common distance
and reddening, we find that the additional metallicity error introduced by the
variation of horizontal branch/red clump magnitude with metallicity and age is
of order +/-0.05 dex, which can be neglected in comparison to the intrinsic
scatter in our method. The results are discussed in the context of abundance
determinations for red giants in Local Group galaxies.Comment: Accepted by MNRAS; 21 pages in LaTeX MNRAS style, 6 tables, 6 figure
Self-similar collapse and the structure of dark matter halos: A fluid approach
We explore the dynamical restrictions on the structure of dark matter halos
through a study of cosmological self-similar gravitational collapse solutions.
A fluid approach to the collisionless dynamics of dark matter is developed and
the resulting closed set of moment equations are solved numerically including
the effect of halo velocity dispersions (both radial and tangential), for a
range of spherically averaged initial density profiles. Our results highlight
the importance of tangential velocity dispersions to obtain density profiles
shallower than in the core regions, and for retaining a memory of the
initial density profile, in self-similar collapse. For an isotropic core
velocity dispersion only a partial memory of the initial density profile is
retained. If tangential velocity dispersions in the core are constrained to be
less than the radial dispersion, a cuspy core density profile shallower than
cannot obtain, in self-similar collapse.Comment: 25 pages, 7 figures, submitted to Ap
Multi-shocks in asymmetric simple exclusions processes: Insights from fixed-point analysis of the boundary-layers
The boundary-induced phase transitions in an asymmetric simple exclusion
process with inter-particle repulsion and bulk non-conservation are analyzed
through the fixed points of the boundary layers. This system is known to have
phases in which particle density profiles have different kinds of shocks. We
show how this boundary-layer fixed-point method allows us to gain physical
insights on the nature of the phases and also to obtain several quantitative
results on the density profiles especially on the nature of the boundary-layers
and shocks.Comment: 12 pages, 8 figure
Baryon Acoustic Oscillations in 2D: Modeling Redshift-space Power Spectrum from Perturbation Theory
We present an improved prescription for matter power spectrum in redshift
space taking a proper account of both the non-linear gravitational clustering
and redshift distortion, which are of particular importance for accurately
modeling baryon acoustic oscillations (BAOs). Contrary to the models of
redshift distortion phenomenologically introduced but frequently used in the
literature, the new model includes the corrections arising from the non-linear
coupling between the density and velocity fields associated with two
competitive effects of redshift distortion, i.e., Kaiser and Finger-of-God
effects. Based on the improved treatment of perturbation theory for
gravitational clustering, we compare our model predictions with monopole and
quadrupole power spectra of N-body simulations, and an excellent agreement is
achieved over the scales of BAOs. Potential impacts on constraining dark energy
and modified gravity from the redshift-space power spectrum are also
investigated based on the Fisher-matrix formalism. We find that the existing
phenomenological models of redshift distortion produce a systematic error on
measurements of the angular diameter distance and Hubble parameter by 1~2%, and
the growth rate parameter by ~5%, which would become non-negligible for future
galaxy surveys. Correctly modeling redshift distortion is thus essential, and
the new prescription of redshift-space power spectrum including the non-linear
corrections can be used as an accurate theoretical template for anisotropic
BAOs.Comment: 18 pages, 10 figure
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