609 research outputs found
Optimal Cosmic-Ray Detection for Nondestructive Read Ramps
Cosmic rays are a known problem in astronomy, causing both loss of data and
data inaccuracy. The problem becomes even more extreme when considering data
from a high-radiation environment, such as in orbit around Earth or outside the
Earth's magnetic field altogether, unprotected, as will be the case for the
James Webb Space Telescope (JWST). For JWST, all the instruments employ
nondestructive readout schemes. The most common of these will be "up the ramp"
sampling, where the detector is read out regularly during the ramp. We study
three methods to correct for cosmic rays in these ramps: a two-point difference
method, a deviation from the fit method, and a y-intercept method. We apply
these methods to simulated nondestructive read ramps with single-sample groups
and varying combinations of flux, number of samples, number of cosmic rays,
cosmic-ray location in the exposure, and cosmic-ray strength. We show that the
y-intercept method is the optimal detection method in the read-noise-dominated
regime, while both the y-intercept method and the two-point difference method
are best in the photon-noise-dominated regime, with the latter requiring fewer
computations.Comment: To be published in PASP. This paper is 12 pages long and includes 15
figure
Evidence for a Bulk Complex Order-Parameter in Y0.9Ca0.1Ba2Cu3O7-delta Thin Films
We have measured the penetration depth of overdoped Y0.9Ca0.1Ba2Cu3O7-delta
(Ca-YBCO) thin films using two different methods. The change of the penetration
depth as a function of temperature has been measured using the parallel plate
resonator (PPR), while its absolute value was obtained from a quasi-optical
transmission measurements. Both sets of measurements are compatible with an
order parameter of the form: Delta*dx2-y2+i*delta*dxy, with Delta=14.5 +- 1.5
meV and delta=1.8 meV, indicating a finite gap at low temperature. Below 15 K
the drop of the scattering rate of uncondensed carriers becomes steeper in
contrast to a flattening observed for optimally doped YBCO films. This decrease
supports our results on the penetration depth temperature dependence. The
findings are in agreement with tunneling measurements on similar Ca-YBCO thin
films.Comment: 11 pages, 4 figure
Signatures of electron-boson coupling in half-metallic ferromagnet MnGe: study of electron self-energy obtained from infrared spectroscopy
We report results of our infrared and optical spectroscopy study of a
half-metallic ferromagnet MnGe. This compound is currently being
investigated as a potential injector of spin polarized currents into germanium.
Infrared measurements have been performed over a broad frequency (50 - 50000
cm) and temperature (10 - 300 K) range. From the complex optical
conductivity we extract the electron self-energy
. The calculation of is based on novel
numerical algorithms for solution of systems of non-linear equations. The
obtained self-energy provides a new insight into electron correlations in
MnGe. In particular, it reveals that charge carriers may be coupled to
bosonic modes, possibly of magnetic origin
Agency theory perspective on public - private - partnerships: international development project
Purpose – Agency theory suggests that divergences will occur when a principal, e.g. client, and agent e.g.
a project manager, interests are different in the execution of a project. The purpose of this paper is to explore
if the agency theory can explain the subtleties integral to the behaviours and relationships between players
delivering a public-private-partnership (PPP) in the context of an international development (ID) project.
The intra-/interpersonal dynamics include governments, non-governmental organisations (NGOs) and private
commercial service providers. The authors develop a conceptual framework and provide evidence from a case
study of the testing of a Road Safety Toolkit in Kenya to explore several propositions.
Design/methodology/approach – Extant literature identified application of the agency theory, and
the development of a conceptual framework. A case study describing an ID project was used to validate the
propositions prior to the expansion of a research instrument for data collection in the field.
Findings – Through the lens of the agency theory and the limitations imposed by exploring a series of
propositions, several insightful conclusions have been derived from the case. ID projects have particular
nuisances that make them unique when compared to the majority of commercial applications. An added
dimension and level of complexity is a consequence of the PPP incorporating government, NGOs and private
corporations. The case exemplified the need for PPP ID projects to build on partner networks to influence and
disseminate outcomes. Some agency problems were far less prominent than would normally be seen in a
commercial project.
Research limitations/implications – The methodologies presented in this paper need to be adapted and
practiced in different kinds of ID projects in order to get confirmatory analytical results. The limitations
imposed by the use of the single case, whilst drawing insightful conclusions, would necessitate greater testing
in the field.
Practical implications – Although the problems of the agency theory are well researched in the operations
management literature, there is limited application to ID projects and no previous research within the context
of a PPP. Therefore, this work is important for greater understanding of the specific issues associated with
project delivery of an ID.
Social implications – Conflicting goals between principals and agents are common for organisations,
which in turn affect inter-relationships on an international footing. The agency theory has had little attention
in the project management field, yet is fundamental to relationships and communication.
Originality/value – There has been little research that explores the agency theory in the context of a PPP
involving governments, NGOs and private commercial service providers, executed as an ID project.
This work, therefore, exhibits new and novel findings
Electronic correlations and unusual superconducting response in the optical properties of the iron-chalcogenide FeTe0.55Se0.45
The in-plane complex optical properties of the iron-chalcogenide
superconductor FeTe0.55Se0.45 have been determined above and below the critical
temperature Tc = 14 K. At room temperature the conductivity is described by a
weakly-interacting Fermi liquid; however, below 100 K the scattering rate
develops a frequency dependence in the terahertz region, signaling the
increasingly correlated nature of this material. We estimate the dc
conductivity just above Tc to be sigma_dc ~ 3500 Ohm-1cm-1 and the superfluid
density rho_s0 ~ 9 x 10^6 cm-2, which places this material close to the scaling
line rho_s0/8 ~ 8.1 sigma_dc Tc for a BCS dirty-limit superconductor. Below Tc
the optical conductivity reveals two gap features at Delta_1,2 ~ 2.5 and ~ 5.1
meV.Comment: Minor revisions, 5 pages, 4 figure
Hybridization gap versus hidden order gap in URuSi as revealed by optical spectroscopy
We present the in-plane optical reflectance measurement on single crystals of
URuAs. The study revealed a strong temperature-dependent spectral
evolution. Above 50 K, the low frequency optical conductivity is rather flat
without a clear Drude-like response, indicating a very short transport life
time of the free carriers. Well below the coherence temperature, there appears
an abrupt spectral weight suppression below 400 cm, yielding evidence
for the formation of a hybridization energy gap arising from the mixing of the
conduction electron and narrow f-electron bands. A small part of the suppressed
spectral weight was transferred to the low frequency side, leading to a narrow
Drude component, while the majority of the suppressed spectral weight was
transferred to the high frequency side centered near 4000 cm. Below the
hidden order temperature, another very prominent energy gap structure was
observed, which leads to the removal of a large part of the Drude component and
a sharp reduction of the carrier scattering rate. The study revealed that the
hybridization gap and the hidden orger gap are distinctly different: they occur
at different energy scales and exhibit completely different spectral
characteristics.Comment: 5 page
Signature of Electronic Correlations in the Optical Conductivity of the Doped Semiconductor Si:P
Electronic transport in highly doped but still insulating silicon at low
temperatures is dominated by hopping between localized states; it serves as a
model system of a disordered solid for which the electronic interaction can be
investigated. We have studied the frequency-dependent conductivity of
phosphorus-doped silicon in the THz frequency range (30 GHz to 3 THz) at low
temperatures K. The crossover in the optical conductivity from a
linear to a quadratic frequency dependence as predicted by Efros and Shklovskii
is observed qualitatively; however, the simple model does not lead to a
quantitative agreement. Covering a large range of donor concentration, our
temperature- and frequency-dependent investigations reveal that electronic
correlation effects between the localized states play an important and complex
role at low temperatures. In particular we find a super-linear frequency
dependence of the conductivity that highlights the influence of the density of
states, i.e. the Coulomb gap, on the optical conductivity. When approaching the
metal-to-insulator transition by increasing doping concentration, the
dielectric constant and the localization length exhibit critical behavior.Comment: 9 pages, 8 figures, 1 tabl
Central star formation in S0 galaxies
As a class, S0 galaxies are characterized by a lack of resolved bright stars in the disk. However, several lines of evidence support the hypothesis that a high rate of star formation is occurring at the centers of some S0 galaxies. Many of the warmest, most powerful far infrared sources in nearby bright galaxies occur in S0 galaxies. (Dressel 1988, Ap. J., 329, L69). The ratios of radio continuum flux to far infrared flux for these S0 galaxies are comparable to the ratios found for spiral galaxy disks and for star-burst galaxies. Very Large Array (VLA) maps of some of these S0 galaxies show that the radio continuum emission originates in the central few kiloparsecs. It is diffuse or clumpy, unlike the radio sources in active S0 galaxies, which are either extremely compact or have jet-lobe structures. Imaging of some of these galaxies at 10.8 microns shows that the infrared emission is also centrally concentrated. Many of the infrared-powerful S0 galaxies are Markarian galaxies. In only one case in this sample is the powerful ultraviolet emission known to be generated by a Seyfert nucleus. Optical spectra of the central few kiloparsecs of these S0 galaxies generally show deep Balmer absorption lines characteristic of A stars, and H beta emission suggestive of gas heated by O stars. A key question to our understanding of these galaxies is whether they really are S0 galaxies, or at least would have been recognized as S0 galaxies before the episode of central star formation began. Some of Nilson's classifications (used here) have been confirmed by Sandage or de Vaucouleurs and collaborators from better plates; some of the galaxies may be misclassified Sa galaxies (the most frequent hosts of central star formation); some are apparently difficult to classify because of mixed characteristics, faint non-S0 features, or peculiarities. More optical imaging is needed to characterize the host galaxies and to study the evolution of their star-forming regions
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