110 research outputs found
A New Shear Estimator for Weak Lensing Observations
We present a new shear estimator for weak lensing observations which properly
accounts for the effects of a realistic point spread function (PSF). Images of
faint galaxies are subject to gravitational shearing followed by smearing with
the instrumental and/or atmospheric PSF. We construct a `finite resolution
shear operator' which when applied to an observed image has the same effect as
a gravitational shear applied prior to smearing. This operator allows one to
calibrate essentially any shear estimator. We then specialize to the case of
weighted second moment shear estimators. We compute the shear polarizability
which gives the response of an individual galaxy's polarization to a
gravitational shear. We then compute the response of the population of
galaxies, and thereby construct an optimal weighting scheme for combining shear
estimates from galaxies of various shapes, luminosities and sizes. We define a
figure of merit --- an inverse shear variance per unit solid angle --- which
characterizes the quality of image data for shear measurement. The new method
is tested with simulated image data. We discuss the correction for anisotropy
of the PSF and propose a new technique involving measuring shapes from images
which have been convolved with a re-circularizing PSF. We draw attention to a
hitherto ignored noise related bias and show how this can be analyzed and
corrected for. The analysis here draws heavily on the properties of real PSF's
and we include as an appendix a brief review, highlighting those aspects which
are relevant for weak lensing.Comment: 39 pages, 9 figure
Tunable Supercurrent Through Semiconductor Nanowires
Nanoscale superconductor-semiconductor hybrid devices are assembled from InAs
semiconductor nanowires individually contacted by aluminum-based superconductor
electrodes. Below 1 K, the high transparency of the contacts gives rise to
proximity-induced superconductivity. The nanowires form superconducting weak
links operating as mesoscopic Josephson junctions with electrically tunable
coupling. The supercurrent can be switched on/off by a gate voltage acting on
the electron density in the nanowire. A variation in gate voltage induces
universal fluctuations in the normal-state conductance which are clearly
correlated to critical current fluctuations. The ac Josephson effect gives rise
to Shapiro steps in the voltage-current characteristic under microwave
irradiation.Comment: 9 pages, 3 figure
Single quantum dot nanowire LEDs
We report reproducible fabrication of InP-InAsP nanowire light emitting
diodes in which electron-hole recombination is restricted to a
quantum-dot-sized InAsP section. The nanowire geometry naturally self-aligns
the quantum dot with the n-InP and p-InP ends of the wire, making these devices
promising candidates for electrically-driven quantum optics experiments. We
have investigated the operation of these nano-LEDs with a consistent series of
experiments at room temperature and at 10 K, demonstrating the potential of
this system for single photon applications
JPCam: A 1.2Gpixel camera for the J-PAS survey
JPCam is a 14-CCD mosaic camera, using the new e2v 9k-by-9k 10microm-pixel
16-channel detectors, to be deployed on a dedicated 2.55m wide-field telescope
at the OAJ (Observatorio Astrofisico de Javalambre) in Aragon, Spain. The
camera is designed to perform a Baryon Acoustic Oscillations (BAO) survey of
the northern sky. The J-PAS survey strategy will use 54 relatively narrow-band
(~13.8nm) filters equi-spaced between 370 and 920nm plus 3 broad-band filters
to achieve unprecedented photometric red-shift accuracies for faint galaxies
over ~8000 square degrees of sky. The cryostat, detector mosaic and read
electronics is being supplied by e2v under contract to J-PAS while the
mechanical structure, housing the shutter and filter assembly, is being
designed and constructed by a Brazilian consortium led by INPE (Instituto
Nacional de Pesquisas Espaciais). Four sets of 14 filters are placed in the
ambient environment, just above the dewar window but directly in line with the
detectors, leading to a mosaic having ~10mm gaps between each CCD. The massive
500mm aperture shutter is expected to be supplied by the Argelander-Institut
fur Astronomie, Bonn. We will present an overview of JPCam, from the filter
configuration through to the CCD mosaic camera. A brief outline of the main
J-PAS science projects will be included.Comment: 11 pages and 9 figure
A DNA microarray survey of gene expression in normal human tissues
BACKGROUND: Numerous studies have used DNA microarrays to survey gene expression in cancer and other disease states. Comparatively little is known about the genes expressed across the gamut of normal human tissues. Systematic studies of global gene-expression patterns, by linking variation in the expression of specific genes to phenotypic variation in the cells or tissues in which they are expressed, provide clues to the molecular organization of diverse cells and to the potential roles of the genes. RESULTS: Here we describe a systematic survey of gene expression in 115 human tissue samples representing 35 different tissue types, using cDNA microarrays representing approximately 26,000 different human genes. Unsupervised hierarchical cluster analysis of the gene-expression patterns in these tissues identified clusters of genes with related biological functions and grouped the tissue specimens in a pattern that reflected their anatomic locations, cellular compositions or physiologic functions. In unsupervised and supervised analyses, tissue-specific patterns of gene expression were readily discernable. By comparative hybridization to normal genomic DNA, we were also able to estimate transcript abundances for expressed genes. CONCLUSIONS: Our dataset provides a baseline for comparison to diseased tissues, and will aid in the identification of tissue-specific functions. In addition, our analysis identifies potential molecular markers for detection of injury to specific organs and tissues, and provides a foundation for selection of potential targets for selective anticancer therapy
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Towards high-resolution as stronomical imaging
This paper is a report from a recent meeting on "the Future of
high-resolution imaging in the visible and infrared", reviewing the
astronomical drivers for development and the technological advances that might
boost performance. Each of the authors listed contributed a section themselves
Supercurrent reversal in quantum dots
When two superconductors become electrically connected by a weak link a
zero-resistance supercurrent can flow. This supercurrent is carried by Cooper
pairs of electrons with a combined charge of twice the elementary charge, e.
The 2e charge quantum is clearly visible in the height of Shapiro steps in
Josephson junctions under microwave irradiation and in the magnetic flux
periodicity of h/2e in superconducting quantum interference devices. Several
different materials have been used to weakly couple superconductors, such as
tunnel barriers, normal metals, or semiconductors. Here, we study supercurrents
through a quantum dot created in a semiconductor nanowire by local
electrostatic gating. Due to strong Coulomb interaction, electrons only tunnel
one-by-one through the discrete energy levels of the quantum dot. This
nevertheless can yield a supercurrent when subsequent tunnel events are
coherent. These quantum coherent tunnelling processes can result in either a
positive or a negative supercurrent, i.e. in a normal or a pi-junction,
respectively. We demonstrate that the supercurrent reverses sign by adding a
single electron spin to the quantum dot. When excited states of the quantum dot
are involved in transport, the supercurrent sign also depends on the character
of the orbital wavefunctions
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