6,223 research outputs found
Enhanced Zeeman splitting in Ga0.25In0.75As quantum point contacts
The strength of the Zeeman splitting induced by an applied magnetic field is
an important factor for the realization of spin-resolved transport in
mesoscopic devices. We measure the Zeeman splitting for a quantum point contact
etched into a Ga0.25In0.75As quantum well, with the field oriented parallel to
the transport direction. We observe an enhancement of the Lande g-factor from
|g*|=3.8 +/- 0.2 for the third subband to |g*|=5.8 +/- 0.6 for the first
subband, six times larger than in GaAs. We report subband spacings in excess of
10 meV, which facilitates quantum transport at higher temperatures.Comment: [Version 2] Revtex4, 11 pages, 3 figures, accepted for publication in
Applied Physics Letter
CFHT AO Imaging of the CLASS Gravitational Lens System B1359+154
We present adaptive optics imaging of the CLASS gravitational lens system
B1359+154 obtained with the Canada-France-Hawaii Telescope (CFHT) in the
infrared K band. The observations show at least three brightness peaks within
the ring of lensed images, which we identify as emission from multiple lensing
galaxies. The results confirm the suspected compound nature of the lens, as
deduced from preliminary mass modeling. The detection of several additional
nearby galaxies suggests that B1359+154 is lensed by the compact core of a
small galaxy group. We attempted to produce an updated lens model based on the
CFHT observations and new 5-GHz radio data obtained with the MERLIN array, but
there are too few constraints to construct a realistic model at this time. The
uncertainties inherent with modeling compound lenses make B1359+154 a
challenging target for Hubble constant determination through the measurement of
differential time delays. However, time delays will offer additional
constraints to help pin down the mass model. This lens system therefore
presents a unique opportunity to directly measure the mass distribution of a
galaxy group at intermediate redshift.Comment: 12 pages including 3 figures; ApJL accepte
Symmetry of two terminal, non-linear electric conduction
The well-established symmetry relations for linear transport phenomena can
not, in general, be applied in the non-linear regime. Here we propose a set of
symmetry relations with respect to bias voltage and magnetic field for the
non-linear conductance of two-terminal electric conductors. We experimentally
confirm these relations using phase-coherent, semiconductor quantum dots.Comment: 4 pages, 4 figure
CLASS B0827+525: `Dark lens' or binary radio-loud quasar?
We present radio, optical, near-infrared and spectroscopic observations of
the source B0827+525. We consider this source as the best candidate from the
Cosmic Lens All-Sky Survey (CLASS) for a `dark lens' system or binary
radio-loud quasar. The system consists of two radio components with somewhat
different spectral indices, separated by 2.815 arcsec. VLBA observations show
that each component has substructure on a scale of a few mas. A deep K-band
exposure with the W.M.Keck-II Telescope reveals emission near both radio
components. The K-band emission of the weaker radio component appears extended,
whereas the emission from the brighter radio component is consistent with a
point source. Hubble Space Telescope F160W-band observations with the NICMOS
instrument confirms this. A redshift of 2.064 is found for the brighter
component, using the LRIS instrument on the W.M.Keck-II Telescope. The
probability that B0827+525 consists of two unrelated compact flat-spectrum
radio sources is ~3%, although the presence of similar substructure in both
component might reduce this.
We discuss two scenarios to explain this system: (i) CLASS B0827+525 is a
`dark lens' system or (ii) B0827+525 is a binary radio-loud quasar. B0827+525
has met all criteria that thus far have in 100% of the cases confirmed a source
as an indisputable gravitational lens system. Despite this, no lens galaxy has
been detected with m_F160W<=23 mag. Hence, we might have found the first binary
radio-loud quasar. At this moment, however, we feel that the `dark lens'
hypothesis cannot yet be fully excluded.Comment: 9 pages, 6 figures; Accepted for publication in Astronomy &
Astrophysics; Full-res. images 1 and 3 can be obtained from L.V.E.
Probing the Sensitivity of Electron Wave Interference to Disorder-Induced Scattering in Solid-State Devices
The study of electron motion in semiconductor billiards has elucidated our
understanding of quantum interference and quantum chaos. The central assumption
is that ionized donors generate only minor perturbations to the electron
trajectories, which are determined by scattering from billiard walls. We use
magnetoconductance fluctuations as a probe of the quantum interference and show
that these fluctuations change radically when the scattering landscape is
modified by thermally-induced charge displacement between donor sites. Our
results challenge the accepted understanding of quantum interference effects in
nanostructures.Comment: 8 pages, 5 figures, Submitted to Physical Review
Measuring Cosmological Parameters with the JVAS and CLASS Gravitational Lens Surveys
The JVAS (Jodrell Bank-VLA Astrometric Survey) and CLASS (Cosmic Lens All-Sky
Survey) are well-defined surveys containing about ten thousand flat-spectrum
radio sources. For many reasons, flat-spectrum radio sources are particularly
well-suited as a population from which one can obtain unbiased samples of
gravitational lenses. These are by far the largest gravitational (macro)lens
surveys, and particular attention was paid to constructing a cleanly-defined
sample for the survey itself and for the underlying luminosity function. Here
we present the constraints on cosmological parameters, particularly the
cosmological constant, derived from JVAS and combine them with constraints from
optical gravitational lens surveys, `direct' measurements of ,
and the age of the universe, and constraints derived from CMB
anisotropies, before putting this final result into the context of the latest
results from other, independent cosmological tests.Comment: LaTeX, 9 pages, 6 PostScript figures, uses texas.sty. To appear in
the Proceedings of the 19th Texas Symposium on Relativistic Astrophysics and
Cosmology (CD-ROM). Paper version available on request. Actual poster (A0 and
A4 versions) available from
http://multivac.jb.man.ac.uk:8000/helbig/research/publications/info/
texas98.htm
The Sinorhizobium meliloti MsbA2 protein is essential for the legume symbiosis
Sinorhizobium meliloti is a beneficial legume symbiont, closely related to Brucella species, which are chronic mammalian pathogens. We discovered that the S. meliloti MsbA2 protein is essential to ensure the symbiotic interaction with the host plant, alfalfa. S. meliloti invades plant cells via plant-derived structures known as infection threads. However, in the absence of MsbA2, S. meliloti remains trapped within abnormally thickened infection threads and induces a heightened plant defence response, characterized by a substantial thickening of the nodule endodermis layer and the accumulation of polyphenolic compounds. The S. meliloti MsbA2 protein is homologous to the Escherichia coli lipopolysaccharide/phospholipid trafficking protein MsbA. However, MsbA2 was not essential for the membrane transport of either lipopolysaccharide or phospholipids in S. meliloti. We determined that the msbA2 gene is transcribed in free-living S. meliloti and that in the absence of MsbA2 the polysaccharide content of S. meliloti is altered. Consequently, we propose a model whereby the altered polysaccharide content of the S. meliloti msbA2 mutant could be responsible for its symbiotic defect by inducing an inappropriate host response. © 2008 SGM
The new two-image gravitational lens system CLASS B2319+051
We report the discovery of a new two-image gravitational lens system from the
Cosmic Lens All-Sky Survey, CLASS B2319+051. Radio imaging with the Very Large
Array (VLA) and Multi-Element Radio-Linked Interferometer Network (MERLIN)
shows two compact components with a flux density ratio of 5:1, separated by
1.36 arcsec. Observations with the Very Long Baseline Array (VLBA) resolve each
of the radio components into a pair of parity-reversed subcomponents. Hubble
Space Telescope (HST) observations with the Near-Infrared Camera and
Multi-Object Spectrometer (NICMOS) show a bright elliptical galaxy (G1)
coincident with the radio position, and a second irregular galaxy (G2) 3.4
arcsec to the northwest. Previous spectroscopic studies have indicated that
these galaxies are at different redshifts: z(G1) = 0.624, z(G2) = 0.588.
Infrared counterparts to the lensed radio components are not detected in the
NICMOS image, and the source redshift has not yet been determined. Preliminary
mass modeling based on the VLBA subcomponent data indicates that the lensing
potential includes a strong external shear contribution. A VLA monitoring
program is currently being undertaken to measure the differential time delay.Comment: 16 pages, 7 figs, several typos corrected, AJ in press (August 2001
A New Quadruple Gravitational Lens System: CLASS B0128+437
High resolution MERLIN observations of a newly-discovered four-image
gravitational lens system, B0128+437, are presented. The system was found after
a careful re-analysis of the entire CLASS dataset. The MERLIN observations
resolve four components in a characteristic quadruple-image configuration; the
maximum image separation is 542 mas and the total flux density is 48 mJy at 5
GHz. A best-fit lens model with a singular isothermal ellipsoid results in
large errors in the image positions. A significantly improved fit is obtained
after the addition of a shear component, suggesting that the lensing system is
more complex and may consist of multiple deflectors. The integrated radio
spectrum of the background source indicates that it is a GigaHertz-Peaked
Spectrum (GPS) source. It may therefore be possible to resolve structure within
the radio images with deep VLBI observations and thus better constrain the
lensing mass distribution.Comment: Accepted for publication in MNRAS. 4 pages, 3 included PostScript
figure
CLASS B1152+199 and B1359+154: Two New Gravitational Lens Systems Discovered in the Cosmic Lens All-Sky Survey
The third phase of the Cosmic Lens All-Sky Survey (CLASS) has recently been
completed, bringing the total number of sources imaged to over 15000 in the
CLASS and JVAS combined survey. In the VLA observations carried out in March
and April of 1998, two new candidate lensed systems were discovered: CLASS
B1152+199 and B1359+154. B1152+199 is a 1.6 arcsecond double, with a background
quasar at z=1.019 lensed by a foreground galaxy at z=0.439. The relatively flat
radio spectra of the lensed images, combined with a previous ROSAT detection of
the source, make B1152+199 a strong candidate for time delay studies at both
radio and X-ray wavelengths. B1359+154 is a quadruply lensed quasar at z=3.235,
with a maximum image separation of 1.7 arcseconds. As yet, the redshift of the
lensing object in this system is undetermined. The steep spectral index of the
source suggests that B1359+154 will not exhibit strong variability, and is
therefore unlikely to be useful for determining the Hubble constant from
measured time delays.Comment: accepted for publication in The Astronomical Journa
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