745 research outputs found
New Consequences of Induced Transparency in a Double-Lambda scheme: Destructive Interference In Four-wave Mixing
We investigate a four-state system interacting with long and short laser
pulses in a weak probe beam approximation. We show that when all lasers are
tuned to the exact unperturbed resonances, part of the four-wave mixing (FWM)
field is strongly absorbed. The part which is not absorbed has the exact
intensity required to destructively interfere with the excitation pathway
involved in producing the FWM state. We show that with this three-photon
destructive interference, the conversion efficiency can still be as high as
25%. Contrary to common belief,our calculation shows that this process, where
an ideal one-photon electromagnetically induced transparency is established, is
not most suitable for high efficiency conversion. With appropriate
phase-matching and propagation distance, and when the three-photon destructive
interference does not occur, we show that the photon flux conversion efficiency
is independent of probe intensity and can be close to 100%. In addition, we
show clearly that the conversion efficiency is not determined by the maximum
atomic coherence between two lower excited states, as commonly believed. It is
the combination of phase-matching and constructive interference involving the
two terms arising in producing the mixing wave that is the key element for the
optimized FWM generation. Indeed, in this scheme no appreciable excited state
is produced, so that the atomic coherence between states |0> and |2> is always
very small.Comment: Submitted to Phys. Rev. A, 7 pages, 4 figure
Thermally activated resonant tunnelling in GaAs/AlGaAs triple barrier heterostructures
We report the observation of a thermally activated resonant tunnelling feature in the current?voltage characteristics (I(V)) of triple barrier resonant tunnelling structures (TBRTS) due to the alignment of the n = 1 confined states of the two quantum wells within the active region. With great renewed interest in tunnelling structures for high frequency (THz) operation, the understanding of device transport and charge accumulation as a function of temperature is critical. With rising sample temperature, the tunnelling current of the observed low voltage resonant feature increases in magnitude showing a small negative differential resistance region which is discernible even at 293 K and is unique to multiple barrier devices. This behaviour is not observed in conventional double barrier resonant tunnelling structures where the transmission coefficient at the Fermi energy is predominantly controlled by an electric field, whereas in TBRTS it is strongly controlled by the 2D to 2D state alignment
Aerial Measurement of Radioxenon Concentration off the West Coast of Vancouver Island following the Fukushima Reactor Accident
In response to the Fukushima nuclear reactor accident, on March 20th, 2011,
Natural Resources Canada conducted aerial radiation surveys over water just off
of the west coast of Vancouver Island. Dose-rate levels were found to be
consistent with background radiation, however a clear signal due to Xe-133 was
observed. Methods to extract Xe-133 count rates from the measured spectra, and
to determine the corresponding Xe-133 volumetric concentration, were developed.
The measurements indicate that Xe-133 concentrations on average lie in the
range of 30 to 70 Bq/m3.Comment: 14 pages, 5 figures, accepted for publication in Journal of
Environmental Radioactivit
On-microscope staging of live cells reveals changes in the dynamics of transcriptional bursting during differentiation
Determining the mechanisms by which genes are switched on and off during development is a key aim of current biomedical research. Gene transcription has been widely observed to occur in a discontinuous fashion, with short bursts of activity interspersed with periods of inactivity. It is currently not known if or how this dynamic behaviour changes as mammalian cells differentiate. To investigate this, using an on-microscope analysis, we monitored mouse α-globin transcription in live cells throughout erythropoiesis. We find that changes in the overall levels of α-globin transcription are most closely associated with changes in the fraction of time a gene spends in the active transcriptional state. We identify differences in the patterns of transcriptional bursting throughout differentiation, with maximal transcriptional activity occurring in the mid-phase of differentiation. Early in differentiation, we observe increased fluctuation in transcriptional activity whereas at the peak of gene expression, in early erythroblasts, transcription is relatively stable. Later during differentiation as α-globin expression declines, we again observe more variability in transcription within individual cells. We propose that the observed changes in transcriptional behaviour may reflect changes in the stability of active transcriptional compartments as gene expression is regulated during differentiation
Four small puzzles that Rosetta doesn't solve
A complete macromolecule modeling package must be able to solve the simplest
structure prediction problems. Despite recent successes in high resolution
structure modeling and design, the Rosetta software suite fares poorly on
deceptively small protein and RNA puzzles, some as small as four residues. To
illustrate these problems, this manuscript presents extensive Rosetta results
for four well-defined test cases: the 20-residue mini-protein Trp cage, an even
smaller disulfide-stabilized conotoxin, the reactive loop of a serine protease
inhibitor, and a UUCG RNA tetraloop. In contrast to previous Rosetta studies,
several lines of evidence indicate that conformational sampling is not the
major bottleneck in modeling these small systems. Instead, approximations and
omissions in the Rosetta all-atom energy function currently preclude
discriminating experimentally observed conformations from de novo models at
atomic resolution. These molecular "puzzles" should serve as useful model
systems for developers wishing to make foundational improvements to this
powerful modeling suite.Comment: Published in PLoS One as a manuscript for the RosettaCon 2010 Special
Collectio
HARP/ACSIS: A submillimetre spectral imaging system on the James Clerk Maxwell Telescope
This paper describes a new Heterodyne Array Receiver Programme (HARP) and
Auto-Correlation Spectral Imaging System (ACSIS) that have recently been
installed and commissioned on the James Clerk Maxwell Telescope (JCMT). The
16-element focal-plane array receiver, operating in the submillimetre from 325
to 375 GHz, offers high (three-dimensional) mapping speeds, along with
significant improvements over single-detector counterparts in calibration and
image quality. Receiver temperatures are 120 K across the whole band and
system temperatures of 300K are reached routinely under good weather
conditions. The system includes a single-sideband filter so these are SSB
figures. Used in conjunction with ACSIS, the system can produce large-scale
maps rapidly, in one or more frequency settings, at high spatial and spectral
resolution. Fully-sampled maps of size 1 square degree can be observed in under
1 hour.
The scientific need for array receivers arises from the requirement for
programmes to study samples of objects of statistically significant size, in
large-scale unbiased surveys of galactic and extra-galactic regions. Along with
morphological information, the new spectral imaging system can be used to study
the physical and chemical properties of regions of interest. Its
three-dimensional imaging capabilities are critical for research into
turbulence and dynamics. In addition, HARP/ACSIS will provide highly
complementary science programmes to wide-field continuum studies, and produce
the essential preparatory work for submillimetre interferometers such as the
SMA and ALMA.Comment: MNRAS Accepted 2009 July 2. 18 pages, 25 figures and 6 table
The JCMT Gould Belt Survey: Evidence for radiative heating in Serpens MWC 297 and its influence on local star formation
We present SCUBA-2 450micron and 850micron observations of the Serpens MWC
297 region, part of the JCMT Gould Belt Survey of nearby star-forming regions.
Simulations suggest that radiative feedback influences the star-formation
process and we investigate observational evidence for this by constructing
temperature maps. Maps are derived from the ratio of SCUBA-2 fluxes and a two
component model of the JCMT beam for a fixed dust opacity spectral index of
beta = 1.8. Within 40 of the B1.5Ve Herbig star MWC 297, the submillimetre
fluxes are contaminated by free-free emission with a spectral index of
1.03+-0.02, consistent with an ultra-compact HII region and polar winds/jets.
Contamination accounts for 73+-5 per cent and 82+-4 per cent of peak flux at
450micron and 850micron respectively. The residual thermal disk of the star is
almost undetectable at these wavelengths. Young Stellar Objects are confirmed
where SCUBA-2 850micron clumps identified by the fellwalker algorithm coincide
with Spitzer Gould Belt Survey detections. We identify 23 objects and use Tbol
to classify nine YSOs with masses 0.09 to 5.1 Msun. We find two Class 0, one
Class 0/I, three Class I and three Class II sources. The mean temperature is
15+-2K for the nine YSOs and 32+-4K for the 14 starless clumps. We observe a
starless clump with an abnormally high mean temperature of 46+-2K and conclude
that it is radiatively heated by the star MWC 297. Jeans stability provides
evidence that radiative heating by the star MWC 297 may be suppressing clump
collapse.Comment: 24 pages, 13 figures, 7 table
4-Aminopyridine-induced epileptogenesis depends on activation of mitogen-activated protein kinase ERK
Extracellular signal-regulated kinases such as ERK1 [p44
mitogen-activated protein kinase (MAPK)] and ERK2 (p42
MAPK) are activated in the CNS under physiological and
pathological conditions such as ischemia and epilepsy. Here,
we studied the activation state of ERK1/2 in rat hippocampal
slices during application of the K+ channel blocker 4-aminopyridine
(4AP, 50 lM), a procedure that enhances synaptic
transmission and leads to the appearance of epileptiform
activity. Hippocampal slices superfused with 4AP-containing
medium exhibited a marked activation of ERK1/2 phosphorylation
that peaked within about 20 min. These effects
were not accompanied by changes in the activation state of
c-Jun N-terminal kinase (JNK), another member of the MAP
kinase superfamily. 4AP-induced ERK1/2 activation
was inhibited by the voltage-gated Na+ channel blocker
tetrodotoxin (1 lM). We also found that application of the ERK
pathway inhibitors U0126 (50 lM) or PD98059 (100 lM)
markedly reduced 4AP-induced epileptiform synchronization,
thus abolishing ictal discharges in the CA3 area. The effects
induced by U0126 or PD98059 were not associated with
changes in the amplitude and latency of the field potentials
recorded in the CA3 area following electrical stimuli delivered
in the dentate hylus. These data demonstrate that activation of
ERK1/2 accompanies the appearance of epileptiform activity
induced by 4AP and suggest a cause-effect relationship
between the ERK pathway and epileptiform synchronization
Dense molecular gas toward W49A: A template for extragalactic starbursts?
The HCN, HCO+, and HNC molecules are commonly used as tracers of dense
star-forming gas in external galaxies, but such observations are spatially
unresolved. Reliably inferring the properties of galactic nuclei and disks
requires detailed studies of sources whose structure is spatially resolved. We
compare the spatial distributions and abundance ratios of HCN, HCO+, and HNC in
W49A, the most massive and luminous star-forming region in the Galactic disk,
based on maps of a 2' (6.6 pc) field at 14" (0.83 pc) resolution of the J=4-3
transitions of HCN, H13CN, HC15N, HCO+, H13CO+, HC18O+ and HNC. The kinematics
of the molecular gas in W49A appears complex, with a mixture of infall and
outflow motions. Both the line profiles and comparison of the main and rarer
species show that the main species are optically thick. Two 'clumps' of
infalling gas appear to be at ~40 K, compared to ~100 K at the source centre,
and may be ~10x denser than the rest of the outer cloud. Chemical modelling
suggests that the HCN/HNC ratio probes the current gas temperature, while the
HCN/HCO+ ratio and the deuterium fractionation were set during an earlier,
colder phase of evolution. The data suggest that W49A is an appropriate
analogue of an extragalactic star forming region. Our data show that the use of
HCN/HNC/HCO+ line ratios as proxies for the abundance ratios is incorrect for
W49A, suggesting the same for galactic nuclei. Our observed isotopic line
ratios such as H13CN/H13CO+ approach our modeled abundance ratios quite well in
W49A. The 4-3 lines of HCN and HCO+ are much better tracers of the dense
star-forming gas in W49A than the 1-0 lines. Our observed HCN/HNC and HCN/HCO+
ratios in W49A are inconsistent with homogeneous PDR or XDR models, indicating
that irradiation hardly affects the gas chemistry in W49A. Overall, the W49A
region appears to be a useful template for starburst galaxies.Comment: Accepted by A&A; 17 pages, 15 figure
The JCMT Gould Belt Survey: A First Look at the Auriga–California Molecular Cloud with SCUBA-2
We present 850 and 450 μm observations of the dense regions within the Auriga–California molecular cloud using SCUBA-2 as part of the JCMT Gould Belt Legacy Survey to identify candidate protostellar objects, measure the masses of their circumstellar material (disk and envelope), and compare the star formation to that in the Orion A molecular cloud. We identify 59 candidate protostars based on the presence of compact submillimeter emission, complementing these observations with existing Herschel/SPIRE maps. Of our candidate protostars, 24 are associated with young stellar objects (YSOs) in the Spitzer and Herschel/PACS catalogs of 166 and 60 YSOs, respectively (177 unique), confirming their protostellar nature. The remaining 35 candidate protostars are in regions, particularly around LkHα 101, where the background cloud emission is too bright to verify or rule out the presence of the compact 70 μm emission that is expected for a protostellar source. We keep these candidate protostars in our sample but note that they may indeed be prestellar in nature. Our observations are sensitive to the high end of the mass distribution in Auriga–Cal. We find that the disparity between the richness of infrared star-forming objects in Orion A and the sparsity in Auriga–Cal extends to the submillimeter, suggesting that the relative star formation rates have not varied over the Class II lifetime and that Auriga–Cal will maintain a lower star formation efficiency
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