502 research outputs found
Coherence lifetimes of excitations in an atomic condensate due to the thin spectrum
We study the quantum coherence properties of a finite sized atomic condensate
using a toy-model and the thin spectrum model formalism. The decoherence time
for a condensate in the ground state, nominally taken as a variational symmetry
breaking state, is investigated for both zero and finite temperatures. We also
consider the lifetimes for Bogoliubov quasi-particle excitations, and contrast
them to the observability window determined by the ground state coherence time.
The lifetimes are shown to exhibit a general characteristic dependence on the
temperature, determined by the thin spectrum accompanying the spontaneous
symmetry breaking ground state
Magnetodielectric coupling and phonon properties of compressively strained EuTiO3 thin films deposited on LSAT
Compressively strained epitaxial (001) EuTiO3 thin films of tetragonal
symmetry have been deposited on (001) (LaAlO3)_0.29-(SrAl_{1/2}Ta_{1/2}O3)_0.71
(LSAT) substrates by reactive molecular-beam epitaxy. Enhancement of the Neel
temperature by 1 K with 0.9% compressive strain was revealed. The polar phonons
ofthe films have been investigated as a function of temperature and magnetic
field by means of infrared reflectance spectroscopy. All three infrared active
phonons show strongly stiffened frequencies compared to bulk EuTiO3 in
accordance with first principles calculations. The phonon frequencies exhibit
gradual softening on cooling leading to an increase in static permittivity. A
new polar phonon with frequency near the TO1 soft mode was detected below 150
K. The new mode coupled with the TO1 mode was assigned as the optical phonon
from the Brillouin zone edge, which is activated in infrared spectra due to an
antiferrodistortive phase transition and due to simultaneous presence of polar
and/or magnetic nanoclusters. In the antiferromagnetic phase we have observed a
remarkable softening of the lowest-frequency polar phonon under an applied
magnetic field, which qualitatively agrees with first principles calculations.
This demonstrates the strong spin-phonon coupling in EuTiO3, which is
responsible for the pronounced dependence of its static permittivity on
magnetic field in the antiferromagnetic phase.Comment: Submitted to Phys. Rev.
Optimal pooling for genome re-sequencing with ultra-high-throughput short-read technologies
New generation sequencing technologies offer unique opportunities and challenges for re-sequencing studies. In this article, we focus on re-sequencing experiments using the Solexa technology, based on bacterial artificial chromosome (BAC) clones, and address an experimental design problem. In these specific experiments, approximate coordinates of the BACs on a reference genome are known, and fine-scale differences between the BAC sequences and the reference are of interest. The high-throughput characteristics of the sequencing technology makes it possible to multiplex BAC sequencing experiments by pooling BACs for a cost-effective operation. However, the way BACs are pooled in such re-sequencing experiments has an effect on the downstream analysis of the generated data, mostly due to subsequences common to multiple BACs. The experimental design strategy we develop in this article offers combinatorial solutions based on approximation algorithms for the well-known max n-cut problem and the related max n-section problem on hypergraphs. Our algorithms, when applied to a number of sample cases give more than a 2-fold performance improvement over random partitioning
Assessing the capability of three different altimetry satellite missions to observe the Northern Current by using a high-resolution model
Over the last 3 decades, satellite altimetry has observed sea surface
height variations, providing a regular monitoring of the surface ocean
circulation. Altimetry measurements have an intrinsic signal-to-noise ratio
that limits the spatial scales of the currents that can be captured. However,
the recent progress made on both altimetry sensors and data processing allows
us to observe smaller geophysical signals, offering new perspectives in
coastal areas where these structures are important.
In this methodological study, we assess the ability of three altimeter
missions with three different technologies to capture the Northern Current
(northwestern Mediterranean Sea) and its variability, namely Jason-2 (Ku-band low-resolution-mode altimeter, launched in 2008), SARAL/AltiKa (Ka-band low-resolution-mode altimeter, launched in 2013) and Sentinel-3A (synthetic
aperture radar altimeter, launched in 2016). Therefore, we use a
high-resolution regional model as a reference.
We focus along the French coast of Provence, where we first show that the
model is very close to the observations of high-frequency radars and gliders
in terms of surface current estimates.
In the model, the Northern Current is observed 15â20âkm from the coast on
average, with a mean core velocity of 0.39âmâsâ1. Its signature in terms of sea
level consists of a drop whose mean value at 6.14ââE is 6.9âcm,
extending over 20âkm. These variations show a clear seasonal pattern, but
high-frequency signals are also present most of the time. In comparison, in
1âHz altimetry data, the mean sea level drop associated with the Northern
Current is overestimated by 3.0âcm for Jason-2, but this overestimation is significantly less with
SARAL/AltiKa and Sentinel-3A (0.3 and 1.4âcm respectively). In terms of
corresponding sea level variability, Jason-2 and SARAL altimetry estimates
are larger than the model reference (+1.3 and +1âcm respectively),
whereas Sentinel-3A shows closer values (â0.4âcm). When we derive
geostrophic surface currents from the satellite sea level variations
without any data filtering, in comparison to the model, the standard
deviations of the velocity values are also very different from one mission to the
other (3.7 times too large for Jason-2 but 2.4 and 2.9 times too large for
SARAL and Sentinel-3A respectively). When low-pass filtering altimetry sea
level data with different cutoff wavelengths, the best agreement between the
model and the altimetry distributions of velocity values are obtained with a
60, 30 and 40â50âkm cutoff wavelength for Jason-2, SARAL and
Sentinel-3A data respectively. This study shows that using a high-resolution model as a reference for altimetry data allows us not only to
illustrate how the advances in the performances of altimeters and in the
data processing improve the observation of coastal currents but also to
quantify the corresponding gain.</p
Microstructural evolution under low shear rates during Rheo processing of LM25 alloy
© ASM InternationalMicrostructural features of LM25 alloy processed by two different routes: (1) conventional casting, and(2)shear casting based on inclined heated surface are studied. The microstructures of the primary phase for the shear-cast samples show rosette or ellipsoidal morphologies. Heat transfer of contacting melt with the
inclined tube surface and shear stress exerted on the layers of the melt as result of gravitational force are crucial parameters for the microstructural evolution. Compared to those produced by conventional casting, shear-cast samples have a much improved tensile strength and ductility due to globular microstructure
Joint analysis of coastal altimetry and high-frequency (HF)Â radar data: observability of seasonal and mesoscale ocean dynamics in the Bay of Biscay
Land-based coastal high-frequency (HF) radar systems provide operational measurements of
coastal surface currents (within 1â3 m depth) with high spatial
(300 mâ10 km) and temporal (ââ€â1 h) sampling resolutions, while the
near-continuous altimetry missions provide information, from 1993 until today,
on geostrophic currents in the global ocean with typical along-track and
temporal sampling resolutions of â>â7 km and â>â9Â days, respectively. During
the last years, the altimetry community has made a step forward in improving
these data in the coastal area, where the data present lower quality than in
the open ocean. The combination of HF radar and altimetry measurements arises
as a promising strategy to improve the continuous monitoring of the coastal
area (e.g. by expanding the measurements made by HF radars to adjacent areas
covered by the altimetry or by validating/confirming improvements brought by
specific coastal algorithms or new altimeter missions). A first step towards
this combination is the comparison of both data sets in overlapping areas.In this study, a HFÂ radar system and two Jason-2 satellite altimetry
products with different processing are compared over the period from
1 January 2009 to 24 July 2015. The results provide an evaluation of the
performance of different coastal altimetry data sets within the study area
and a better understanding of the ocean variability contained in the
HFÂ radar and altimetry data sets. Both observing systems detect the main
mesoscale processes within the study area (the Iberian Poleward Current and
mesoscale eddies), and the highest correlations between radar and altimetry
(up to 0.64) occur in the slope where the Iberian Poleward Current
represents a significant part of the variability in the circulation.
Besides, the use of an Ekman model, to add the wind-induced current
component to the altimetry-derived geostrophic currents, increases the
agreement between both data sets (increasing the correlation by around 10 %).</p
Coherent Pair Production by Photons in the 20-170 GeV Energy Range Incident on Crystals and Birefringence
The cross section for coherent pair production by linearly polarised photons
in the 20-170 GeV energy range was measured for photon aligned incidence on
ultra-high quality diamond and germanium crystals. The theoretical description
of coherent bremsstrahlung and coherent pair production phenomena is an area of
active theoretical debate and development. However, under our experimental
conditions, the theory predicted the combined cross section and polarisation
experimental observables very well indeed. In macroscopic terms, our experiment
measured a birefringence effect in pair production in a crystal. This study of
this effect also constituted a measurement of the energy dependent linear
polarisation of photons produced by coherent bremsstrahlung in aligned
crystals. New technologies for manipulating high energy photon beams can be
realised based on an improved understanding of QED phenomena at these energies.
In particular, this experiment demonstrates an efficient new polarimetry
technique. The pair production measurements were done using two independent
methods simultaneously. The more complex method using a magnet spectrometer
showed that the simpler method using a multiplicity detector was also viable.Comment: 10 pages, 13 figures, 1 table, REVTeX4 two column, Version for
publicatio
Linear to Circular Polarisation Conversion using Birefringent Properties of Aligned Crystals for Multi-GeV Photons
We present the first experimental results on the use of a thick aligned Si
crystal acting as a quarter wave plate to induce a degree of circular
polarisation in a high energy linearly polarised photon beam. The linearly
polarised photon beam is produced from coherent bremsstrahlung radiation by 178
GeV unpolarised electrons incident on an aligned Si crystal, acting as a
radiator. The linear polarisation of the photon beam is characterised by
measuring the asymmetry in electron-positron pair production in a Ge crystal,
for different crystal orientations. The Ge crystal therefore acts as an
analyser. The birefringence phenomenon, which converts the linear polarisation
to circular polarisation, is observed by letting the linearly polarised photons
beam pass through a thick Si quarter wave plate crystal, and then measuring the
asymmetry in electron-positron pair production again for a selection of
relative angles between the crystallographic planes of the radiator, analyser
and quarter wave plate. The systematics of the difference between the measured
asymmetries with and without the quarter wave plate are predicted by theory to
reveal an evolution in the Stokes parameters from which the appearance of a
circularly polarised component in the photon beam can be demonstrated. The
measured magnitude of the circularly polarised component was consistent with
the theoretical predictions, and therefore is in indication of the existence of
the birefringence effect.Comment: 12 pages, 12 figures, 1 table, REVTeX4 two column, Version for
publicatio
Results on the Coherent Interaction of High Energy Electrons and Photons in Oriented Single Crystals
The CERN-NA-59 experiment examined a wide range of electromagnetic processes
for multi-GeV electrons and photons interacting with oriented single crystals.
The various types of crystals and their orientations were used for producing
photon beams and for converting and measuring their polarisation.
The radiation emitted by 178 GeV unpolarised electrons incident on a 1.5 cm
thick Si crystal oriented in the Coherent Bremsstrahlung (CB) and the
String-of-Strings (SOS) modes was used to obtain multi-GeV linearly polarised
photon beams.
A new crystal polarimetry technique was established for measuring the linear
polarisation of the photon beam. The polarimeter is based on the dependence of
the Coherent Pair Production (CPP) cross section in oriented single crystals on
the direction of the photon polarisation with respect to the crystal plane.
Both a 1 mm thick single crystal of Germanium and a 4 mm thick multi-tile set
of synthetic Diamond crystals were used as analyzers of the linear
polarisation.
A birefringence phenomenon, the conversion of the linear polarisation of the
photon beam into circular polarisation, was observed. This was achieved by
letting the linearly polarised photon beam pass through a 10 cm thick Silicon
single crystal that acted as a "quarter wave plate" (QWP) as suggested by N.
Cabibbo et al.Comment: Presented at International workshop "Relativistic Channeling and
Related Coherent Phenomena", Frascati (Rome) 23-26 March 200
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