541 research outputs found
Strain analysis of multiferroic BiFeO3-CoFe2O4 nanostructures by Raman scattering
We report a Raman scattering investigation of columnar BiFeO3-CoFe2O4
(BFO-CFO) epitaxial thin film nanostructures, where BFO pillars are embedded in
a CFO matrix. The feasibility of a strain analysis is illustrated through an
investigation of two nanostructures with different BFO-CFO ratios. We show that
the CFO matrix presents the same strain state in both nanostructures, while the
strain state of the BFO pillars depends on the BFO/CFO ratio with an increasing
tensile strain along the out-of-plane direction with decreasing BFO content.
Our results demonstrate that Raman scattering allows monitoring strain states
in complex 3D multiferroic pillar/matrix composites.Comment: revised version submitted to Appl. Phys. Let
Phonons in the multiferroic langasite BaNbFeSiO : evidences for symmetry breaking
The chiral langasite BaNbFeSiO is a multiferroic
compound. While its magnetic order below T=27 K is now well characterised,
its polar order is still controversial. We thus looked at the phonon spectrum
and its temperature dependence to unravel possible crystal symmetry breaking.
We combined optical measurements (both infrared and Raman spectroscopy) with ab
initio calculations and show that signatures of a polar state are clearly
present in the phonon spectrum even at room temperature. An additional symmetry
lowering occurs below 120~K as seen from emergence of softer phonon modes in
the THz range. These results confirm the multiferroic nature of this langasite
and open new routes to understand the origin of the polar state
Lattice and spin excitations in multiferroic h-YMnO3
We used Raman and terahertz spectroscopies to investigate lattice and
magnetic excitations and their cross-coupling in the hexagonal YMnO3
multiferroic. Two phonon modes are strongly affected by the magnetic order.
Magnon excitations have been identified thanks to comparison with neutron
measurements and spin wave calculations but no electromagnon has been observed.
In addition, we evidenced two additional Raman active peaks. We have compared
this observation with the anti-crossing between magnon and acoustic phonon
branches measured by neutron. These optical measurements underly the unusual
strong spin-phonon coupling
Construction of the Pauli-Villars-regulated Dirac vacuum in electromagnetic fields
Using the Pauli-Villars regularization and arguments from convex analysis, we
construct solutions to the classical time-independent Maxwell equations in
Dirac's vacuum, in the presence of small external electromagnetic sources. The
vacuum is not an empty space, but rather a quantum fluctuating medium which
behaves as a nonlinear polarizable material. Its behavior is described by a
Dirac equation involving infinitely many particles. The quantum corrections to
the usual Maxwell equations are nonlinear and nonlocal. Even if photons are
described by a purely classical electromagnetic field, the resulting vacuum
polarization coincides to first order with that of full Quantum
Electrodynamics.Comment: Final version to appear in Arch. Rat. Mech. Analysi
Potentiel du contrôle ultrasonore d’une plaque masquée par des écrans parallèles
National audienceThe in-service inspection of internal structures of future liquid sodium-cooled fast reactors isan important issue of the CEA. The ultrasonic methods can overcome the sodium opacity,are insensitive to radiation and are adapted to high temperatures. Moreover, they allow tocontrol from outside the circuit sodium; this communication regards this point. Thepropagation of ultrasounds is studied in a plate masked by n plates immersed in liquidsodium to control it. The need to obtain a maximum of propagating energy directed the studytowards Lamb waves in layered solid-fluid structures. The model developed is based on themethod of transfer matrix and leads to the optimization of the pair (angle, frequency). Anexperimental study validates the model on a set of stainless steel plates immersed in water.The possibilities for the control of a plate behind n screens are then analyzed and discussed.L'inspection en service de structures internes des réacteurs à neutrons rapides refroidis ausodium liquide constitue une problématique importante au CEA. Les méthodes ultrasonorespermettent de s’affranchir du caractère opaque du sodium, sont insensibles au rayonnementet sont adaptées aux hautes températures. De plus, elles permettent des contrôles depuisl’extérieur du circuit sodium ; c’est dans ce cadre que s’insère cette communication. Il s’agitd’étudier la propagation des ultrasons dans une plaque masquée par n plaques immergéesen sodium afin de la contrôler. La nécessité de propager le maximum d'énergie a orientél'étude vers les ondes de Lamb dans des structures multicouches solide-fluide. Lamodélisation repose sur la méthode des matrices de transfert et conduit à l’optimisation ducouple (angle, fréquence). Une étude expérimentale valide ce modèle sur un ensemble deplaques d’acier inoxydable immergées dans l’eau. Les possibilités de contrôle d’une plaquederrière n écrans sont analysées et discutées
Genetic diversity and the emergence of ethnic groups in Central Asia
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licens
Magnetic and structural properties of the iron silicide superconductor LaFeSiH
The magnetic and structural properties of the recently discovered
pnictogen/chalcogen-free superconductor LaFeSiH (~K) have been
investigated by Fe synchrotron M{\"o}ssbauer source (SMS) spectroscopy,
x-ray and neutron powder diffraction and Si nuclear magnetic resonance
spectroscopy (NMR). No sign of long range magnetic order or local moments has
been detected in any of the measurements and LaFeSiH remains tetragonal down to
2 K. The activated temperature dependence of both the NMR Knight shift and the
relaxation rate is analogous to that observed in strongly overdoped
Fe-based superconductors. These results, together with the
temperature-independent NMR linewidth, show that LaFeSiH is an homogeneous
metal, far from any magnetic or nematic instability, and with similar Fermi
surface properties as strongly overdoped iron pnictides. This raises the
prospect of enhancing the of LaFeSiH by reducing its carrier
concentration through appropriate chemical substitutions. Additional SMS
spectroscopy measurements under hydrostatic pressure up to 18.8~GPa found no
measurable hyperfine field
SMARTPOP: Inferring the impact of social dynamics on genetic diversity through high speed simulations
Background: Social behavior has long been known to influence patterns of genetic diversity, but the effect of social processes on population genetics remains poorly quantified - partly due to limited community-level genetic sampling (which is increasingly being remedied), and partly to a lack of fast simulation software to jointly model genetic evolution and complex social behavior, such as marriage rules.Results: To fill this gap, we have developed SMARTPOP - a fast, forward-in-time genetic simulator - to facilitate large-scale statistical inference on interactions between social factors, such as mating systems, and population genetic diversity. By simultaneously modeling genetic inheritance and dynamic social processes at the level of the individual, SMARTPOP can simulate a wide range of genetic systems (autosomal, X-linked, Y chromosomal and mitochondrial DNA) under a range of mating systems and demographic models. Specifically designed to enable resource-intensive statistical inference tasks, such as Approximate Bayesian Computation, SMARTPOP has been coded in C++ and is heavily optimized for speed and reduced memory usage.Conclusion: SMARTPOP rapidly simulates population genetic data under a wide range of demographic scenarios and social behaviors, thus allowing quantitative analyses to address complex socio-ecological questions. © 2014 Guillot and Cox; licensee BioMed Central Ltd
A Randomised Trial Comparing Genotypic and Virtual Phenotypic Interpretation of HIV Drug Resistance: The CREST Study
OBJECTIVES: The aim of this study was to compare the efficacy of different HIV drug resistance test reports (genotype and virtual phenotype) in patients who were changing their antiretroviral therapy (ART). DESIGN: Randomised, open-label trial with 48-week followup. SETTING: The study was conducted in a network of primary healthcare sites in Australia and New Zealand. PARTICIPANTS: Patients failing current ART with plasma HIV RNA > 2000 copies/mL who wished to change their current ART were eligible. Subjects were required to be > 18 years of age, previously treated with ART, have no intercurrent illnesses requiring active therapy, and to have provided written informed consent. INTERVENTIONS: Eligible subjects were randomly assigned to receive a genotype (group A) or genotype plus virtual phenotype (group B) prior to selection of their new antiretroviral regimen. OUTCOME MEASURES: Patient groups were compared for patterns of ART selection and surrogate outcomes (plasma viral load and CD4 counts) on an intention-to-treat basis over a 48-week period. RESULTS: Three hundred and twenty seven patients completing > one month of followup were included in these analyses. Resistance tests were the primary means by which ART regimens were selected (group A: 64%, group B: 62%; p = 0.32). At 48 weeks, there were no significant differences between the groups for mean change from baseline plasma HIV RNA (group A: 0.68 log copies/mL, group B: 0.58 log copies/mL; p = 0.23) and mean change from baseline CD4+ cell count (group A: 37 cells/mm(3), group B: 50 cells/mm(3); p = 0.28). CONCLUSIONS: In the absence of clear demonstrated benefits arising from the use of the virtual phenotype interpretation, this study suggests resistance testing using genotyping linked to a reliable interpretive algorithm is adequate for the management of HIV infection
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