1,115 research outputs found
Evidence of diffusive fractal aggregation of TiO2 nanoparticles by femtosecond laser ablation at ambient conditions
The specific mechanisms which leads to the formation of fractal
nanostructures by pulsed laser deposition remain elusive despite intense
research efforts, motivated mainly by the technological interest in obtaining
tailored nanostructures with simple and scalable production methods. Here we
focus on fractal nanostructures of titanium dioxide, , a strategic
material for many applications, obtained by femtosecond laser ablation at
ambient conditions. We model the fractal formation through extensive Monte
Carlo simulations based on a set of minimal assumptions: irreversible sticking
and size independent diffusion. Our model is able to reproduce the fractal
dimensions and the area distributions of the nanostructures obtained in the
experiments for different densities of the ablated material. The comparison of
theory and experiment show that such fractal aggregates are formed after
landing of the ablated material on the substrate surface by a diffusive
mechanism. Finally we discuss the role of the thermal conductivity of the
substrate and the laser fluence on the properties of the fractal
nanostructures. Our results represent an advancement towards controlling the
production of fractal nanostructures by pulsed laser deposition.Comment: 21 page
Effect of phase noise on useful quantum correlations in Bose Josephson junctions
In a two-mode Bose Josephson junction the dynamics induced by a sudden quench
of the tunnel amplitude leads to the periodic formation of entangled states.
For instance, squeezed states are formed at short times and macroscopic
superpositions of phase states at later times. The two modes of the junction
can be viewed as the two arms of an interferometer; use of entangled states
allows to perform atom interferometry beyond the classical limit. Decoherence
due to the presence of noise degrades the quantum correlations between the
atoms, thus reducing phase sensitivity of the interferometer. We consider the
noise induced by stochastic fluctuations of the energies of the two modes of
the junction. We analyze its effect on squeezed states and macroscopic
superpositions and study quantitatively the amount of quantum correlations
which can be used to enhance the phase sensitivity with respect to the
classical limit. To this aim we compute the squeezing parameter and the quantum
Fisher information during the quenched dynamics. For moderate noise intensities
we show that these useful quantum correlations increase on time scales beyond
the squeezing regime. This suggests multicomponent superpositions as
interesting candidates for high-precision atom interferometry
Noise in Bose Josephson junctions: Decoherence and phase relaxation
Squeezed states and macroscopic superpositions of coherent states have been
predicted to be generated dynamically in Bose Josephson junctions. We solve
exactly the quantum dynamics of such a junction in the presence of a classical
noise coupled to the population-imbalance number operator (phase noise),
accounting for, for example, the experimentally relevant fluctuations of the
magnetic field. We calculate the correction to the decay of the visibility
induced by the noise in the non-Markovian regime. Furthermore, we predict that
such a noise induces an anomalous rate of decoherence among the components of
the macroscopic superpositions, which is independent of the total number of
atoms, leading to potential interferometric applications.Comment: Fig 2 added; version accepted for publicatio
Eco-friendly gas mixtures for Resistive Plate Chambers based on Tetrafluoropropene and Helium
Due to the recent restrictions deriving from the application of the Kyoto
protocol, the main components of the gas mixtures presently used in the
Resistive Plate Chambers systems of the LHC experiments will be most probably
phased out of production in the coming years. Identifying possible replacements
with the adequate characteristics requires an intense R&D, which was recently
started, also in collaborations across the various experiments. Possible
candidates have been proposed and are thoroughly investigated. Some tests on
one of the most promising candidate - HFO-1234ze, an allotropic form of
tetrafluoropropane- have already been reported. Here an innovative approach,
based on the use of Helium, to solve the problems related to the too elevate
operating voltage of HFO-1234ze based gas mixtures, is discussed and the
relative first results are shown.Comment: 9 pages, 6 figures, 1 tabl
An improvement of SPME-based sampling technique to collect volatile organic compounds from Quercus Ilex at the environmental level
Biogenic Volatile Organic Compounds (BVOCs) include many chemical compounds emitted by plants into the atmosphere. These compounds have a great effect on biosphere–atmosphere interactions and may affect the concentration of atmospheric pollutants, with further consequences on human health and forest ecosystems. Novel methods to measure and determine BVOCs in the atmosphere are of compelling importance considering the ongoing climate changes. In this study, we developed a fast and easy-to-handle analytical methodology to sample these compounds in field experiments using solid-phase microextraction (SPME) fibers at the atmospheric level. An improvement of BVOCs adsorption from SPME fibers was obtained by coupling the fibers with fans to create a dynamic sampling system. This innovative technique was tested sampling Q. ilex BVOCs in field conditions in comparison with the conventional static SPME sampling technique. The results showed a great potential of this dynamic sampling system to collect BVOCs at the atmosphere level, improving the efficiency and sensitivity of SPME fibers. Indeed, our novel device was able to reduce the sampling time, increase the amount of BVOCs collected through the fibers and add information regarding the emissions of these compounds at the environmental level
Characterizing stellar populations in spiral disks
It is now possible to measure detailed spectral indices for stellar
populations in spiral disks. We propose to interpret these data using
evolutionary synthesis models computed from the Star Formation Histories
obtained from chemical evolutionary models. We find that this technique is a
powerful tool to discriminate between old and young stellar populations. We
show an example of the power of Integral Field spectroscopy in unveiling the
spatial distribution of populations in a barred galaxy.Comment: 5 pages, to be published in "Science Perspectives for 3D
Spectroscopy", Eds. M. Kissler-Patig, M.M. Roth and J.R. Walsh
(Springer-Verlag, ESO astrophysics symposia series
An Update on the Phenotype, Genotype and Neurobiology of ADCY5-Related Disease.
Adenylyl cyclase 5 (ADCY5)-related phenotypes comprise an expanding disease continuum, but much remains to be understood about the underlying pathogenic mechanisms of the disease. ADCY5-related disease comprises a spectrum of hyperkinetic disorders involving chorea, myoclonus, and/or dystonia, often with paroxysmal exacerbations. Hypotonia, developmental delay, and intellectual disability may be present. The causative gene encodes adenylyl cyclase, the enzyme responsible for the conversion of adenosine triphosphate (ATP) to cyclic adenosine-3',5'-monophosphate (cAMP). cAMP is a second messenger that exerts a wide variety of effects via several intracellular signaling pathways. ADCY5 is the most commonly expressed isoform of adenylyl cyclase in medium spiny neurons (MSNs) of the striatum, and it integrates and controls dopaminergic signaling. Through cAMP pathway, ADCY5 is a key regulator of the cortical and thalamic signaling that control initiation of voluntary movements and prevention of involuntary movements. Gain-of-function mutations in ADCY5 have been recently linked to a rare genetic disorder called ADCY5-related dyskinesia, where dysregulation of the cAMP pathway leads to reduced inhibitory activity and involuntary hyperkinetic movements. Here, we present an update on the neurobiology of ADCY5, together with a detailed overview of the reported clinical phenotypes and genotypes. Although a range of therapeutic approaches has been trialed, there are currently no disease-modifying treatments. Improved in vitro and in vivo laboratory models will no doubt increase our understanding of the pathogenesis of this rare genetic movement disorder, which will improve diagnosis, and also facilitate the development of precision medicine approaches for this, and other forms of hyperkinesia
Stochastic processes, galactic star formation, and chemical evolution. Effects of accretion, stripping, and collisions in multiphase multi-zone models
This paper reports simulations allowing for stochastic accretion and mass
loss within closed and open systems modeled using a previously developed
multi-population, multi-zone (halo, thick disk, thin disk) treatment. The star
formation rate is computed as a function of time directly from the model
equations and all chemical evolution is followed without instantaneous
recycling. Several types of simulations are presented here: (1) a closed system
with bursty mass loss from the halo to the thick disk, and from the thick to
the thin disk, in separate events to the thin disk; (2) open systems with
random environmental (extragalactic) accretion, e.g. by infall of high velocity
clouds directly to the thin disk; (3) schematic open system single and multiple
collision events and intracluster stripping. For the open models, the mass of
the Galaxy has been explicitly tracked with time. We present the evolution of
the star formation rate, metallicity histories, and concentrate on the light
elements. We find a wide range of possible outcomes, including an explanation
for variations in the Galactic D/H ratio, and highlight the problems for
uniquely reconstructing star forming histories from contemporary abundance
measurements.Comment: 12 pages, 12 Postscript figures, uses A&A style macros. Accepted for
publication by Astronomy & Astrophysic
Influence of participation, facilitator styles, and metacognitive reflection on knowledge building in online university courses
Understanding how to foster knowledge building in online and blended learning environments is a key for computer-supported collaborative learning research. Knowledge building is a deeply constructivist pedagogy and kind of inquiry learning focused on theory building. A strong indicator of engagement in knowledge building activity is the socio-cognitive dynamic of epistemic agency, in which students exercise a higher level of agency for setting forth their ideas and negotiating fit with those of others rather than relying on their teacher. The purpose of this paper is to investigate the influence of (a) levels of participation, (b) facilitator styles and (c) metacognitive reflection on knowledge building in two blended, post-secondary education contexts. A study of a total of 67 undergraduate students suggest that high levels of participation, a supportive facilitator style, and ample opportunities for metacognitive reflection on the students’ own participation strategies are most conducive for fostering epistemic agency for knowledge building. Implications of these results for research and instructional design of online courses are discussed
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