2,693 research outputs found
Fast and rewritable colloidal assembly via field synchronized particle swapping
We report a technique to realize reconfigurable colloidal crystals by using
the controlled motion of particle defects above an externally modulated
magnetic substrate. The transport of particles is induced by applying a uniform
rotating magnetic field to a ferrite garnet film characterized by a periodic
lattice of magnetic bubbles. For filling factor larger than one colloid per
bubble domain, the particle current arises from propagating defects where
particles synchronously exchange their position when passing from one occupied
domain to the next. The amplitude of an applied alternating magnetic field can
be used to displace the excess particles via a swapping mechanism, or to
mobilize the entire colloidal system at a predefined speed
Magnetostrictive behaviour of thin superconducting disks
Flux-pinning-induced stress and strain distributions in a thin disk
superconductor in a perpendicular magnetic field is analyzed. We calculate the
body forces, solve the magneto-elastic problem and derive formulas for all
stress and strain components, including the magnetostriction . The
flux and current density profiles in the disk are assumed to follow the Bean
model. During a cycle of the applied field the maximum tensile stress is found
to occur approximately midway between the maximum field and the remanent state.
An effective relationship between this overall maximum stress and the peak
field is found.Comment: 8 pages, 6 figures, submitted to Supercond. Sci. Technol., Proceed.
of MEM03 in Kyot
Dramatic role of critical current anisotropy on flux avalanches in MgB2 films
Anisotropic penetration of magnetic flux in MgB2 films grown on vicinal
sapphire substrates is investigated using magneto-optical imaging. Regular
penetration above 10 K proceeds more easily along the substrate surface steps,
anisotropy of the critical current being 6%. At lower temperatures the
penetration occurs via abrupt dendritic avalanches that preferentially
propagate {\em perpendicular} to the surface steps. This inverse anisotropy in
the penetration pattern becomes dramatic very close to 10 K where all flux
avalanches propagate in the strongest-pinning direction. The observed behavior
is fully explained using a thermomagnetic model of the dendritic instability.Comment: 4 pages, 5 figure
Finger patterns produced by thermomagnetic instability in superconductors
A linear analysis of thermal diffusion and Maxwell equations is applied to
study the thermomagnetic instability in a type-II superconducting slab. It is
shown that the instability can lead to formation of spatially nonuniform
distributions of magnetic field and temperature. The distributions acquire a
finger structure with fingers perpendicular to the screening current direction.
We derive the criterion for the instability, and estimate its build-up time and
characteristic finger width. The fingering instability emerges when the
background electric field is larger than a threshold field, , and the
applied magnetic field exceeds a value . Numerical
simulations support the analytical results, and allow to follow the development
of the fingering instability beyond the linear regime. The fingering
instability may be responsible for the nucleation of dendritic flux patterns
observed in superconducting films using magneto-optical imaging.Comment: 8 pages, 6 figures, accepted to Phys. Rev. B; (new version: minor
changes
Local threshold field for dendritic instability in superconducting MgB2 films
Using magneto-optical imaging the phenomenon of dendritic flux penetration in
superconducting films was studied. Flux dendrites were abruptly formed in a 300
nm thick film of MgB2 by applying a perpendicular magnetic field. Detailed
measurements of flux density distributions show that there exists a local
threshold field controlling the nucleation and termination of the dendritic
growth. At 4 K the local threshold field is close to 12 mT in this sample,
where the critical current density is 10^7 A/cm^2. The dendritic instability in
thin films is believed to be of thermo-magnetic origin, but the existence of a
local threshold field, and its small value are features that distinctly
contrast the thermo-magnetic instability (flux jumps) in bulk superconductors.Comment: 6 pages, 6 figures, submitted to Phys. Rev.
Flaws in design, analysis and interpretation of Pfizer's antifungal trials of voriconazole and uncritical subsequent quotations
We have previously described how a series of trials sponsored by Pfizer of its antifungal drug, fluconazole, in cancer patients with neutropenia handicapped the control drug, amphotericin B, by flaws in design and analysis. We describe similar problems in two pivotal trials of Pfizer's new antifungal agent, voriconazole, published in a prestigious journal. In a non-inferiority trial, voriconazole was significantly inferior to liposomal amphothericin B, but the authors concluded that voriconazole was a suitable alternative. The second trial used amphothericin B deoxycholate as comparator, but handicapped the drug by not requiring pre-medication to reduce infusion-related toxicity or substitution with electrolytes and fluid to reduce nephrotoxicity, although the planned duration of treatment was 84 days. Voriconazole was given for 77 days on average, but the comparator for only 10 days, which precludes a meaningful comparison. In a random sample of 50 references to these trials, we found that the unwarranted conclusions were mostly uncritically propagated. It was particularly surprising that relevant criticism raised by the FDA related to the first trial was only quoted once, and that none of the articles noted the obvious flaws in the design of the second trial. We suggest that editors ensure that the abstract reflects fairly on the remainder of the paper, and that journals do not impose any time limit for accepting letters that point out serious weaknesses in a study that have not been noted before
BioConcens: Biomass and bioenergy production agriculture – consequences for soil fertility, environment, spread of animal parasites and socio-economy
The research programme called “international research cooperation and organic integrity” was commenced for a period 2006-2010. It is coordinated by DARCOF (The Danish Research Centre for Organic Farming). The whole programme, with acronym DARCOF III, consists of 15 projects (http://www.darcof.dk/research/darcofiii/index.html). One of them is BIOCONCENS - Biomass and bioenergy production in organic farming – consequences for soil fertility, environment, spread of animal parasites and socio-economy (http://www.bioconcens.elr.dk/uk/). The production of bioenergy in organic agriculture (OA) can reduce its dependency of fossil fuels and decrease green house gasses emission; consequently it will increase sustainability of organic farms. Biorefinery concept based on co-production of biogas, bioethanol and protein fodder in organic farming will be developed within the BIOCONCENS project and the background for the project and the different work packages will be presented in this paper
A Model for Prejudiced Learning in Noisy Environments
Based on the heuristics that maintaining presumptions can be beneficial in
uncertain environments, we propose a set of basic axioms for learning systems
to incorporate the concept of prejudice. The simplest, memoryless model of a
deterministic learning rule obeying the axioms is constructed, and shown to be
equivalent to the logistic map. The system's performance is analysed in an
environment in which it is subject to external randomness, weighing learning
defectiveness against stability gained. The corresponding random dynamical
system with inhomogeneous, additive noise is studied, and shown to exhibit the
phenomena of noise induced stability and stochastic bifurcations. The overall
results allow for the interpretation that prejudice in uncertain environments
entails a considerable portion of stubbornness as a secondary phenomenon.Comment: 21 pages, 11 figures; reduced graphics to slash size, full version on
Author's homepage. Minor revisions in text and references, identical to
version to be published in Applied Mathematics and Computatio
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