5,124 research outputs found
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
Dendritic flux patterns in MgB2 films
Magneto-opitcal studies of a c-oriented epitaxial MgB2 film with critical
current density 10^7 A/cm^2 demonstrate a breakdown of the critical state at
temperatures below 10 K [cond-mat/0104113]. Instead of conventional uniform and
gradual flux penetration in an applied magnetic field, we observe an abrupt
invasion of complex dendritic structures. When the applied field subsequently
decreases, similar dendritic structures of the return flux penetrate the film.
The static and dynamic properties of the dendrites are discussed.Comment: Accepted to Supercond. Sci. Techno
Evidence of discrete scale invariance in DLA and time-to-failure by canonical averaging
Discrete scale invariance, which corresponds to a partial breaking of the
scaling symmetry, is reflected in the existence of a hierarchy of
characteristic scales l0, c l0, c^2 l0,... where c is a preferred scaling ratio
and l0 a microscopic cut-off. Signatures of discrete scale invariance have
recently been found in a variety of systems ranging from rupture, earthquakes,
Laplacian growth phenomena, ``animals'' in percolation to financial market
crashes. We believe it to be a quite general, albeit subtle phenomenon. Indeed,
the practical problem in uncovering an underlying discrete scale invariance is
that standard ensemble averaging procedures destroy it as if it was pure noise.
This is due to the fact, that while c only depends on the underlying physics,
l0 on the contrary is realisation-dependent. Here, we adapt and implement a
novel so-called ``canonical'' averaging scheme which re-sets the l0 of
different realizations to approximately the same value. The method is based on
the determination of a realization-dependent effective critical point obtained
from, e.g., a maximum susceptibility criterion. We demonstrate the method on
diffusion limited aggregation and a model of rupture.Comment: 14 pages, 6 figures, in press in Int. J. Mod. Phys.
Current-induced dendritic magnetic instability in superconducting MgB2 films
Magneto-optical imaging reveals that in superconducting films of MgB2 a
transport current creates avalanche-like flux dynamics where highly branching
dendritic penetration patterns are formed. The instability is triggered when
the current exceeds a threshold value, and the superconductor, shaped as a long
strip, is initially in the critical state. The instability exists up to 19 K,
which is a much wider temperature range than in previous experiments, where
dendrites were formed by applying a magnetic field. The instability is believed
to be of thermo-magnetic origin indicating that thermal stabilization may
become crucial in applications of MgB2.Comment: 3 pages, 3 figures, resubmitted to Appl.Phys.Let
Shape Distortion by Irreversible Flux-Pinning-Induced Magnetostriction
Exact analytical results are obtained for the flux-pinning-induced
magnetostriction in cylindrical type-II superconductors placed in parallel
magnetic field. New modes of irreversible deformation are found: In contrast to
the circular cylinder where shape is conserved, it is shown that a square
cross-section deforms with considerable distortion. During a field cycle both
concave, convex, and even more complicated distortions are predicted. Strong
implications for dilatometric measurements on crystals are emphasized. The main
results are valid for any critical-state model, j_c = j_c(B).Comment: 4 pages, 4 graph
The role of antibiotics in the treatment of chronic prostatitis: A consensus statement
Practical guidelines for the diagnosis and treatment of chronic prostatitis are presented. Chronic prostatitis is classified as chronic bacterial prostatitis (culture-positive) and chronic inflammatory prostatitis (culture-negative). If chronic bacterial prostatitis is suspected, based on relevant symptoms or recurrent UTIs, underlying urological conditions should be excluded by the following tests: rectal examination, midstream urine culture and residual urine. The diagnosis should be confirmed by the Meares and Stamey technique. Antibiotic therapy is recommended for acute exacerbations of chronic prostatitis, chronic bacterial prostatitis and chronic inflammatory prostatitis, if there is clinical, bacteriological or supporting immunological evidence of prostate infection. Unless a patient presents with fever, antibiotic treatment should not be initiated immediately except in cases of acute prostatitis or acute episodes in a patient with chronic bacterial prostatitis. The work-up, with the appropriate investigations should be done first, within a reasonable time period which, preferably, should not be longer than 1 week. During this period, nonspecific treatment, such as appropriate analgesia to relieve symptoms, should be given. The minimum duration of antibiotic treatment should be 2-4 weeks. If there is no improvement in symptoms, treatment should be stopped and reconsidered. However, if there is improvement, it should be continued for at least a further 2-4 weeks to achieve clinical cure and, hopefully, eradication of the causative pathogen. Antibiotic treatment should not be given for 6-8 weeks without an appraisal of its effectiveness. Currently used antibiotics are reviewed. Of these, the fluoroquinolones ofloxacin and ciprofloxacin are recommended because of their favourable antibacterial spectrum and pharmacokinetic profile. A number of clinical trials are recommended and a standard study design is proposed to help resolve some outstanding issues
Following autophagy step by step
Autophagy is an evolutionarily conserved lysosomal degradation route for soluble components of the cytosol and organelles. There is great interest in identifying compounds that modulate autophagy because they may have applications in the treatment of major diseases including cancer and neurodegenerative disease. Hundeshagen and colleagues describe this month in BMC Biology a screening assay based on flow cytometry that makes it possible to track distinct steps in the autophagic process and thereby identify novel modulators of autophagy
Theory of Networked Minority Games based on Strategy Pattern Dynamics
We formulate a theory of agent-based models in which agents compete to be in
a winning group. The agents may be part of a network or not, and the winning
group may be a minority group or not. The novel feature of the present
formalism is its focus on the dynamical pattern of strategy rankings, and its
careful treatment of the strategy ties which arise during the system's temporal
evolution. We apply it to the Minority Game (MG) with connected populations.
Expressions for the mean success rate among the agents and for the mean success
rate for agents with neighbors are derived. We also use the theory to
estimate the value of connectivity above which the Binary-Agent-Resource
system with high resource level goes into the high-connectivity state.Comment: 24 pages, 3 figures, submitted to PR
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.
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
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