8,991 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
Stochastics theory of log-periodic patterns
We introduce an analytical model based on birth-death clustering processes to
help understanding the empirical log-periodic corrections to power-law scaling
and the finite-time singularity as reported in several domains including
rupture, earthquakes, world population and financial systems. In our
stochastics theory log-periodicities are a consequence of transient clusters
induced by an entropy-like term that may reflect the amount of cooperative
information carried by the state of a large system of different species. The
clustering completion rates for the system are assumed to be given by a simple
linear death process. The singularity at t_{o} is derived in terms of
birth-death clustering coefficients.Comment: LaTeX, 1 ps figure - To appear J. Phys. A: Math & Ge
Oscillatory regimes of the thermomagnetic instability in superconducting films
The stability of superconducting films with respect to oscillatory precursor
modes for thermomag- netic avalanches is investigated theoretically. The
results for the onset threshold show that previous treatments of
non-oscillatory modes have predicted much higher thresholds. Thus, in film
supercon- ductors, oscillatory modes are far more likely to cause
thermomagnetic breakdown. This explains the experimental fact that flux
avalanches in film superconductors can occur even at very small ramping rates
of the applied magnetic field. Closed expressions for the threshold magnetic
field and temperature, as well oscillation frequency, are derived for different
regimes of the oscillatory thermomagnetic instability.Comment: 5 pages, 5 figure
Dendritic flux avalanches in rectangular superconducting films -- numerical simulations
Dendritic flux avalanches is a frequently encountered instability in the
vortex matter of type II superconducting films at low temperatures. Previously,
linear stability analysis has shown that such avalanches should be nucleated
where the flux penetration is deepest. To check this prediction we do numerical
simulations on a superconducting rectangle. We find that at low substrate
temperature the first avalanches appear exactly in the middle of the long
edges, in agreement with the predictions. At higher substrate temperature,
where there are no clear predictions from the theory, we find that the location
of the first avalanche is decided by fluctuations due to the randomly
distributed disorder.Comment: 3 pages, 2 figure
Individual differences in white matter microstructure reflect variation in functional connectivity during action choice.
The relation between brain structure and function is of fundamental importance in neuroscience. Comparisons between behavioral and brain imaging measures suggest that variation in brain structure correlates with the presence of specific skills[1-3]. Behavioral measures, however, reflect the integrated function of multiple brain regions. Rather than behavior, a physiological index of function could be a more sensitive and informative measure with which to compare structural measures. Here, we test for a relationship between a physiological measure of functional connectivity between two brain areas during a simple decision making task and a measure of structural connectivity. Paired-pulse transcranial magnetic stimulation indexed functional connectivity between two regions important for action choices: premotor and motor cortex. Fractional anisotropy (FA), a marker of microstructural integrity, indexed structural connectivity. Individual differences in functional connectivity during action selection show highly specific correlations with FA in localised regions of white matter interconnecting regions including the premotor and motor cortex. Probabilistic tractography[4, 5], a technique for identifying fibre pathways from diffusion-weighted imaging (DWI), reconstructed the anatomical networks linking the component brain regions involved in making decisions. These findings demonstrate a relationship between individual differences in functional and structural connectivity within human brain networks central to action choice
Diversity of flux avalanche patterns in superconducting films
The variety of morphologies in flux patterns created by thermomagnetic
dendritic avalanches in type-II superconducting films is investigated using
numerical simulations. The avalanches are triggered by introducing a hot spot
at the edge of a strip-shaped sample, which is initially prepared in a
partially penetrated Bean critical state by slowly ramping the transversely
applied magnetic field. The simulation scheme is based on a model accounting
for the nonlinear and nonlocal electrodynamics of superconductors in the
transverse geometry. By systematically varying the parameters representing the
Joule heating, heat conduction in the film, and heat transfer to the substrate,
a wide variety of avalanche patterns is formed, and quantitative
characterization of areal extension, branch width etc. is made. The results
show that branching is suppressed by the lateral heat diffusion, while large
Joule heating gives many branches, and heat removal into the substrate limits
the areal size. The morphology shows significant dependence also on the initial
flux penetration depth.Comment: 6 pages, 6 figure
Ray optics in flux avalanche propagation in superconducting films
Experimental evidence of wave properties of dendritic flux avalanches in
superconducting films is reported. Using magneto-optical imaging the
propagation of dendrites across boundaries between a bare NbN film and areas
coated by a Cu-layer was visualized, and it was found that the propagation is
refracted in full quantitative agreement with Snell's law. For the studied film
of 170 nm thickness and a 0.9 mkm thick metal layer, the refractive index was
close to n=1.4. The origin of the refraction is believed to be caused by the
dendrites propagating as an electromagnetic shock wave, similar to damped modes
considered previously for normal metals. The analogy is justified by the large
dissipation during the avalanches raising the local temperature significantly.
Additional time-resolved measurements of voltage pulses generated by segments
of the dendrites traversing an electrode confirm the consistency of the adapted
physical picture.Comment: 4 pages, 4 figure
- …