191 research outputs found
Onset of dendritic flux avalanches in superconducting films
We report a detailed comparison of experimental data and theoretical
predictions for the dendritic flux instability, believed to be a generic
behavior of type-II superconducting films. It is shown that a thermo-magnetic
model published very recently [Phys. Rev. B 73, 014512 (2006)] gives an
excellent quantitative description of key features like the instability onset
(first dendrite appearance) magnetic field, and how the onset field depends on
both temperature and sample size. The measurements were made using
magneto-optical imaging on a series of different strip-shaped samples of MgB2.
Excellent agreement is also obtained by reanalyzing data previously published
for Nb.Comment: 4 pages, 5 figure
Interaction between superconducting vortices and Bloch wall in ferrite garnet film
Interaction between a Bloch wall in a ferrite-garnet film and a vortex in a
superconductor is analyzed in the London approximation. Equilibrium
distribution of vortices formed around the Bloch wall is calculated. The
results agree quantitatively with magneto-optical experiment where an in-plane
magnetized ferrite-garnet film placed on top of NbSe2 superconductor allows
observation of individual vortices. In particular, our model can reproduce a
counter-intuitive attraction observed between vortices and a Bloch wall having
the opposite polarity. It is explained by magnetic charges appearing due to
discontinuity of the in-plane magnetization across the wall.Comment: 4 pages, 5 figure
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
Hydrodynamic Instability of the Flux-antiflux Interface in Type-II Superconductors
The macroturbulence instability observed in fluxline systems during
remagnetization of superconductors is explained. It is shown that when a region
with flux is invaded by antiflux the interface can become unstable if there is
a relative tangential flux motion. This condition occurs at the interface when
the viscosity is anisotropic, e.g., due to flux guiding by twin boundaries in
crystals. The phenomenon is similar to the instability of the tangential
discontinuity in classical hydrodynamics. The obtained results are supported by
magneto-optical observations of flux distribution on the surface of a YBCO
single crystal with twins.Comment: 12 pages, 3 figures, submitted to Physical Review Letter
Coccidioidomycosis in Chicken Pullets in Jos, Plateau State, Nigeria: A Case Report
No Abstract
Experiments in vortex avalanches
Avalanche dynamics is found in many phenomena spanning from earthquakes to
the evolution of species. It can be also found in vortex matter when a type II
superconductor is externally driven, for example, by increasing the magnetic
field. Vortex avalanches associated with thermal instabilities can be an
undesirable effect for applications, but "dynamically driven" avalanches
emerging from the competition between intervortex interactions and quenched
disorder constitute an interesting scenario to test theoretical ideas related
with non-equilibrium dynamics. However, differently from the equilibrium phases
of vortex matter in type II superconductors, the study of the corresponding
dynamical phases - in which avalanches can play a role - is still in its
infancy. In this paper we critically review relevant experiments performed in
the last decade or so, emphasizing the ability of different experimental
techniques to establish the nature and statistical properties of the observed
avalanche behavior.Comment: To be published in Reviews of Modern Physics April 2004. 17 page
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Studies on Plasmodium falciparum pfmdr1 gene assoiciated with chloroquine resistance in Yunnnan province.
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