56 research outputs found
Measurement and calculation of levitation forces between magnets and granular superconductors
Recent developments indicate that exploitation of the phenomenon of magnetic levitation may become one of the most important near-term applications of high-T(sub c) superconductivity. Because of this, the interaction between a strong permanent magnet(PM) and bulk high-T(sub c) superconductor (HTSC) is currently a subject of much interest. We have studied central features of the mechanics of PM-HTSC systems of simple geometries. Here we report experimental results for the components of the levitation force, their associated stiffness and mechanical ac-loss. To analyze the observed behavior a theoretical framework based on critical-state considerations is developed. It will be shown that all the mechanical properties can be explained consistently at a quantitative level wing a minimum of model parameters
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
Superconductor strip with transport current: Magneto-optical study of current distribution and its relaxation
The dynamics of magnetic flux distributions across a YBaCuO strip carrying
transport current is measured using magneto-optical imaging at 20 K. The
current is applied in pulses of 40-5000 ms duration and magnitude close to the
critical one, 5.5 A. During the pulse some extra flux usually penetrates the
strip, so the local field increases in magnitude. When the strip is initially
penetrated by flux, the local field either increases or decreases depending
both on the spatial coordinate and the current magnitude. Meanwhile, the
current density always tends to redistribute more uniformly. Despite the
relaxation, all distributions remain qualitatively similar to the Bean model
predictions.Comment: RevTeX, 9 pages, 9 figures, submitted to Supercond. Sci. Technol.
Revision: MO image and more refs are adde
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
Unified order-disorder vortex phase transition in high-Tc superconductors
The diversity of vortex melting and solid-solid transition lines measured in
different high-T superconductors is explained, postulating a unified
order-disorder phase transition driven by both thermally- and disorder-induced
fluctuations. The temperature dependence of the transition line and the nature
of the disordered phase (solid, liquid, or pinned liquid) are determined by the
relative contributions of these fluctuations and by the pinning mechanism. By
varying the pinning mechanism and the pinning strength one obtains a spectrum
of monotonic and non-monotonic transition lines similar to those measured in
BiSrCaCuO, YBaCuO,
NdCeCuO,
BiPbSrCaCuO and (LaSr)CuOComment: To be published in Phys. Rev. B Rapid Com
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.
Faraday rotation spectra of bismuth-substituted ferrite garnet films with in-plane magnetization
Single crystalline films of bismuth-substituted ferrite garnets have been
synthesized by the liquid phase epitaxy method where GGG substrates are dipped
into the flux. The growth parameters are controlled to obtain films with
in-plane magnetization and virtually no domain activity, which makes them
excellently suited for magnetooptic imaging. The Faraday rotation spectra were
measured across the visible range of wavelengths. To interprete the spectra we
present a simple model based on the existence of two optical transitions of
diamagnetic character, one tetrahedral and one octahedral. We find excellent
agreement between the model and our experimental results for photon energies
between 1.77 and 2.53 eV, corresponding to wavelengths between 700 and 490 nm.
It is shown that the Faraday rotation changes significantly with the amount of
substituted gallium and bismuth. Furthermore, the experimental results suggest
that the magnetooptic response changes linearly with the bismuth substitution.Comment: 15 pages, 6 figures, published in Phys. Rev.
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