395 research outputs found
Theory of measurements of electrodynamic properties in anisotropic superconductors in tilted magnetic fields. Part I: flux flow and Campbell regimes
The vortex dynamics of uniaxial anisotropic superconductors in magnetic
fields applied with arbitrary orientation is theoretically studied. Focus is on
the model for electrical transport experiments in the linear regime. Relevant
vortex parameters, like the viscous drag, the vortex mobility and pinning
constant (with point pins), together with the flux flow and Campbell
resistivities, are derived in tensor form, in the very different free flux flow
and pinned Campbell regimes. The applicability to the various tensor quantities
of the well-known scaling laws for the angular dependence on the field
orientation is commented. Moreover, it is shown that the experiments do not
generally yield the intrinsic values of the anisotropic viscosity and pinning
constant. Explicit expressions relating measured and intrinsic quantities are
given.Comment: 13 pages, 3 figure
Theory of measurements of electrodynamic properties in anisotropic superconductors in tilted magnetic fields. Part II: high frequency regimes
The model for high frequency electrodynamics in anisotropic type-II
superconductors in the vortex state is studied considering arbitrary
orientations between the applied field, the applied current and the anisotropy
axis. An anisotropic treatment is provided for the vortex dynamics, taking into
account all the phenomena relevant at high frequency, which include flux flow,
pinning and creep. The coupling between vortex motion and high frequency
currents is included, providing an entirely tensor model of the electromagnetic
response to high frequency fields. Examples of data analysis of angular
measurements are presented, showing how to derive the angular dependence of the
material properties from the measured anisotropic response. Finally, the
expression of the measured angle-dependent surface impedance in the largely
used thin film geometry is computed.Comment: 14 pages, 3 figure
The superconducting proposal for the CS magnet system of FAST: a preliminary analysis of the heat load due to AC losses
FAST (Fusion Advanced Studies Torus), the Italian proposal of a Satellite
Facility to ITER, is a compact tokamak (R = 1.82 m, a = 0.64 m,
triangularity = 0.4) able to investigate non-linear dynamics effects
of -particle behavior in burning plasmas and to test technical
solutions for the first wall/divertor directly relevant for ITER and DEMO.
Currently, ENEA is investigating the feasibility of a superconducting solution
for the magnet system. This paper focuses on the analysis of the CS (Central
Solenoid) magnet thermal behavior. In particular, considering a superconducting
solution for the CS which uses the room available in the resistive design and
referring to one of the most severe scenario envisaged for FAST, the heat load
of the CS winding pack due to AC losses is preliminarily evaluated. The results
provide a tentative baseline for the definition of the strand requirements and
conductor design, that can be accepted in order to fulfil the design
requirements.Comment: 15 pages, 5 figure
Effect of nanosize BaZrO3 inclusions on vortex parameters in YBaCuO
We report on the field dependence of the microwave complex resistivity data
in YBaCuO/BaZrO films grown by PLD at various BaZrO
content. The data, analyzed within a recently developed general framework for
the mixed-state microwave response of superconductors, yield the field
dependence of the fluxon parameters such as the vortex viscosity and the
pinning constant. We find that pinning undergoes a change of regime when the
BaZrO content in the target increases from 2.5 mol.% to 5 mol.%.
Simultaneously, the vortex viscosity becomes an increasing function of the
applied magnetic field. We propose a scenario in which flux lines are pinned as
bundles, and a crossover from dilute point pins to dense c-axis correlated
defects takes place between 2.5 and 5 mol.% in the BZO concentration. Our data
are inconsistent with vortices occupying mainly the BaZrO sites at low
fields, and suggest instead that vortices occupy both BaZrO sites and
interstitials in the YBaCuO matrix, even at low fields.Comment: Presented at EUCAS 2009, to be published in J. Phys.:Conf. Serie
Vortex state microwave response in superconducting cuprates and MgB
We investigate the physics of the microwave response in
YBaCuO, SmBaCuO and MgB
in the vortex state. We first recall the theoretical basics of vortex-state
microwave response in the London limit. We then present a wide set of
measurements of the field, temperature, and frequency dependences of the vortex
state microwave complex resistivity in superconducting thin films, measured by
a resonant cavity and by swept-frequency Corbino disk. The combination of these
techniques allows for a comprehensive description of the microwave response in
the vortex state in these innovative superconductors. In all materials
investigated we show that flux motion alone cannot take into account all the
observed experimental features, neither in the frequency nor in the field
dependence. The discrepancy can be resolved by considering the (usually
neglected) contribution of quasiparticles to the response in the vortex state.
The peculiar, albeit different, physics of the superconducting materials here
considered, namely two-band superconductivity in MgB and superconducting
gap with lines of nodes in cuprates, give rise to a substantially increased
contribution of quasiparticles to the field-dependent microwave response. With
careful combined analysis of the data it is possible to extract or infer many
interesting quantities related to the vortex state, such as the
temperature-dependent characteristic vortex frequency and vortex viscosity, the
field dependence of the quasiparticle density, the temperature dependence of
the -band superfluid density in MgBComment: 51 pages, 27 figures, to appear as a book chapter (Nova Science
When Your âProblemâ Becomes Mine: Adult Female Siblingsâ Perspectives of Having a Brother with a Disability
This study is a retrospective look at adult female siblingsâ perspectives of their childhoods and present identities based on having a brother with a disability. This paper focuses on siblingsâ experiences within educational and public domains, and how such experiences have shaped their personalities and career choices. Qualitative findings were organized into four central themes, each with subcate-gories: (a) Siblingsâ Fear and Worries, in-school bullying, school friends coming to the home; (b) Anger and Resentment, towards the perceived advantages of hav-ing a disability, towards teachers; (c) Positive Impacts, acceptance of others in varied public contexts, advocacy; and (d) Career Choices, entering the teaching profession. Findings are discussed in relation to both the emotional and educa-tional significance for siblings
Strong Reduction of the Field-Dependent Microwave Surface Resistance in YBCO with BaZrO_3 Inclusions
We present measurements of the magnetic field dependent microwave surface
resistance in laser-ablated YBaCuO films on SrTiO
substrates. BaZrO crystallites were included in the films using composite
targets containing BaZrO inclusions with mean grain size smaller than 1
m. X-ray diffraction showed single epitaxial relationship between
BaZrO and YBaCuO. The effective surface resistance was
measured at 47.7 GHz for 6090 K and 00.8 T. The magnetic
field had a very different effect on pristine YBaCuO and
YBaCuO/BaZrO, while for 0 only a reduction of
in the YBaCuO/BaZrO film was observed,
consistent with dc measurements. At low enough , in moderate fields
YBaCuO/BaZrO exhibited an intrinsic thin film
resistance lower than the pure film. The results clearly indicate that
BaZrO inclusions determine a strong reduction of the field-dependent
surface resistance. From the analysis of the data in the framework of simple
models for the microwave surface impedance in the mixed state we argue that
BaZrO inclusions determine very steep pinning potentials.Comment: LaTeX, 6 pages, 4 figures, uses jpconf.cls and jpconf11.clo class
files, talk given at EUCAS 2007, submitted to J. Phys.: Conf. Serie
New aspects of microwave properties of Nb in the mixed state
We present a study of the frequency dependence of the vortex dynamics in a
conventional superconductor. We have employed a swept-frequency, Corbino-disk
technique to investigate the temperature (3.6K-Tc) and high-field (from Hc2/2
to Hc2) microwave complex resistivity in Nb thin (20-40 nm) films as a function
of the frequency (1-20 GHz). We have found several previously unnoticed
features: (i) a field-dependent depinning frequency in the GHz range; (ii)
deviations from the accepted frequency dependence, that can be ascribed to some
kind of vortex creep; (iii) the presence of switching phenomena, reminiscent of
vortex instabilities. We discuss the possible origin of the features here
reported.Comment: 5 pages, 3 figures, presented at VORTEX VI Conference, to appear on
Physica
Vortex state microwave resistivity in Tl-2212 thin films
We present measurements of the field induced changes in the 47 GHz complex
resistivity, , in TlBaCaCuO
(TBCCO) thin films with 105 K, prepared on CeO buffered
sapphire substrates. At low fields (10 mT) a very small irreversible
feature is present, suggesting a little role of intergranular phenomena. Above
that level exhibits a superlinear dependence with the
field, as opposed to the expected (at high frequencies) quasilinear behaviour.
We observe a crossover between predominantly imaginary to predominantly real
(dissipative) response with increasing temperature and/or field. In addition,
we find the clear scaling property , where the scaling field maps closely the melting
field measured in single crystals. We discuss our microwave results in terms of
loss of flux lines rigidity.Comment: 8 pages, 3 figures, proceedings of 9th HTSHFF, accepted for
publication on J. Supercon
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