278 research outputs found
Application of Percolation Theory to Current Transfer in Granular Superconductors
We investigate the description of current transfer in polycrystalline
superconductors by percolation theory and its limitations. Various computer
models that have been proposed are reviewed and related to the experimental and
theoretical framework. While some conductor properties can be well described by
percolation theory and models, we argue that a conceptual gap exists between
experiment and theory. This gap has to be bridged by finding relations between
electromagnetic and statistical parameters. We derive various such relations
and compare them with recent simulation data. In particular, we suggest a new
scaling law between the two fundamental variables current and probability.Comment: submitted to European Physical Journal B, April 2002, 18 pages
revised and re-submitted June 2002; added remarks and references [15-18] on
Correlated Percolation; corrected minor errors; updated some reference
The Normal State Resistivity of Grain Boundaries in YBa2Cu3O7-delta
Using an optimized bridge geometry we have been able to make accurate
measurements of the properties of YBa2Cu3O7-delta grain boundaries above Tc.
The results show a strong dependence of the change of resistance with
temperature on grain boundary angle. Analysis of our results in the context of
band-bending allows us to estimate the height of the potential barrier present
at the grain boundary interface.Comment: 11 pages, 3 figure
The critical current of YBa2Cu3O7-d Low Angle Grain Boundaries
Transport critical current measurements have been performed on 5 degree
[001]-tilt thin film YBa2Cu3O7-delta single grain boundaries with magnetic
field rotated in the plane of the film, phi. The variation of the critical
current has been determined as a function of the angle between the magnetic
field and the grain boundary plane. In applied fields above 1 T the critical
current, j_c, is found to be strongly suppressed only when the magnetic field
is within an angle phi_k of the grain boundary. Outside this angular range the
behavior of the artificial grain boundary is dominated by the critical current
of the grains. We show that the phi dependence of j_c in the suppressed region
is well described by a flux cutting model.Comment: To be published in PRL, new version with minor changes following
referees report
Effect of varying material anisotropy on critical current anistropy in vicinal YBaCuO thin films
The high cuprate superconductors are noted for their anisotropic
layered structure, certain of these materials indeed tend toward the limit of a
Lawrence-Doniach superconductor. However, YBaCuO has a
smaller anisotropy than would be expected from its interlayer spacing. This is
due to the cuprate chains in the structure. To investigate the influence of the
chain oxygen on transport properties critical current versus applied field
angle measurements were performed on fully oxygenated and de-oxygenated
YBaCuO thin films and optimally oxygenated
YCaBaCuO thin films. The films were grown
on 10 mis-cut SrTiO substrates to enable the intrinsic vortex
channelling effect to be observed. The form of the vortex channelling minimum
observed in field angle dependent critical current studies on the films was
seen to depend on film oxygenation. The vortex channelling effect is dependent
on a angular dependent cross-over to a string-pancake flux line lattice. The
results obtained appear to be consistent with the prediction of Blatter et al.
[Rev. Mod. Phys., 66 (4): 1125 (1994)] that increased superconducting
anisotropy leads to the kinked string-pancake lattice existing over a smaller
angular range.Comment: To be submitted to AP
Critical currents in vicinal YBaCuO films
Most measurements of critical current densities in
YBaCuO thin films to date have been performed on films
where the \textit{c}-axis is grown normal to the film surface. With such films,
the analysis of the dependence of on the magnetic field angle is complex.
The effects of extrinsic contributions to the angular field dependence of
, such as the measurement geometry and disposition of pinning centres, are
convoluted with those intrinsically due to the anisotropy of the material. As a
consequence of this, it is difficult to distinguish between proposed FLL
structure models on the basis of angular critical current density measurements
on \textit{c}-axis films. Films grown on mis-cut (vicinal) substrates have a
reduced measurement symmetry and thus provide a greater insight into the
critical current anisotropy. In this paper previous descriptions of the
magnetic field angle dependence of in YBaCuO are
reviewed. Measurements on YBaCuO thin films grown on a
range of vicinal substrates are presented and the results interpreted in terms
of the structure and dimensionality of the FLL in YBaCuO.
There is strong evidence for a transition in the structure of the flux line
lattice depending on magnetic field magnitude, orientation and temperature. As
a consequence, a simple scaling law can not, by itself, describe the observed
critical current anisotropy in YBaCuO. The experimentally
obtained behaviour of YBCO is successfully described in terms of
a kinked vortex structure for fields applied near parallel to the \textit{a-b}
planes.Comment: 10 pages, 12 figures, Submitted to PR
Nanoscale grains, high irreversibility field, and large critical current density as a function of high energy ball milling time in C-doped magnesium diboride
Magnesium diboride (MgB2) powder was mechanically alloyed by high energy ball
milling with C to a composition of Mg(B0.95C0.05)2 and then sintered at 1000 C
in a hot isostatic press. Milling times varied from 1 minute to 3000 minutes.
Full C incorporation required only 30-60 min of milling. Grain size of sintered
samples decreased with increased milling time to less than 30 nm for 20-50 hrs
of milling. Milling had a weak detrimental effect on connectivity. Strong
irreversibility field (H*) increase (from 13.3 T to 17.2 T at 4.2 K) due to
increased milling time was observed and correlated linearly with inverse grain
size (1/d). As a result, high field Jc benefited greatly from lengthy powder
milling. Jc(8 T, 4.2 K) peaked at > 80,000 A/cm2 with 1200 min of milling
compared with only ~ 26,000 A/cm2 for 60 min of milling. This non-compositional
performance increase is attributed to grain refinement of the unsintered powder
by milling, and to the probable suppression of grain growth by milling-induced
MgO nano-dispersions.Comment: 12 pages, 11 figure
The abolition of the General Teaching Council for England and the future of teacher discipline
With the abolition of the General Teaching Council for England in the 2011 Education Act, this article considers the future of teacher discipline in England. It provides a critique of the changes to the regulation of teacher misconduct and incompetence that draws on a Foucauldian framework, especially concerning the issue of public displays of discipline and the concomitant movement to more hidden forms. In addition, the external context of accountability that accompanies the reforms to teacher discipline are considered including the perfection of the panoptic metaphor presented by the changes to Ofsted practices such as the introduction of zero-notice inspections. The article concludes that the reforms will further move teachers from being occupational professionals to being organisational professionals marking them apart from comparable professions in medicine and law
On narrowing coated conductor film: emergence of granularity-induced field hysteresis of transport critical current
Critical current density Jc in polycrystalline or granular superconducting
material is known to be hysteretic with applied field H due to the focusing of
field within the boundary between adjacent grains. This is of concern in the
so-called coated conductors wherein superconducting film is grown on a
granular, but textured surface of a metal substrate. While previous work has
mainly been on Jc determined using induced or magnetization currents, the
present work utilizes transport current via an applied potential in strip
geometry. It is observed that the effect is not as pronounced using transport
current, probably due to a large difference in criterion voltage between the
two types of measurements. However, when the films are narrowed by patterning
into 200-, 100-, or 80-micron, the hysteresis is clearly seen, because of the
forcing of percolation across higher-angle grain boundaries. This effect is
compared for films grown on ion-beam-assisted-deposited (IBAD) YSZ substrate
and those grown on rolling-assisted-biaxially-textures substrates (RABiTS)
which have grains that are about ten times larger. The hysteresis is more
pronounced for the latter, which is more likely to have a weak grain boundary
spanning the width of the microbridge. This is also of concern to applications
in which coated conductors will be striated in order to reduce of AC losses.Comment: text-only: 10 pages, plus 5 figures on 5 page
Atmospheric conditions and their effect on ball-milled magnesium diboride
Magnesium diboride bulk pellets were fabricated from pre-reacted MgB2 powder
ball milled with different amounts of exposure to air. Evidence of increased
electron scattering including increased resistivity, depressed Tc, and enhanced
Hc2 of the milled and heat treated samples were observed as a result of
increased contact with air. These and other data were consistent with alloying
with carbon as a result of exposure to air. A less clear trend of decreased
connectivity associated with air exposure was also observed. In making the case
that exposure to air should be considered a doping process, these results may
explain the wide varibability of "undoped" MgB2 properties extant in the
literature.Comment: Work presented at ASC 2006 in Seattl
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