58 research outputs found
The behavior of grain boundaries in the Fe-based superconductors
The Fe-based superconductors (FBS) are an important new class of
superconducting materials. As with any new superconductor with a high
transition temperature and upper critical field, there is a need to establish
what their applications potential might be. Applications require high critical
current densities, so the usefulness of any new superconductor is determined
both by the capability to develop strong vortex pinning and by the absence or
ability to overcome any strong current-limiting mechanisms of which grain
boundaries in the cuprates are a cautionary example. In this review we first
consider the positive role that grain boundary properties play in the metallic,
low temperature superconductors and then review the theoretical background and
current experimental data relating to the properties of grain boundaries in FBS
polycrystals, bi-crystal thin films, and wires. Based on this evidence, we
conclude that grain boundaries in FBS are weak linked in a qualitatively
similar way to grain boundaries in the cuprate superconductors, but also that
the effects are a little less marked. Initial experiments with the textured
substrates used for cuprate coated conductors show similar benefit for the
critical current density of FBS thin films too. We also note that the
particular richness of the pairing symmetry and the multiband parent state in
FBS may provide opportunities for grain boundary modification as a better
understanding of their pairing state and grain boundary properties are
developed.Comment: To appear in Reports on Progress in Physic
New Fe-based superconductors: properties relevant for applications
Less than two years after the discovery of high temperature superconductivity
in oxypnictide LaFeAs(O,F) several families of superconductors based on Fe
layers (1111, 122, 11, 111) are available. They share several characteristics
with cuprate superconductors that compromise easy applications, such as the
layered structure, the small coherence length, and unconventional pairing, On
the other hand the Fe-based superconductors have metallic parent compounds, and
their electronic anisotropy is generally smaller and does not strongly depend
on the level of doping, the supposed order parameter symmetry is s wave, thus
in principle not so detrimental to current transmission across grain
boundaries. From the application point of view, the main efforts are still
devoted to investigate the superconducting properties, to distinguish intrinsic
from extrinsic behaviours and to compare the different families in order to
identify which one is the fittest for the quest for better and more practical
superconductors. The 1111 family shows the highest Tc, huge but also the most
anisotropic upper critical field and in-field, fan-shaped resistive transitions
reminiscent of those of cuprates, while the 122 family is much less anisotropic
with sharper resistive transitions as in low temperature superconductors, but
with about half the Tc of the 1111 compounds. An overview of the main
superconducting properties relevant to applications will be presented. Upper
critical field, electronic anisotropy parameter, intragranular and
intergranular critical current density will be discussed and compared, where
possible, across the Fe-based superconductor families
Enhancement of the high-field critical current density of superconducting MgB2 by proton irradiation
A relatively high critical temperature, Tc, approaching 40 K, places the
recently-discovered superconductor magnesium diboride (MgB2) intermediate
between the families of low- and copper-oxide-based high-temperature
superconductors (HTS). Supercurrent flow in MgB2 is unhindered by grain
boundaries, unlike the HTS materials. Thus, long polycrystalline MgB2
conductors may be easier to fabricate, and so could fill a potentially
important niche of applications in the 20 to 30 K temperature range. However,
one disadvantage of MgB2 is that in bulk material the critical current density,
Jc, appears to drop more rapidly with increasing magnetic field than it does in
the HTS phases. The magnitude and field dependence of Jc are related to the
presence of structural defects that can "pin" the quantised magnetic vortices
that permeate the material, and prevent them from moving under the action of
the Lorentz force. Vortex studies suggest that it is the paucity of suitable
defects in MgB2 that causes the rapid decay of Jc with field. Here we show that
modest levels of atomic disorder, induced by proton irradiation, enhance the
pinning, and so increase Jc significantly at high fields. We anticipate that
chemical doping or mechanical processing should be capable of generating
similar levels of disorder, and so achieve technologically-attractive
performance in MgB2 by economically-viable routes.Comment: 5 pages, 4 figures, to be published in Nature (in press
Thin Film Magnesium Boride Superconductor with Very High Critical Current Density and Enhanced Irreversibility Field
The discovery of superconductivity at 39 K in magnesium diboride offers the
possibility of a new class of low-cost, high-performance superconducting
materials for magnets and electronic applications. With twice the critical
temperature of Nb_3Sn and four times that of Nb-Ti alloy, MgB_2 has the
potential to reach much higher fields and current densities than either of
these technological superconductors. A vital prerequisite, strongly linked
current flow, has already been demonstrated even at this early stage. One
possible drawback is the observation that the field at which superconductivity
is destroyed is modest. Further, the field which limits the range of practical
applications, the irreversibility field H*(T), is ~7 T at liquid helium
temperature (4.2 K), significantly lower than ~10 T for Nb-Ti and ~20 T for
Nb_3Sn. Here we show that MgB_2 thin films can exhibit a much steeper
temperature dependence of H*(T) than is observed in bulk materials, yielding
H*(4.2 K) above 14 T. In addition, very high critical current densities at 4.2
K, 1 MA/cm_2 at 1 T and 10_5 A/cm_2 at 10 T, are possible. These data
demonstrate that MgB_2 has credible potential for high-field superconducting
applications.Comment: 4 pages pdf, submitted to Nature 3/20/0
To use or not to use cool superconductors?
The high critical temperature and magnetic field in cuprates and Fe-based
superconductors are not enough to assure applications at higher temperatures.
Making these superconductors useful involves complex and expensive technologies
to address many conflicting physics and materials requirements
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