241 research outputs found
The contribution of 211 particles to the mechanical reinforcement mechanism of 123 superconducting single domains
Hardness and fracture toughness of Dy-123 single-domains were studied by
Vickers micro-indentation. A significant anisotropy of the mechanical
properties was observed. Hardness tests give higher values when performed in
(001) planes rather than in planes parallel to the c-axis. Moreover cracks
pattern around the indentation follows preferential orientation in planes
parallel to the c-axis whereas a classical ''four-cracks'' pattern is observed
in the (001) planes. It has been possible to show the crucial role played by
the 211-particles in the deviating mechanism of cracks and the relevance of the
211-particle distribution high homogeneity in the material.Comment: 14 pages, including 5 figures and 1 Table. submitted to Supercond.
Sci. Techno
Current percolation and anisotropy in polycrystalline MgB
The influence of anisotropy on the transport current in MgB
polycrystalline bulk samples and wires is discussed. A model for the critical
current density is proposed, which is based on anisotropic London theory, grain
boundary pinning and percolation theory. The calculated currents agree
convincingly with experimental data and the fit parameters, especially the
anisotropy, obtained from percolation theory agree with experiment or
theoretical predictions.Comment: 5 pages, accepted for publication in Physical Review Letters
(http://prl.aps.org/
Influence of nonlocal electrodynamics on the anisotropic vortex pinning in
We have studied the pinning force density Fp of YNi_2B_2C superconductors for
various field orientations. We observe anisotropies both between the c-axis and
the basal plane and within the plane, that cannot be explained by usual mass
anisotropy. For magnetic field , the reorientation structural
transition in the vortex lattice due to nonlocality, which occurs at a field
, manifests itself as a kink in Fp(H). When , Fp is
much larger and has a quite different H dependence, indicating that other
pinning mechanisms are present. In this case the signature of nonlocal effects
is the presence of a fourfold periodicity of Fp within the basal plane.Comment: 4 pages, 3 figure
Strongly linked current flow in polycrystalline forms of the new superconductor MgB2
The discovery of superconductivity at 39 K in MgB2[1] raises many issues. One
of the central questions is whether this new superconductor resembles a
high-temperature-cuprate superconductor or a low-temperature metallic
superconductor in terms of its current carrying characteristics in applied
magnetic fields. In spite of the very high transition temperatures of the
cuprate superconductors, their performance in magnetic fields has several
drawbacks[2]. Their large anisotropy restricts high bulk current densities to
much less than the full magnetic field-temperature (H-T) space over which
superconductivity is found. Further, weak coupling across grain boundaries
makes transport current densities in untextured polycrystalline forms low and
strongly magnetic field sensitive[3,4]. These studies of MgB2 address both
issues. In spite of the multi-phase, untextured, nano-scale sub-divided nature
of our samples, supercurrents flow throughout without the strong sensitivity to
weak magnetic fields characteristic of Josephson-coupled grains[3].
Magnetization measurements over nearly all of the superconducting H-T plane
show good temperature scaling of the flux pinning force, suggestive of a
current density determined by flux pinning. At least two length scales are
suggested by the magnetization and magneto optical (MO) analysis but the cause
of this seems to be phase inhomogeneity, porosity, and minority insulating
phase such as MgO rather than by weakly coupled grain boundaries. Our results
suggest that polycrystalline ceramics of this new class of superconductor will
not be compromised by the weak link problems of the high temperature
superconductors, a conclusion with enormous significance for applications if
higher temperature analogs of this compound can be discovered
A general scaling relation for the critical current density in Nb3Sn
We review the scaling relations for the critical current density (Jc) in
Nb3Sn wires and include recent findings on the variation of the upper critical
field (Hc2) with temperature (T) and A15 composition. We highlight deficiencies
in the Summers/Ekin relations, which are not able to account for the correct
Jc(T) dependence. Available Jc(H) results indicate that the magnetic field
dependence for all wires can be described with Kramer's flux shear model, if
non-linearities in Kramer plots are attributed to A15 inhomogeneities. The
strain (eps) dependence is introduced through a temperature and strain
dependent Hc2*(T,eps) and Ginzburg- Landau parameter kappa1(T,eps) and a strain
dependent critical temperature Tc(eps). This is more consistent than the usual
Ekin unification, which uses two separate and different dependencies on Hc2*(T)
and Hc2*(eps). Using a correct temperature dependence and accounting for the
A15 inhomogeneities leads to a remarkable simple relation for Jc(H,T,eps).
Finally, a new relation for s(eps) is proposed, based on the first, second and
third strain invariants.Comment: Accepted Topical Review for Superconductor, Science and Technolog
Core pinning by intragranular nanoprecipitates in polycrystalline MgCNi_3
The nanostructure and magnetic properties of polycrystalline MgCNi_3 were
studied by x-ray diffraction, electron microscopy, and vibrating sample
magnetometry. While the bulk flux-pinning force curve F_p(H) indicates the
expected grain-boundary pinning mechanism just below T_c = 7.2 K, a systematic
change to pinning by a nanometer-scale distribution of core pinning sites is
indicated by a shift of F_p(H) with decreasing temperature. The lack of scaling
of F_p(H) suggests the presence of 10 to 20% of nonsuperconducting regions
inside the grains, which are smaller than the diameter of fluxon cores 2xi at
high temperature and become effective with decreasing temperature when xi(T)
approaches the nanostructural scale. Transmission electron microscopy revealed
cubic and graphite nanoprecipitates with 2 to 5 nm size, consistent with the
above hypothesis since xi(0) = 6 nm. High critical current densities, more than
10^6 A/cm^2 at 1 T and 4.2 K, were obtained for grain colonies separated by
carbon. Dirty-limit behavior seen in previous studies may be tied to electron
scattering by the precipitates, indicating the possibility that strong core
pinning might be combined with a technologically useful upper critical field if
versions of MgCNi_3 with higher T_c can be found.Comment: 5 pages, 6 figures, submitted to PR
High magnetic field scales and critical currents in SmFeAs(O,F) crystals: promising for applications
Superconducting technology provides most sensitive field detectors, promising
implementations of qubits and high field magnets for medical imaging and for
most powerful particle accelerators. Thus, with the discovery of new
superconducting materials, such as the iron pnictides, exploring their
potential for applications is one of the foremost tasks. Even if the critical
temperature Tc is high, intrinsic electronic properties might render
applications rather difficult, particularly if extreme electronic anisotropy
prevents effective pinning of vortices and thus severely limits the critical
current density, a problem well known for cuprates. While many questions
concerning microscopic electronic properties of the iron pnictides have been
successfully addressed and estimates point to a very high upper critical field,
their application potential is less clarified. Thus we focus here on the
critical currents, their anisotropy and the onset of electrical dissipation in
high magnetic fields up to 65 T. Our detailed study of the transport properties
of optimally doped SmFeAs(O,F) single crystals reveals a promising combination
of high (>2 x 10^6 A/cm^2) and nearly isotropic critical current densities
along all crystal directions. This favorable intragrain current transport in
SmFeAs(O,F), which shows the highest Tc of 54 K at ambient pressure, is a
crucial requirement for possible applications. Essential in these experiments
are 4-probe measurements on Focused Ion Beam (FIB) cut single crystals with
sub-\mu\m^2 cross-section, with current along and perpendicular to the
crystallographic c-axis and very good signal-to-noise ratio (SNR) in pulsed
magnetic fields. The pinning forces have been characterized by scaling the
magnetically measured "peak effect"
A Review of the Properties of Nb3Sn and Their Variation with A15 Composition, Morphology and Strain State
This article gives an overview of the available literature on simplified,
well defined (quasi-)homogeneous laboratory samples. After more than 50 years
of research on superconductivity in Nb3Sn, a significant amount of results are
available, but these are scattered over a multitude of publications. Two
reviews exist on the basic properties of A15 materials in general, but no
specific review for Nb3Sn is available. This article is intended to provide
such an overview. It starts with a basic description of the Niobium-Tin
intermetallic. After this it maps the influence of Sn content on the the
electron-phonon interaction strength and on the field-temperature phase
boundary. The literature on the influence of Cu, Ti and Ta additions will then
be briefly summarized. This is followed by a review on the effects of grain
size and strain. The article is concluded with a summary of the main results.Comment: Invited Topical Review for Superconductor, Science and Technology.
Provisionally scheduled for July 200
Temperature dependence and mechanisms for vortex pinning by periodic arrays of Ni dots in Nb films
Pinning interactions between superconducting vortices in Nb and magnetic Ni
dots were studied as a function of current and temperature to clarify the
nature of pinning mechanisms. A strong current dependence is found for a square
array of dots, with a temperature dependent optimum current for the observation
of periodic pinning, that decreases with temperature as (1-T/Tc)3/2. This same
temperature dependence is found for the critical current at the first matching
field with a rectangular array of dots. The analysis of these results allows to
narrow the possible pinning mechanisms to a combination of two: the interaction
between the vortex and the magnetic moment of the dot and the proximity effect.
Moreover, for the rectangular dot array, the temperature dependence of the
crossover between the low field regime with a rectangular vortex lattice to the
high field regime with a square configuration has been studied. It is found
that the crossover field increases with decreasing temperature. This dependence
indicates a change in the balance between elastic and pinning energies,
associated with dynamical effects of the vortex lattice in the high field
range.Comment: 12 text pages (revtex), 6 figures (1st jpeg, 2nd-6th postscript)
accepted in Physical Review
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