134 research outputs found
Disorder Induced Effects on the Critical Current Density of Iron Pnictide BaFe_1.8 Co_0.2 As_2 single crystals
Investigating the role of disorder in superconductors is an essential part of
characterizing the fundamental superconducting properties as well as assessing
potential applications of the material. In most cases, the information
available on the defect matrix is poor, making such studies difficult, but the
situation can be improved by introducing defects in a controlled way, as
provided by neutron irradiation. In this work, we analyze the effects of
neutron irradiation on a Ba(FeCo)As single crystal. We
mainly concentrate on the magnetic properties which were determined by
magnetometry. Introducing disorder by neutron irradiation leads to significant
effects on both the reversible and the irreversible magnetic properties, such
as the transition temperature, the upper critical field, the anisotropy, and
the critical current density. The results are discussed in detail by comparing
them with the properties in the unirradiated state.Comment: accepted for Ph
Critical current anisotropy in Nd-1111 single crystals and the infuence of neutron irradiation
We report on angle-resolved magnetization measurements on
NdFeAsOF (Nd-1111) single crystals. The field dependence of
the critical current density, , is non-monotonous in these crystals at all
orientations and temperatures due to the fishtail effect, which strongly
influences the angular dependence of . The currents decrease as the field
is tilted from the crystallographic c-axis at low fields, but increase at high
fields. A peak occurs in the angular dependence of at intermediate
fields. The critical currents are significantly enhanced after irradiation with
fast neutrons and the fishtail disappears. The different current anisotropies
at low and high fields, however, persist. We discuss the data in the framework
of the anisotropic scaling approach and propose a transition from dominant
pinning by large defects of low density at low fields to pinning by small
defects of high density at high fields in the pristine crystal. Strong pinning
dominates at all fields after the irradiation, and the angular dependence of
can be described by anisotropic scaling only after an appropriate
extension to this pinning regime
Disorder effects on the superconducting properties of BaFeCoAs single crystals
Single crystals of superconducting BaFeCoAs were exposed
to neutron irradiation in a fission reactor. The introduced defects decrease
the superconducting transition temperature (by about 0.3 K) and the upper
critical field anisotropy (e.g. from 2.8 to 2.5 at 22 K) and enhance the
critical current densities by a factor of up to about 3. These changes are
discussed in the context of similar experiments on other superconducting
materials
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/
Studies of cracking behavior in melt-processed YBCO bulk superconductors
An important phenomenon in bulk superconductors fabricated by top-seeded-melt growth (TSMG) is the formation of cracks due to the inherent brittleness of the YBa2Cu3O7-δ (Y-123) phase matrix. These form during the fabrication of the superconducting monolith and play an important role in the limitation of current flow. However, cracks may also form during cooling cycles of the sample to liquid nitrogen temperatures. In this investigation, macrocracks along the c-direction, in particular were analyzed microscopically before and after cooling. In addition we attempt to resolve the c-axis macrocrack formation pattern using the magnetoscan technique
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