Splayed Columnar Defects Research Articles

Anisotropic Critical Current Densities in Ba0.6K0.4Fe2As2 with Splayed Columnar Defects

Enhancement and non-monotonic field dependence of critical current density (Jc ), called anomalous peak effect, were observed in Ba1−x K x Fe2As2 with splayed columnar defects. In principle, anisotropy of in-plane Jc should be present in this superconductor since the introduced splayed columnar defects introduce anisotropy in the plane. This study reports in-plane anisotropy of Jc observed in 2.6 GeV U irradiated Ba0.6K0.4Fe2As2 with splay angles of ±15° and ±20°. Results show that Jc component parallel to the splay plane is larger than the perpendicular component, and the parallel component contribute to the anomalous peak effect at around B Φ/3 more.

Effects of Asymmetric Splayed Columnar Defects on the Anomalous Peak Effect in Ba0.6K0.4Fe2As2

In order to enhance the critical current density ($J_c$) of superconductors, introduction of columnar defects (CDs) through swift-particle irradiation is effective. By dispersing the direction of CDs (splayed CDs), not only further enhancement of $J_c$ has been confirmed but also an anomalous peak effect (APE) in $J_c$ at a certain magnetic field determined by the irradiation dose was observed. It has been proposed that the APE arises from the suppression of kink motion and/or the effects of vortex entanglement in systems with splayed CDs. In this study, we measure $J_c$ properties of optimally K-doped Ba-122-type iron-based superconductor Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ single crystals with splayed CDs that are introduced asymmetrically with respect to the $c$-axis by irradiating 2.6 GeV U ions. We discuss the significance of the average direction of splayed CDs to the APE. We also discuss the relationship between the APE and the entanglement of vortices.

Effects of Splayed Columnar Defects on Critical Current Density in CaKFe4As4

Introduction of columnar defects to superconductors through particle irradiation enhances their critical current density (J c) . Further enhancement of J c by dispersing the direction of columnar defects has been confirmed. Moreover, in such systems with splayed columnar defects, an anomalous peak effect in J c at a certain magnetic field determined by the irradiation dose accompanied by the in-plane anisotropy of J c between those parallel and perpendicular to the splay direction were observed. We introduce splayed columnar defects to CaKFe4As4 single crystals, which was recently found as a new type of iron-based superconductors (1144-type IBS), by irradiating 2.6 GeV U and 320 MeV Au ions and measure their J c properties.

Anisotropy of critical current densities in Ba1−xKxFe2As2 and Ba(Fe1−xCox)2As2 with splayed columnar defects

Effects of bimodal splayed columnar defects in Ba1−xKxFe2As2 and Ba(Fe1−xCox)2As2 single crystals through 2.6 GeV U irradiation and 220 MeV Au irradiation are investigated. Magnetic field profiles measured by magneto-optical imaging clarified the presence of in-plane anisotropy of the critical current density (Jc) in 2.6 GeV U irradiated crystals, where Jc parallel to the splay plane is larger than that perpendicular to the splay plane. However, the in-plane anisotropy of Jc is absent in 220MeV Au irradiated crystals. Possible origin of this discrepancy is discussed based on the structure of introduced defects.

Field-driven transition in the Ba1−xKxFe2As2 superconductor with splayed columnar defects

Through 2.6 GeV U irradiations, we have induced bimodal splayed columnar defects in Ba$_{1-x}$K$_x$Fe$_2$As$_2$ single crystals with splay angles, $\pm 5 ^\circ$, $\pm 10 ^\circ$, $\pm 15 ^\circ$, and $\pm 20 ^\circ$. Critical current densities through magnetization measurements were carefully evaluated, where a splay angle of $\pm 5 ^\circ$ brought about the highest $J_\mathrm{c}$. Mageto-optical images close to $T_\mathrm{c}$ indicates highly anisotropic discontinuity lines in the remnant state, and with anisotropy increasing with greater splay angles. Moreover, amongst those with splayed columnar defects, anomalous non-monotonic field dependences of $J_\mathrm{c}$ and $S$ with an extrema at some fraction of the matching field are observed. We discuss that such $J_\mathrm{c}$ enhancement arises from a field-driven coupling transition in which intervortex interactions reorganize the vortex structure to be accommodated into columnar defects, thereby increasing pinning at higher fields.

Improved critical currents in a Bi-2223/Ag coil using splayed columnar defects

We have irradiated a double pancake coil of Bi-2223/Ag sheathed tape with high energy protons, which yield splayed columnar defects in the superconductor core of the tape. We report the critical current I c and current–voltage power law n value as a function of magnetic field at temperatures of 75–64 K for the coil and for a short sample taken from the coil. A novel apparatus using permanent magnets is used to apply a magnetic field in the coil radial direction. Both the coil and short sample show a strong enhancement in I c for magnetic fields along the c axis (tape normal or radial direction for the coil) compared to the performance of unirradiated tape. These results represent the first time a real superconducting device has been irradiated with high energy protons for the purpose of introducing splayed columnar defects to enhance performance.

Effects of fission-fragment damage on vortex dimensionality in silver-sheathedBi2Sr2Ca2Cu3Oxtapes

We report on the vortex dimensionality of uranium-doped Ag/Bi2223 tapes, before and after irradiation to a thermal-neutron fluence. The effective activation energies as a function of current density and applied field were calculated from dynamic magnetisation relaxation measurements. A dimensional crossover from a 3D elastic creep regime to 2D plastic creep was observed in the nonirradiated tape at an applied magnetic field Hcr = 0.37 T, with an associated change in the flux hop velocity and temperature dependence of the activation energy. After introduction of the fission-fragment damage by irradiation, a shift in the crossover to Hcr = 0.65 T was observed. These results indicate an enhancement of the c-axis vortex coherence by the introduced splayed columnar defects, explaining the greater pinning efficiency of the uranium-fission method in Bi2223 rather than the less anisotropic Y123. Conflicting results obtained for the irradiated tape in the absence of any temperature scaling of the activation energies demonstrate the importance of the inclusion of scaling in the magnetisation relaxation analysis.

Vortex wandering among splayed columnar defects

We investigate the scaling properties of single flux lines in a random pinning landscape consisting of splayed columnar defects. Such correlated defects can be injected into Type II superconductors by inducing nuclear fission or via direct heavy ion irradiation. The result is often very efficient pinning of the vortices which gives, e.g., a strongly enhanced critical current. The wandering exponent \zeta and the free energy exponent \omega of a single flux line in such a disordered environment are obtained analytically from scaling arguments combined with extreme-value statistics. In contrast to the case of point disorder, where these exponents are universal, we find a dependence of the exponents on details in the probability distribution of the low lying energies of the columnar defects. The analytical results show excellent agreement with numerical transfer matrix calculations in two and three dimensions.

In situ measurements of transport characteristics in heavy-ion irradiated YBa 2Cu 3O y under magnetic field

We produced splayed columnar defects in a c-axis oriented YBa 2Cu 3O y thin film by irradiating 200 MeV Au ions at low temperature (∼80 K) from two different directions; ±15° with respect to the c-axis. Then the transport critical current density J c as a function of magnetic field direction θ with respect to c-axis was measured in situ. A symmetric peak for J c was observed in θ-dependence of J c around θ=0°.

Stability of supercurrents in a BiSrCaCuO (Bi-2212) high- Tcsuperconductor with artificially created defects

Recent results in a systematic study of the stability of supercurrents in Bi-2212 tapes with randomly oriented, highly splayed columnar defects are presented as a function of defect density. The defects were artificially created by fission fragments of bismuth nuclei, fissioned by irradiation with energetic protons (∼0.8 GeV). Significant enhancements in the persistent current density Jpare observed at all temperatures and fields. Also, a marked shift of the irreversibility line towards higher fields and temperatures improves considerably the capacity of the material for practical applications. Moreover, a significant decrease in the logarithmic decay rate S=dln (Jp) /dln(t) indicates a strong stabilization of the persistent currents. All features point to high effectiveness of this artificial pinning mechanism. However, the optimal proton fluence needs yet to be established.

A path to high-field performance of cuprate superconductors above 100 K with fission induced defects

We demonstrate a method by which we expand the useful range of cuprate superconductors to above 100 K and enhance persistent currents by orders of magnitude in fields of several Tesla — namely fission of Hg nuclei within Hg-cuprates with 0.8 GeV protons. The fission process allows “doping” these cuprates with strongly pinning splayed columnar defects. The technique could be technologically relevant, since it is not limited by the short penetration range of heavier particles (or ions) and could permit modification of larger superconducting objects, such as magnets.

Enhanced critical current densities in (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 2/O/sub 10/ silver-sheathed tapes by splayed columnar defects after 600 MeV proton irradiation

Splayed columnar defects are efficient pinning centers for magnetic vortices in high-T/sub c/ superconductors. Irradiation with 600 MeV protons as primary projectiles generates heavy fission fragments in (Bi, Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ via a p-Bi nuclear reaction. The high-Z secondary projectiles are sufficiently energetic to create randomly oriented columnar defects with important effects on the magnetic and transport critical current densities of (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10/ Ag-sheathed tapes. These effects are discussed on the basis of the splayed defect topology.

Generation and annealing of splayed columnar defects in (Bi, Pb)2 Sr2Ca2Cu3Ox silver-sheathed tapes after 600 MeV proton irradiation

Splayed columnar defects are efficient pinning centers for magnetic vortices in High-T-c Superconductors. Irradiation with 600 MeV protons as primary projectiles generates heavy fission fragments via a pBi nuclear reaction. These secondary projectiles create randomly oriented columnar defects with important effects on the magnetic and transport critical current densities of (Bi, Pb)(2)Sr2Ca2Cu3Ox Ag-sheathed tapes.