Configurations Of Columnar Defects Research Articles

Critical current density of YBCO films with different configurations of columnar defects in longitudinal magnetic field

Critical current density (Jc) properties in longitudinal magnetic fields were investigated for YBa2Cu3Oy thin films with columnar defects (CDs), where different configurations of CDs were systematically installed into the films by using heavy-ion irradiation: a parallel configuration of CDs aligned along the c-axis and two bimodal splay configurations composed of CDs crossing at ±θi relative to the c-axis, where the splay plane defined by the two irradiation angles is perpendicular or parallel to the transport current direction. The unirradiated film under the longitudinal magnetic field shows a Jc peak in the magnetic field dependence of Jc, which is 1.1 times higher than the self-field Jc. For the irradiated films with the parallel CD configuration, on the other hand, the Jc is lower than that for the unirradiated film in all magnetic fields and the value of Jc decreases with increasing CD density. Such degradation effect by CDs under longitudinal magnetic field was observed even for the bimodal splay configurations. These results are attributed to local meandering of current flow induced by CDs extending through the film thickness, which deteriorates the force-free condition.

Vortex pinning by a Gaussian splay of columnar defects inYBa2Cu3O7

A particular configuration of columnar defects (Gaussian splay) has been installed in a ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$ single crystal by irradiation with heavy ions. The influence of this kind of splay on the pinning efficiency has been very controversial in the literature. In the present study, the Gaussian splay has been achieved by defocusing the incident ion beam through a metallic foil. Its pinning efficiency has been compared to that of a standard configuration of tracks parallel to the c axis. In the largest part of the H-T plane, it is shown that the persistent current densities are actually higher in the Gaussian splay than in the parallel configuration.

Effects of crossed columnar defects on vortex pinning in Bi Sr CaCu O

To investigate the existence of a splay effect in Bi2Sr2CaCu2O8 (Bi-2212), vortex pinning has been studied in different configurations of strongly inclined columnar defects (75 ° from the c axis), installed by heavy-ion irradiation. It is shown that the symmetry of the track setting with respect to the field direction is a more influent parameter than the presence of a dispersion in the track directions. We claim that the enhanced pinning efficiency which is observed in some splayed configurations of columnar defects in Bi-2212 can be interpreted without invoking a splay effect.

Pinning enhancement in Bi 2Sr 2CaCu 2O 8 by tilting columnar defects from the c axis

Single crystals of Bi 2Sr 2CaCu 2O 8 have been irradiated by Pb ions at different angles, θ i , from the c axis. The pinning efficiency of these different configurations of columnar defects has been investigated by SQUID magnetometry with the magnetic field applied along the c axis. A regime was found, at high temperatures and low fields, where the critical current density is lower for θ i =0° than for very high angles, like θ i =75°. This enhancement can be related to an increase of the individual pinning energy resulting from the larger size of the track projection in the ab planes. Several track configurations are compared to take into account the additional influence of the variation of track density in the ab planes owing to the ion beam inclination.

Effects of the splayed disorder on the vortex pinning by columnar defects in superconducting oxides

Splayed configurations of columnar defects with different characteristic angles θi relative to the $$\vec c$$ have been achieved in single crystals of YBCO, (Tl,Bi)-1212 and Bi-2212. An increased pinning efficiency consistents with the splayed model of Hwa et al, has been observed with θi=10° for YBCO and (Tl,Bi)-1212. For the more anisotropic compound Bi-2212, higher angles up to θi=75° are necessary to obtain the same result, but these last results are more delicate to interpret.

Vortex motion through the splayed configurations of columnar defects

We report large current enhancements inY Ba 2 Cu 3 O 7-σ with splayed columnar defects. An optimalrelative splay angle is Θ opt = 10° for the planar splay distribution. The vortex motion through gaussian splay appears to be controlled by the large-angle tails of the distribution. The regimes are observed where vortex motion isenhanced rather thansuppressed by splay.