V. Berseth

Kinks and vortex-twister dynamics in type-II superconductors

Abstract We report magneto-optical observations of moving helicoidal vortex structures in high purity YBa2Cu3O7−δ single cyrstals. We found that the dynamics of these ‘vortex-twisters’ is mainly controlled by localized instabilities (kinks) which stream along the helices. The kinks allow the motion of the twisters, or the annihilation of twisters with opposite chirality.

Pinning in BSCCO above the ordinary irreversibility line

Frequency-dependent observations of magnetic flux structures are used to show that pinning plays a principal role in the whole mixed state in Bi2Sr2CaCu2O8 (BSCCO) single crystals. We speculate that the random pinning force on the moving vortices may dominate over thermal fluctuations and considerably modify the position of the vortex lattice phase transition.

Pinning-to-barrier crossover in YBa2Cu3O7−δ single crystals

The behaviour of magnetic flux in high purity untwinned YBa2Cu3O7−δ (YBCO) single crystals is visualised by means of the magneto-optical technique. It is observed that after zero field-cooling at high temperatures nearTc, flux penetrates directly to the sample center over a flux free edge area, in contrast to the usual Bean-like flux penetration from the edges. This fact clearly shows that volume pinning becomes negligible compared with the edge barrier. The role of the recently rediscovered geometrical barrier in the crystal magnetisation is discussed.

Morphology of growth of Bi2Sr2CaCu2O8 single crystals

A good correlation of twins on the basal surface of flux-grown Bi2Sr2CaCu2Ox (BSCCO) single crystals with surface. growth steps is observed, the b-axis being perpendicular to the steps and, thus, parallel to the growth direction. It is found that mono-twin BSCCO single crystals produced by the travelling solvent floating zone method also grow preferentially along b, i.e. nearly perpendicularly to the boule axis, contrary to the common belief. This new understanding of the morphology of growth explains the nature of major defects in these crystals, which considerably change their measured superconducting properties, in a different way.A good correlation of twins on the basal surface of flux-grown Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub X} (BSCCO) single crystals with surface growth steps is observed, the b-axis being perpendicular to the steps and, thus, parallel to the growth direction. It is found that mono-twin BSCCO single crystals produced by the travelling solvent floating zone method also grow preferentially along b, i.e. nearly perpendicularly to the boule axis, contrary to the common belief. This new understanding of the morphology of growth explains the nature of major defects in these crystals, which considerably change their measured superconducting properties, in a different way.

Instabilities of the force-free current configurations

Using the magneto-optic technique, it is shown that inductively induced force-free current configurations in high purity YBa2Cu3O7−δ single crystals become unstable above a certain well-defined amplitude and frequency of the variation of the applied perpendicular field.

Spatially resolved resistivity near the vortex lattice phase transition in Bi2Sr2CaCu2O8+δ single crystals

Abstract Using a multiterminal contact configuration, we investigate the local variations of the resistivity drop near the vortex lattice first order phase transition in a very homogeneous Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO) single crystal.

Influence of columnar defects on the thermodynamic properties of BSCCO

Amorphous columnar defects strongly affect the reversible magnetization of Bi2Sr2CaCu2O8+δ single crystals both in the vortex solid, where the change reflects the change in vortex energy due to pinning, and in the vortex liquid, where the randomly positioned columns disrupt the interaction between superconducting fluctuations.