Anisotropic flux dynamics in single-crystalline and melt-textured YBa2Cu3O7- delta.

Abstract

Direct transport measurements of the angular dependencies of the critical current density for single-crystalline and melt-textured ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ have been carried out in the temperature range between 77 K and ${\mathit{T}}_{\mathit{c}}$ and applied magnetic fields up to 2.0 T. In contrast to the melt-textured samples, the ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta}) dependencies for the twinned single crystals reveal a minimum in the H\ensuremath{\parallel}c configuration (\ensuremath{\theta}=0). The nonmonotonic ${\mathit{J}}_{\mathit{c}}$(H) behavior (fishtail effect) appeared to be much more pronounced in tilted magnetic fields. The fishtail as well as the ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta}) minimum at H\ensuremath{\parallel}c were shown to depend strongly on the ${\mathit{E}}_{\mathit{c}}$ criterion for ${\mathit{J}}_{\mathit{c}}$ definition. An ${\mathit{E}}_{\mathit{c}}$ increase by two orders of magnitude suppresses the fishtail and transforms the ${\mathit{J}}_{\mathit{c}}$(\ensuremath{\theta}) minimum to a maximum. Two alternative approaches for the flux dynamics were used: (i) thermally activated-vortex-hopping theories and (ii) the classical mean-field approximation (MFA). The MFA analyses of the I-V curves for the different \ensuremath{\theta}, H, and T were much more consistent and informative. In the case of the flux-line lattice (FLL) moving in the pinning potential there should exist a region (defined by the level of dissipation) between creep-controlled FLL motion and Bardeen-Stephen flow.In this region a key role is played by not only the depth of the pinning wells but also the features of the pinning potential, elastic, and viscous properties of the FLL. A scaling behavior predicted by the MFA analysis was observed for the low-field I-V curves. A breakdown of scaling was detected when the ``fishtail effect'' arose. The fishtail effect is shown to have a dynamic nature. It is controlled by vortex interaction with random pointlike disorder and influenced strongly by the FLL relaxation if the relaxation and pinning times are comparable. The twins are supposed to promote a correlated disorder and inhibit the three-dimensional pointlike random disorder as a dominant state for the pinning of the FLL. This is a reason for the ${\mathit{J}}_{\mathit{c}}$ minimum at H\ensuremath{\parallel}c, observed at the low level of dissipation.