Anistropic flux dynamics in YBa2Cu3O7−δ single crystals

PHYSIOA®

Physica C 235-240 (1994)3059-3060 North-Holland

Anisotropic flux dynamics in YBa2Cu307_ 6 single crystals V. F. Solovjov a, V. M. pana,b, H. C. Freyhardtb alnstitute for Metal Physics, 36 Vernadsky blvd., 252142 Kiev, Ukraine blnstitute for Metal Physics, University of Goettingen, HospitalstraBe 3-7, D-37073, Germany

Direct transport measurements of the angular dependencies of critical current density for single-crystalline YBa2Cu307. 8 have been carried out in wide ranges of temperatures and applied magnetic fields. The consistent model of the highly anysotropic flux dynamics as well as the origin of the "fishtail" effect in field dependencies of critical current was suggested on the basis of scaling analysis of current-voltage characteristics. Anisotropy effects in the flux dynamics for YBa2Cu3OT. 6 single crystals were studied by means of transport current measured I-V curves in a wide range of angles, external fields and temperatures. The single crystals are twinned (twin space is 0,2-0.5~m) and shown the sharp transition at 89-90 K with ATc=0.1-0.3 K. A short pulse technique has allowed to record I-V curves of the extremely small-sized samples. A pulse duration was 10Bs, its amplitude up to 10 Amps. A voltage measurement accuracy of 0.2 BV was limited by a noise level. The angular and field Jc dependencies are analysed with the shape of the I-V curves along. The Jc(O) plots (Fig.l) reveal a remarkable minimum at HUC-configuradon ((9=0) and symmetrical maxima at ®--+20°. The nonmonotonic Jc(H) behavior ("fishtail-effect") appeared to be more pronounced in tilted magnetic fields. It was observed clearly that the fishtail is influenced by the field component directed along the C-axis. Two alternative approaches were utilized for the flux motion description: thermallyactivated-hopping-based approach (vortex glass, Bose glass, collective creep) and classical meanfield approximation (MFA) for an elastic continuum in a random pinning potential. The MFA analysis of the I-V curves for the different ®, H and T is proven to be much more consistent. We argue that in the case of the 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 the key

role play not only the depth of the pinning wells but the form of pinning potential, elastic and viscous properties of the FLL. Scaling behavior predicted by 41.6 - • x l 0 1.4 ~/e xI

0921-4534/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSDI 0921-4534(94)02095-7

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the MFA analysis was clearly observed for the lowfield I-V curves (Fig.2).We also observed a breakdown of scaling when the fishtail-effect is arisen. The fishtail as well as the Jc(®) minimum at H[IC were shown to be dependent strongly upon the E ccriterion used for Jc definition. The E c increase by two order of magnitude suppresses the fishtail and transforms Jc(®) minimum to maximum (Figs.3,4). The scaling breakdown and fishtail-effect appeared to be highly dependent upon the relationship of the relaxation and pinning times. The pinning time is defined by the velocity of flux line travelling over the pinning site and by the character of its potential. while the relaxation time is determined by the elastic FLL response on an external impact. When the E c criterion is high enough it means the FLL is moving so fast that it can not be trapped anymore to randomly distributed and quickly changing minima of the pinning potential caused by the point-like quenched disorder. The scaling is recovering and the fishtail is vanishing. However the extended defects (twins) still are capable to pin the FLL by the correlated disorder created. It is a reason for the Jc(®) minimum to be transformed to maximum.

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