VORTEX DYNAMICS IN YBA2CU3O7-DELTA SUPERCONDUCTING FILMS - EXPERIMENTAL EVIDENCE FOR AN INSTABILITY IN THE VORTEX SYSTEM AT HIGH CURRENT DENSITIES

摘要

Current-voltage characteristics and the temperature dependence of the electrical resistivity of YBa2Cu3O7-delta thin films were investigated in magnetic fields up to 5 T. For a given magnetic field, both voltage and resistivity jumps could be observed at high driving forces when the current density and the temperature reached the critical values J* and T*, respectively. Excellent scaling of the experimental data was obtained by describing the field and temperature dependence by J*(H,T) = J*(T)/(1+H/H-0)(alpha) and J*(T)-(1-T/T-c0)(3/2) with 1/3 less than or equal to alpha less than or equal to 3/4 and H-0 and T-c0 being constants. These results indicate a well-defined vortex instability closely resembling that predicted by Larkin and Ovchinnikov (LO) for high flux flow velocities. In the present case, however, by referring to the current-voltage (I-V) characteristics, this instability can be demonstrated to occur even at temperatures well below T-g defined by separating negatively from positively curved I-V characteristics. Analyzing the experimentally observed instability according to LO theory, the temperature dependence of the inelastic quasiparticle scattering time has been obtained. As a new aspect, the corresponding critical flux-flow velocity exhibits a significant field dependence for small magnetic fields, while for fields above 2 T this dependence seems to disappear. [References: 29]