Vortex Instability and Vortex-Glass Transition in Bi_2Sr_2CaCu_2O_8 and YBa_2Cu_3O_7 Thin Films

摘要

We investigated the current-voltage (I-V) curves of high-T_c superconductors at very low and very high dissipation levels. In the limit of low driving currents the barriers for vortex movement become infinite in the vortex-glass state. Using long measurement bridges up to 0.5 m we were able to sample an electric field range from 1 V/m down to 10_(-8) V/m in one experimental setup. The resulting I-V curves allowed us to verify an excellent glass scaling of the I-V curves, which revealed an increased dynamical exponent of the glass transition. We also found a considerable dependence of the vortex-glass scaling on the probed electric-field range. At very high dissipation levels the I-V curves of type II superconductors in magnetic fields can show voltage jumps due to flux-flow instabilities at high vortex-velocities. They have been investigated in Bi_2Sr_2CaCu_2O_8 and YBa_2Cu_3O_7. The results have been interpreted in the framework of the theory of Larkin and Ovchinnikov. The extensions introduced by Bezugly and Shklovskij account for unavoidable quasiparticle heating during the measurement. The influence of avoidable heating effects on the I-V curves was studied experimentally by pulsed measurements with a time resolution in the microsecond range. In Bi_2Sr_2CaCu_2O_8 we found the instability in the vortex-liquid phase. However, in YBa_2Cu_3O_7 a remarkable conincidence between the vortex-glass phase, which manifests itself at low dissipation, and the existence of the high dissipative flux-flow instability was observed.