Nonequilibrium Vortex Dynamics and Resistance Fluctuations in Superconductors.
In Part I of this thesis, we studied vortex avalanches in superconducting niobium films where vortices move in the form of avalanches at low temperatures. The size of the avalanches has a broad distribution that can be fitted with power-laws, which may be hints of self-organized criticality. The avalanche dynamics are independent of the ramp rate, indicating the external driving force is always slow compared to the vortex system's intrinsic dynamics. By varying the temperature or external field, the vortex dynamics undergo a sharp transition between avalanche-dominant and smooth behaviors. Simultaneous measurements with two probes positioned at different places on the sample revealed that some avalanches are localized and that some are global, which is consistent with a broad distribution of vortex bundle sizes. In Part II, we used transport and 1/f noise measurements to study the nature of the normal-state conduction in YBCO thin films and grainboundaries. In YBCO thin films with different oxygen content, x, we found a sharp minimum of the normalized 1/f noise power at x = 6.5. We attribute this behavior to vacancy creation in Cu-O chains and discuss a simple model based on parallel conduction paths through both chains and planes. In YBCO grainboundaries, we found a characteristic temperature dependence of the resistance fluctuations and a linear temperature dependence of the grain boundary resistance. These observations are quantitatively compared with a model of many, parallel conduction paths across the boundary, taking into account both temperature-independent and thermally activated transport processes.
- Pub Date:
- January 1995
- Physics: Condensed Matter