Flux Flow Noise in Superconductors and Chaos in Josephson Systems.
This dissertation includes three parts of work. The first part is flux flow noise measurements in type II superconductors. We have used an RF SQUID to measure the motion of flux flow caused by the presence of an electric current inside In(,0.9) -Pb(,0.1) alloy sheets. Two different regions of flux flow have been investigated. In the quasilinear region, the amplitude of the ac signal across the SQUID output has small value and smooth variation as the current I is changed. Power spectra are featured by the form of A(1+((pi)f/f(,c))('2))('-1). The characteristic frequency f(,c) extracted from data shows a linear relationship with (I-I(,c)). A simple model is developed to explain the above phenomena. Bundle sizes and the density of bundles are obtained. In the near onset region, the amplitude of the ac signal has large value and shows peak structures. Power spectra in this region assume the form of l/f, which are explained by modifying the model we developed. The second part is a study of chaos in a Josephson junction simulator system. With only an ac driving current the system can enter chaotic state from periodic state via period-doubling bifurcations or intermittent transitions. All the universal constants associated with these two routes are measured and in agreement with theoretical predictions. When the system is driven by both ac and dc currents, it can enter chaos via quasiperiodicity. Hopping between different states is a very important mechanism for chaos in this case. The experimentally measured return maps show some generic features which are similar to those obtained from the one-dimensional circle map calculations. A self -similarity structure is found in the current step plateaus in low (beta)(,c) region and a fractal dimension D = 0.91 for spaces between those plateaus is obtained in a narrow region. The last part includes a study of interactions between six phase slip centers which coexist within 10 (mu)m long indium filaments; and the results of two strongly coupled microbridges in the presence of microwave radiation. Strong distortions including negative-resistance regions are found in I-V curves.
- Pub Date:
- Physics: Condensed Matter