Research of Superconductive Chaos Generator Device "Josephson Tetrode"

在本研究项目中，对由五个Josephson弱链结组成的四端超导器件“Josephson tetrode”中混沌的产生进行了数值模拟。这种新型超导器件可应用于数百千兆赫频率下的超快速随机信号生成。在约瑟夫逊四极中，两个弱连接结是串联的，另外三个连接结是并联的。当其中一个正常电阻发生变化时，计算了跨结电压的动态变化。利用分岔图、三维吸引子和庞加莱截面证实了混沌的产生。分岔图可以解释为混沌的准周期击穿情形。混沌产生的机制是交叉结点上三个独立电压之间的非线性频率混合。约瑟夫森四极是由五个台阶边缘型约瑟夫森结组成的。因此，将二维金兹堡-朗道方程应用于阶边Josephson结，并解析导出了其解。计算了临界电流的流相关系和温度依赖关系。

In this research project, the generation of chaos in "Josephson tetrode", a four-terminal superconductive device made of five Josephson weak-link junctions was numerically demonstrated. This novel superconductive device can be applied to ultra-fast random signal generations at frequencies of hundreds of gigahertz.In the Josephson tetrode, two weak-link junctions are series-connected and other three junctions are parallel-connected. The dynamics of electrical voltages across the junctions were calculated when one of the normal resistances was varied. The generation of chaos was confirmed by using a bifurcation diagram, three-dimensional attractors and the Poincare sections.The bifurcation diagram can be interpreted as the quasi-periodicity-breakdown scenerio to chaos. The mechanism of chaos generation is a nonlinear frequency mixing among three independent voltages across the junctions.The Josephson tetrode was made of five step-edge type Josephson junctions. So two-dimensional Ginzburg-Landau equation was applied to the step-edge Josephson junction and the solution was analytically derived. The current-phase relationship and temperature dependence of the critical current were calculated.