Research of Superconductive Chaos Generator Device "Josephson Tetrode"

在本研究项目中，数值演示了由5个约瑟夫森弱连接结组成的四端超导器件“约瑟夫森四极管”中混沌的产生。在约瑟夫森四极管中，两个弱连接结串联，另外三个结并联，可用于产生频率为数百千兆赫的超快随机信号。当其中一个法向电阻变化时，计算了结两端电压的动态变化。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场景。混沌产生的机制是三个独立电压之间的非线性混频，约瑟夫森四极管由五个阶梯边型约瑟夫森结组成。因此，将二维Ginzburg-Landau方程应用于阶跃边约瑟夫森结，并进行了解析推导。计算了临界电流的电流-相关系和温度依赖性。

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.