Research of Superconductive Chaos Generator Device "Josephson Tetrode"

在本研究计画中，数值模拟了由五个约瑟夫森弱连接结所组成的四端超导元件“约瑟夫森四极管”中混沌的产生。该器件采用两个弱连接结串联、三个弱连接结并联的结构，可用于产生频率为几百GHz的超快随机信号。当其中一个正常电阻变化时，计算了跨结的电压的动态。利用分岔图、三维吸引子和Poincare截面证实了混沌的产生，分岔图可以解释为混沌的准周期性破裂现象。混沌产生的机理是三个独立的跨接电压之间的非线性频率混合。为此，将二维Ginzburg-Landau方程应用于阶跃边约瑟夫森结，并得到了解析解。计算了临界电流的电流-相位关系和温度依赖性。

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.