Observation of the magnetic flux quantum that decides three dimensional magnetic field dependence of a superconducting junction

约瑟夫森电流的调制特性通常通过在与约瑟夫森结平面平行的方向上施加磁场来测量。通过对外加磁场的二维扫描，研究了约瑟夫森电流的调制特性。然而，众所周知，当磁通被困在超导薄膜中时，会对超导电子器件的特性产生不良影响。利用超导薄膜中捕获的磁通量量子测量了约瑟夫森结二维磁场依赖性的变化。这一次，我们通过测量I_c对垂直外加磁场的依赖关系，详细地研究了超导薄膜中磁通量被捕获的条件。当施加大于2400A/m的垂直磁场于Josephson结时，Josephson电流发生不可逆调制。这是因为磁通量被困在Nb超导薄膜中。接下来，我们在搜索线圈附近施加垂直磁场后将样品振动至零，并通过观察搜索线圈的输出电压来研究Nb超导薄膜的磁化情况。在约瑟夫森结处施加小于1800A/m的垂直磁场时，没有观察到搜索线圈的输出电压，也无法观察到捕获的磁通量。在约瑟夫森结处施加大于2400A/m的垂直磁场时，观察到搜索线圈输出电压为20 μV，并确定了捕获磁通。这些结果表明，由于捕获磁通，约瑟夫森电流被不可逆地调制。

Modulation characteristics of Josephson current is usually measured by applying the magnetic field in the direction parallel to the plane of a Josephson junction. We have studied the modulation characteristics of Josephson current by two-dimensional scan of the external magnetic field. However, it is generally known that when the magnetic flux is trapped into superconducting thin film, it has a bad influence on characteristics of superconducting electronic devices. We have measured the change in two-dimensional magnetic field dependence of a Josephson junction by magnetic flux quantum trapped in superconducting thin film. This time, we investigated the condition that the magnetic flux is trapped in the superconducting thin film in detail by measuring the dependence of I_c upon the perpendicular applied magnetic field. When the perpendicular magnetic field of more than 2400A/m was applied to the Josephson junction, Josephson current was modulated irreversibly. This is because the magnetic flux is trapped in Nb superconducting thin film. Next, we vibrated the sample near the search coil after a perpendicular magnetic field was applied and was reduced to zero, and investigated the magnetization of the Nb superconducting thin film by observing the output voltage of the search coil. When the perpendicular magnetic field of less than 1800A/m was applied to the Josephson junction, the output voltage of the search coil was not observed, and the trapped magnetic flux could not be observed. When the perpendicular magnetic field of more than 2400A/m was applied to the Josephson junction, the output voltage of the search coil of 20 μV was observed, and we confirmed the trapped magnetic flux. These results indicate that the Josephson current was irreversibly modulated due to trapped magnetic flux.