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

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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.