Development of high-frequency current probe utilizing high T_c Josephson junction

在本研究中，我们开发了用于高频电流探头的高T_c超导磁传感器。我们的研究结果总结如下：我们开发了利用双晶结的磁传感器。利用结的电流-电压特性随磁场的变化，可以将磁场转换为电压，转换效率为dV/dB。与SQUID相比，本传感器可以测量磁场的绝对值。磁传感器采用30度双晶结制成。为了优化转换效率dV/dB，从理论和实验两方面研究了该结的I-V曲线。阐明了影响转换效率的关键因素，如结宽与约瑟夫森穿透深度之比λ_j、磁通聚焦效应和温度依赖性。并与传统感应线圈串联阵列的性能进行了比较。当传感器面积为100 μm × 50 μm时，结阵列的灵敏度在7 GHz以内比感应线圈高。此外，与感应线圈的情况不同，灵敏度与频率无关。因此，使用目前的传感器可以进行宽带测量。用磁传感器制成电流探头，测量高频电流产生的磁场。成功地测量了频率为130 MHz的电流。随着测量系统的改进，我们将演示对GHz信号的测量。

In this research, we developed high T_c superconducting magnetic sensor for high-frequency current probe. Our results are summarized as follows.1. We developed the magnetic sensor utilizing bicrystal junctions. Using the change of the current-voltage characteristics of the junction with the magnetic field, we can convert the magnetic field to the voltage with the conversion efficiency of dV/dB. Compared to the SQUID, the present sensor can measure the absolute value of the magnetic field.2. The magnetic sensor is made using the 30-degree bicrystal junction. The I-V curve of the junction is studied both theoretically and experimentally in order to optimize the conversion efficiency dV/dB. Key factors which determine the conversion efficiency, such as the ration of the junction width to the Josephson penetration depth λ_j, flux focusing effect, and temperature dependence, are clarified.3. Performance of the series array of the junction is compared with that of the conventional induction coil. When the sensor area is 100 μm x 50 μm, the sensitivity of the junction array is more sensitive up to 7 GHz than the induction coil. Moreover, the sensitivity is independent of the frequency, unlike the case of the induction coil. Therefore, wide-band measurement is possible with the present sensor.4. Current probe is made with the magnetic sensor, and the magnetic field produced by the high frequency current is measured. The current with frequency of 130 MHz is successfully measured. With the improvement of the measurement system, we will demonstrate the measurement of GHz signal.