Uncertainty Evaluation of Electrical Parameters of High-Temperature Superconductor Josephson Junctions

High-temperature superconductor quantum voltage standard operates at liquid nitrogen temperature, which is favorable for wide-spread application of quantum voltage standard. Josephson voltage is proportional to the applied microwave frequency, which should be matched to the eigen-frequency of the junction. Therefore, the normal state resistance and critical current of the junction should be accurately evaluated to obtain its eigen-frequency. High-temperature superconductor bicrystal Josephson junctions are fabricated on LSAT substrates, and 4-probe method is used to measure their I-V characteristic curves, from which the normal-state resistance and the critical current are extracted. Due to thermal-noise rounding effect and self-heating, the I-V characteristics of high-temperature superconductor Josephson junctions are usually distorted, thus the normal-state resistance and intrinsic critical current values are varied. In this work, geometric-mean criterion and RSJ model are combined for proper evaluation of both parameters and their uncertainty level. Experimental results show that the 4-µm bicrystal junctions on LSAT substrates is a SNS junction and conforms to the RSJ model, with a normal-state resistance value of 0.06397 Ω and a synthetic relative standard uncertainty of 6.1%. The intrinsic critical current is 3.3214 mA with a synthesize relative standard uncertainty of 6.0%. This work will support the development of a practical high-temperature superconducting quantum voltage standard device.