A Measurement Method of Critical Current of Un-Shunted Josephson Junction

The measurement of critical current I-c, as an important parameter of un-shunted Josephson junctions (JJ), is critical for designing superconductor analog devices or digital circuits. However, precise measurement of I-c from current-voltage (I-V) characteristic curves often faces difficulties, such as the disturbance of environmental magnetic and electromagnetic waves (EM) noise, which significantly decreases the I-c value from ideal critical current I-c(0) and increases the uncertainty of I-c measured value (>10 %). In this article, a test procedure is proposed for measuring the I-V curve of un-shunted JJ under a magnetic shielding environment, effectively avoiding deficiencies of conventional methods such as poor uncertainty and lower I-c measurement value. The uncertainty of I-c is investigated based on different superconducting JJs, and the results show that the combined uncertainty of I-c is less than 2% (the current step <1 mu A). Also, a formula expressing the relationship of I-c with the radius of JJ is proposed, which may provide a good way of predicting I-c for practical experimental measurements. The effectiveness of the procedure and the formula are both verified by experiments based on the Nb/Al-AlOx/Nb samples. The method is helpful for round-robin standard measurement on I-c of un-shunted JJ and shortens the investigation process feedback time of the RSFQ integrated circuit.