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
μ
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-AlO
_x
/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.