The maximum zero-voltage tunnel current and its fluctuations for lead—lead oxide—lead Josephson tunnel junctions

The maximum zero-voltage tunnel current I
0
and its fluctuations were measured for Josephson tunnel junctions as a function of the normal state resistance
R
in the range of 50-300 Ω at
T = 4.2
K. The junctions, based on pure lead, have areas of about 100 µm
²
, corresponding to capacitances
C
of about 3 pF. The results obtained from a simple noise model, describing the junction in the zero-voltage state as a parallel resonant circuit with parameters
C, L \sim \Phi_{0}/2\piI_{0S}
, and
R
, are in good agreement with the experiments. It is shown that the deviation of the experimental results for high-resistivity junctions from the theory of Ambegaokar and Baratoff is due to thermal noise currents originating in the normal state resistance
R
.