Specific capacitance of Josephson tunnel junctions

The specific capacitance of several types of Josephson tunnel junctions has been measured by observing resonances in lightly-damped 2-junction interferometers. The capacitance was calculated using the resonance voltage, obtained by analyzing steps in the I-V characteristics, and the interferometer inductance, which was measured directly by the injection of a control current. Using this technique, the specific capacitance C
s
was determined for tunnel junctions with Pb-In-Au alloy base electrodes and Pb-Bi counterelectrodes, as well as for junctions made on Nb films with Pb-In-Au counterelectrodes. In both cases, barriers were produced by rf plasma oxidation. Junctions with Josephson current densities j
1
between 200 and 5000 A/cm
²
were investigated. It was found that 1/C
s
decreased with
\log j_{1}
, and that at j
1
= 1000 A/cm
²
, C
s
was 4.2 ±0.3 μF/cm
²
for the Pb-alloy junctions and 13.4 ±1 μF/cm
²
for the Nb junctions. These results are discussed in relation to available data on oxide thickness and dielectric constant.