Environmental Effect on Ultrasmall Josephson Junctions

In order to study the effect of environment on ultrasmall Josephson junctions, we measured the temperature dependence of the zero-bias resistance of single ultrasmall Josephson junctions with different lead resistances. The normal-state resistance of the junctions was chosen to be around the resistance quantum R Q = h/4e 2 = 6.45 kΩ. We found that the temperature dependence was strongly dependent on the lead resistance. This result indicates that there exists a clear environmental effect on ultrasmall Josephson junctions. The observed environmental effect can be explained in terms of the difference of bias conditions or the noise effect on superconductivity, both of which can be effectively controlled by the lead resistance. The marked noise effect was also manifested by the magnetic field dependence of the V–I characteristics of linear junction arrays with tunnel resistances much lower than R Q.