Charge fluctuations and zero-bias resistance in small-capacitance tunnel junctions

The zero-bias resistance in small capacitance tunnel junctions was recently measured by Cleland, Schmidt, and Clarke and these authors used a quantum Langevin model to explain the data [Phys. Rev. B 45, 2950 (1992)]. Their model takes into account Nyquist noise generated in the leads but here we extend it to include all sources of Nyquist noise with emphasis on the noise generated in the junction itself. This enables us to get better agreement with the experimental data, particularly for junctions with high tunnel resistance and high lead resistance.