Discrete lorentzian structure in low frequency voltage noise spectra of very small area Josephson tunnel junctions

We report the results of low frequency voltage noise measurements on very small area (A = 0.05 to 1 μm²) Nb-Nb2O5-PbBi tunnel junctions as a function of tunnel junction quality and over a wide range of voltage and temperature. We find that the noise spectrum Svin our bandwidth (1Hz to 25.6kHz) is largely composed of a small number (2-3) of distinct Lorentzian components each of which can be characterized by its own voltage and temperature dependent magnitude and mean rate. These lorentzian components provide a powerful means to probe the actual microscopic fluctuation events which lead to 1/f noise in larger devices. The ensemble average spectrum of our devices trends about a 1/f frequency dependence and has a magnitude proportional to (IRj)²and 1/A. We have used our results to establish the intrinsic low frequency energy sensitivity of d.c. SQUIDs made with shunted tunnel junctions.