Low-frequency noise and discrete charge trapping in small-area tunnel junction dc SQUID's

We present measurements of the noise properties of very small-area, high current density tunnel junction dc superconducting quantum interference devices (SQUID's). The low-frequency noise spectra display broad curves and bumps consistent with the presence of individual Lorentzian features superimposed on a background which is always much flatter than 1/f. When these features are most prominent, the voltage across the SQUID exhibits discrete switching behavior which we attribute to the trapping and untrapping of single electrons within the tunneling barrier. This observation, along with other evidence presented, suggests that critical current fluctuations displaying tunneling kinetics dominate the low-frequency noise of these ultrasmall devices.