LOW-FREQUENCY NOISE PROPERTIES OF ALL-THIN FILM SUPERCONDUCTING POINTCONTACTS

We report the results of low frequency voltage noise measurements of a n all-thin film superconducting point contact (SPC). The SPC is prepar ed in liquid helium by application of a voltage pulse to a Nb-MgO-NbN edge sandwich. Increasing the amplitude of the pulse we may observe a gradual transition of a junction from the tunneling to metallic transp ort regime. We study the magnitude of low frequency noise during this transition and find that noise decreases quickly with decreasing ratio of subgap-to-supergap resistance. This result is contrasted with an o pposite finding of C. T. Rogers and R. A. Buhrman [IEEE Trans. Magn. M AG-21, 126 (1985)] for the limit of a ''leaky'' tunnel junction. This comparison suggests that the excess subgap conductivity observed in ou r samples is caused by an Andreev reflection.