Millimeter-wave response and linewidth of Josephson oscillations in YBa2Cu3O7 step-edge junctions

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Abstract
We have studied the response of YBa2Cu3O7 step‐edge junctions to low‐intensity millimeter‐wave radiation in the temperature range from 4 to 80 K. The linewidth of the Josephson oscillations derived from the resonant part of the response at voltages V≂(h/2e)f is shown to be determined by thermal fluctuations at liquid nitrogen temperatures. At lower temperatures the observed linewidth increases indicating that low‐frequency fluctuations become dominant in the junction as the temperature is reduced. Due to an inhomogeneous spatial distribution of the current the step‐edge junction might be considered as a multijunction multiloop interferometer and the excess noise can be discussed in terms of spontaneous transitions between the different states of these interferometers.