MILLIMETER-WAVE RESPONSE AND LINEWIDTH OF JOSEPHSON OSCILLATIONS IN YBA2CU3O7 STEP-EDGE JUNCTIONS

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 congruent-to (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.