NEAR-MILLIMETER-WAVE RESPONSE OF HIGH-TC RAMP-TYPE JOSEPHSON-JUNCTIONS

We have studied the response of a YBCO/PBCO/YBCO ramp-type junction to coherent radiation at 176 and 270 GHz. The I-V characteristic of the junction closely resembles the prediction of the RSJ model. The I(c)R( n) product of the junction is 0.25 mV at 5 K. The millimeter-wave radi ation is coupled to the junction via a quasioptical structure that foc uses the radiation onto the junction through a yttrium-stabilized ZrO2 substrate. At 176 GHz, we have observed as many as six Shapiro steps at the maximum power level of our Gunn oscillator-pumped frequency dou bler. Shapiro steps are still clearly seen up to 65 K. The amplitudes of the zeroth, first, and second Shapiro steps, as functions of the sq uare root of the radiation power, agree remarkably well with a Bessel function fit, indicating the junction is voltage-biased at the radiati on frequency (rf). At 270 GHz, due to a combination of the heavy rf lo ss in the ZrO2 substrate and the lack of radiation power, we have obse rved only the first Shapiro step.