Josephson harmonic mixing and internal oscillation-mixing in YBCO superconducting weak link at liquid nitrogen temperatures

Using a superconducting weak link (bridge) made of YBCO, the authors have successfully observed Josephson harmonic mixing between a local oscillation (LO) at X band and a signal at Ka band. Specifically, fourth-order harmonic mixing between 35.47 GHz and 8.843 GHz was observed. For a given signal level, the IF (intermediate frequency) responses depended strongly on both LO level and DC voltage bias across the junction. For zero bias, there is still IF output which is characteristic of even-order harmonic mixing. When the LO power level is such that the zero voltage current is suppressed to the second zero, the IF responses are maximum if the junction is biased between the zeroth and first steps induced by the LO. Also studied is the internal oscillation-mixing. An incoming signal at either X or Ka band gives rise to IF signal peaks if the junction is biased at voltages such that the corresponding internal Josephson frequencies are equal to, or harmonics of, the incoming signal frequency