THE AC JOSEPHSON EFFECT AND SUBMILLIMETER WAVE MIXING WITH A WEAK-LINK ARRAY OF GRAIN BOUNDARIES FORMED IN YBCO FILM

For application of 100-320 GHz frequency signals to constrictions of YBCO polycrystalline films, Shapiro steps were observed at the exact voltages Vn=n((h) over bar omega/2e) expected for a single junction, where n is an integer. The constriction is formed with network of grain boundary Josephson weak-links, however, our result is different from the coherent step response of N*N array (S.P.Benz, Phys. Rev. Lett. 64, 693, 1990) which arises at N times voltage of (h) over bar omega/2e. At temperature from 53 to 65 K, the heterodyne harmonic mixing experiment was carried out for 318.09304 GHz signal. Peaks of IF signal ( 95.2 MHz) output were 105.99928 GHz local signal. The result suggests that the wave function is continuous throughout the weak-link array of YBCO polycrystlline film, and the frequency of oscillating pair current depends on the total potential difference across the array. It is clear that a network of grain boundaries works as a single Josephson element, and can be used for practical devices operating at submillimeter-wave fequencies. A simple model and experiments are described.