Current-phase relation of high-T/sub c/ bicrystal and step-edge Josephson junctions

The real and imaginary part of the rf impedance of a phase-biased weak link based on YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films were investigated experimentally at 77 K. The investigated weak links were step-edge or bicrystal Josephson junctions incorporated into a washer type superconducting ring with a small inductance L=80...300 pH. The ring was inductively coupled to a tank circuit with a resonant frequency of about 23 MHz. The phase shift /spl alpha/ between the rf voltage and rf current through the tank circuit was found to be a convenient measure of the change of inductance of a weak link. From the dependence of /spl alpha/ on the magnetic flux in the ring one can deduce the current-phase relation I(/spl phi/)=I/sub c/ f(/spl phi/) of the weak link. The influence of thermal noise on f(/spl phi/) was estimated numerically using a Gaussian distribution of the noise amplitude. The experimentally observed dependence /spl alpha/(/spl phi/) for the bicrystal weak link can be sufficiently good described with f(/spl phi/)=sin (/spl phi/) and with a reasonable value of the noise amplitude. The step-edge junction shows remarkable deviation from the sinusoidal current-phase relation.