THE ORIGIN OF THE HALF-INTEGRAL CONSTANT VOLTAGE STEPS IN HIGH-T-C GRAIN-BOUNDARY JUNCTION

Half-integral constant voltage steps have been observed recently on th e I-V curve of the high-T-c grain-boundary junctions in the low-freque ncy regime. Instead of the nonsinusoidal current phase relationship ac ross the Josephson junction, we propose a phenomenological model that can satisfactorily explain the appearance of half-integral constant vo ltage steps. We regard the junction as being comprised of superconduct ing microbridges separated by normal regions. Those normal regions and current leakage might cause the junction to trap the magnetic flux an d induce a circulating current. The results of numerical simulations, based on the model mentioned above, are in agreement with the I-V char acteristics of the RF biased grain-boundary junctions. Moreover, we ha ve made a detailed comparison of the numerical and experimental data f or the microwave power dependence of the half-integral steps.