DYNAMICS OF SINGLE-PLAQUETTE JOSEPHSON-JUNCTION ARRAYS

We have studied, both analytically and numerically, the response of a single-plaquette square array to a combined ac and dc current. The array consists of 2x2 superconductors coupled by overdamped Josephson junctions. Our study shows that even a single-plaquette array exhibits all the essential features found in the large arrays; in zero magnetic field, or with a current in the diagonal direction, the array does not show any fractional Shapiro step. On the other hand, in a transverse external field except with diagonal application of a current, the dc I-V characteristics exhibit very rich structure, i.e., a step at every rational number of 2e [V] /hnu. The step widths DELTAI(n) calculated for n=1/2, 1/3, 1/4, 1/5, and 1/6 with varying filling factor f show varied characteristic features.