RF RESPONSE OF SINGLE-PLAQUETTE JOSEPHSON-JUNCTION ARRAYS

We derive an equation for a single-plaquette Josephson-junction array for an arbitrary number of flux quanta per unit cell f. We show that for f=1/2 this equation is equivalent to that derived by Rzchowski et al. for a 2x2 array in the f=1/2 ground state. We find that in the presence of an rf drive the system exhibits both integer and fractional Giant Shapiro steps at [V]=nhv/4e, where nu is the rf frequency and n=1,2,3,..., for all values of the flux quantum f. In addition, very small subharmonic steps at [V]=nhnu/2em, where m=1,2,3,... and not equal to n, are also observed for all f; however, these steps are found to be consistently smaller than the integer and half-integer fractional steps concurrently present in the I-V characteristics. We discuss these results and the array dynamics which lead to the described behavior.