On the Origin of Fractional Shapiro Steps in Systems of Josephson Junctions with Few Degrees of Freedom

We investigate the origin of fractional Shapiro steps in arrays consisting of a few overdamped Josephson junctions. We show that when the symmetry reduces the equations to that of a single junction equation, only integer steps appear. Otherwise, fractional steps will appear when the evolution equations contain second (or higher) order derivatives or non-sinusoidal terms. We make a point of distinguishing the last two possibilities in the generation of the fractional steps.