Dynamical states in annular Josephson junctions: Amplitude dependence of zero field steps on the magnetic field

In this work we report on experiments about the dynamical resonance properties of annular Josephson junctions of intermediate (with respect to the Josephson penetration depth) size. We investigate the dynamical states of the first four resonances appearing in the current-voltage characteristic. The experiment conditions permit a simultaneous observation of Zero Field Steps and Fiske Steps and interplay between them. Two sets of measurements were performed: in the absence of trapped flux, with trapped flux. The magnetic field induces either a sharp or a continuous transition from the Zero Field to Fiske Steps. We interpret this observed behaviours in terms of dynamical states involving fluxon – antifluxon pairs or states involving fluxons of the same polarity. We show that is possible to fit always the Fiske Step behaviour beyond a magnetic field threshold by using a theory strictly valid for small annular junctions and that the same theory can also reproduce, in some specific case, the Zero Field Step behaviour.