Shapiro Steps under Resonance Irradiation

We consider Josephson junction coupled to a high- quality parallel resonance circuit under microwave irradiation on the resonance frequency. The phase- locking of Josephson oscillations manifesting itself through Shapiro steps is studied using analytic theory developed for the circuit. Substantial change in amplitude of the oscillating part of Josephson phase within the phase-locking range determines specificity of the step behavior. This analysis is compared with results of numerical simulation.