The influence of magnetic vortices motion on the inverse ac Josephson effect in asymmetric arrays

We report on the influence a preferential magnetic vortices motion has on the magnitude of the inverse ac Josephson effect (the appearance of dc current Shapiro steps) and the coherent operation of asymmetrical parallel arrays of YBa₂Cu₃O_7−δ Josephson junctions (JJ) irradiated with microwave (MW) radiation in the presence of an applied magnetic field B. The preferential direction of motion of the Josephson vortices is due to the asymmetry-induced ratchet effect and has a dramatic impact: for a particular positive dc bias current I when the flux-flow is robust multiple pronounced Shapiro-steps are observed consistent with a coherent operation of the array. This suggests an efficient emission/detection of MW in related applications. In contrast, when we reverse the direction of I, the flux-flow is reduced and the Shapiro steps are strongly suppressed due to a highly incoherent operation that suggests an inefficient emission/detection of MW. Remarkably, by changing B slightly, the situation is reversed: Shapiro steps are now suppressed for a positive I while well pronounced for a reverse current − I. Our results suggest that a preferential vortex-flow has a very significant impact on the coherent MW operation of superconducting devices consisting of either multiple JJs or an asymmetrically biased single long JJ. This is particularly relevant in the case of flux-flow oscillators for sub-terahertz integrated-receivers, flux-driven Josephson (travelling-wave) parametric amplifiers, or on-chip superconducting MW generators, which usually operate at bias currents in the Shapiro step region.