Shapiro step responses in the flux-flow state of bi-2212 intrinsic Josephson junctions with cooperation of pancake vortices

Flux-flow properties of one-dimensionally long Bi-2212 intrinsic Josephson junction stacks are studied. Quasioptically irradiated millimeter waves drives Josephson vortices and reveals current steps in equal voltage intervals corresponding to the Josephson relation. Differential conductance properties and the power dependence of the current step height are also measured. The experimental results indicate that the responses are consistent with the Shapiro step. Periodic potentials produced by pancake vortices are considered to be effective to realize the in-phase collective motion of the Josephson vortices.