Microwave-induced flow of vortices in long Josephson junctions

We report an experimental and numerical study of microwave-induced flow of vortices in long Josephson junctions at zero dc magnetic field. Our intriguing observation is that applying an ac bias of a small frequency $f⪡{f}_{p}$ and sufficiently large amplitude changes the current-voltage characteristics ($I-V$ curve) of the junction in a way similar to the effect of dc magnetic field, well known as the flux-flow behavior. The characteristic voltage $V$ of this low voltage branch increases with the power $P$ of microwave radiation as ${V}_{s}{\propto}{P}^{{\alpha}}$ with the index ${\alpha}{\simeq}0.5$. Experiments using a low-temperature laser scanning microscope unambiguously indicate the motion of Josephson vortices driven by microwaves. Numerical simulations agree with the experimental data and show strongly irregular vortex motion. We explain our results by exploiting an analogy between the microwave-induced vortex flow in long Josephson junctions and incoherent multiphoton absorption in small Josephson junctions in the presence of large thermal fluctuations. In the case of long Josephson junctions the spatially temporal chaos in the vortex motion mimics the thermal fluctuations. ... mehrIn accordance with this analogy, a control of the intensity of chaos in a long junction by changing its damping constant leads to a pronounced change in the shape of the $I-V$ curve. Our results provide a possible explanation to previously measured but not yet understood microwave-driven properties of intrinsic Josephson junctions in high-temperature superconductors.