Magneto-optical observations of vortices in anisotropic and nanostructured superconductors

Magneto-optical imaging technique is applied to visualize vortex distribution in superconducting networks and in highly anisotropic superconductor Bi₂Sr₂CaCu₂O_8+y under tilted magnetic fields. We have succeeded in visualizing the vortex distribution in a finite-size superconducting network with the unit cell size as small as 6 x 6 μm². We have also visualized the ratchet effect of pancake vortices induced by the time-asymmetric motion of Josephson vortices. The ratchet effect appears most clearly at the similar condition as the vortex lensing effect. The ratchet efficiency diminishes strongly as a function of frequency and it is almost completely suppressed above 1 kHz. Such a strong frequency dependence of the vortex ratchet effect is discussed based on the skin effect controlled by c-axis resistivity