Flux penetrations into two- and three-dimensional nanostructured superconductors

We have fabricated two- and three-dimensional nanostructured superconductors, and observed vortex penetrations by magneto-optical imaging. In the case of two-dimensional superconducting networks with square holes on a square lattice, anomalous diagonal penetrations are widely observed. Two kinds of diagonal vortex penetrations at high and low temperatures have been interpreted as originating from the repulsive interaction of vortices and sharp fan-shaped vortex penetration from the corners of the square holes, respectively. In the case of three-dimensional stack of superconducting strip arrays with double and triple layers, vortex avalanches have been observed in a wide temperature and dimension ranges due to enhanced demagnetization effect. While spotlike avalanches are observed when the overlap of strips is small, anomalous linear avalanches traversing many strips in different layers are observed when the overlap is large. In triple-layer strip arrays, in addition to the spotlike and linear avalanches, vortex penetrations along the line of strips are also observed. Origins of the anomalous diagonal penetration and vortex avalanches are discussed.