Response of fractal penetration of magnetic flux to disorder landscape in superconducting films

TL;DR

This study investigates how magnetic flux penetrates superconducting films with different defect sizes, revealing self-affine and fractal behaviors linked to disorder and self-organized criticality.

Contribution

It demonstrates the transition from self-affine to fractal flux penetration patterns depending on defect size distribution in superconducting films.

Findings

Self-affine flux fronts with roughness exponent ~0.66 in small defect films.

Fractal flux contours with dimension ~1.33 in large defect films.

Different flux penetration patterns indicate self-organized criticality at multiple scales.

Abstract

Magnetic flux front and induction contours in superconducting YBa2Cu3O7-d films with defect size s ~ l (superconducting coherence length) and s > l are studied by magneto-optical imaging. Robust self-affine spatial correlation was observed using scaling analysis in the small pinning-dominated (s ~ l) disorderd films. The roughness exponent was determined to be ~ 0.66, independent of numbers of defects (or the film thickness). When the disorder landscape also included a distribution of large defects (s >> l), the flux front and induction contours exhibited self-similarity, with a fractal dimension to be ~ 1.33 using the box-counting method. The remarkably different flux penetration patterns were shown to be the manifestation of self-organized criticality at different length scales.