Avalanche-driven fractal flux distributions in NbN superconducting films

TL;DR

This study investigates flux avalanches in NbN superconducting films, revealing temperature-dependent dendritic patterns with fractal characteristics and identifying thresholds for stability loss and recovery.

Contribution

It provides new insights into the temperature and field dependence of flux avalanches and fractal structures in NbN superconductors using magneto-optical imaging.

Findings

Flux avalanches form dendritic structures below 5.5 K.

Dendrite size and fractal dimension increase with temperature.

Stability is restored above specific temperature and field thresholds.

Abstract

Flux distributions in thin superconducting NbN films placed in a perpendicular magnetic field have been studied using magneto-optical imaging. Below 5.5 K the flux penetrates in the form of abrupt avalanches resulting in dendritic structures. Magnetization curves in this regime exhibit extremely noisy behavior. Stability is restored both above a threshold temperature T* and applied field H*, where H* is smaller for increasing field than during descent. The dendrite size and morphology are strongly T dependent, and fractal analysis of the first dendrites entering into a virgin film shows that dendrites formed at higher T have larger fractal dimension.