Flux Dendrites of Opposite Polarity in Superconducting MgB$_2$ rings observed with magneto-optical imaging

TL;DR

This study uses magneto-optical imaging to observe flux dendrites of opposite polarity in superconducting MgB$_2$ rings, revealing complex interactions and trigger mechanisms during flux penetration and perforation.

Contribution

It provides the first direct visualization of opposite-polarity flux dendrites and their interactions in MgB$_2$ rings, highlighting new trigger mechanisms and flux dynamics.

Findings

Opposite-polarity flux dendrites can trigger anti-dendrites.

Dendrites cause perforations and flux redistribution in MgB$_2$ rings.

Trigger mechanisms include non-local magnetic and local thermal effects.

Abstract

Magneto-optical imaging was used to observe flux dendrites with opposite polarities simultaneously penetrate superconducting, ring-shaped MgB$_2$ films. By applying a perpendicular magnetic field, branching dendritic structures nucleate at the outer edge and abruptly propagate deep into the rings. When these structures reach close to the inner edge, where flux with opposite polarity has penetrated the superconductor, they occasionally trigger anti-flux dendrites. These anti-dendrites do not branch, but instead trace the triggering dendrite in the backward direction. Two trigger mechanisms, a non-local magnetic and a local thermal, are considered as possible explanations for this unexpected behaviour. Increasing the applied field further, the rings are perforated by dendrites which carry flux to the center hole. Repeated perforations lead to a reversed field profile and new features of dendrite activity when the applied field is subsequently reduced.