Suppression of magnetic flux avalanches and recovery of the critical state in superconducting NbN films

TL;DR

This study demonstrates that coating NbN superconducting films with copper suppresses thermo-magnetic instabilities, allowing the recovery of the critical state and preserving shielding currents, as confirmed by magneto-optical imaging.

Contribution

The paper introduces a method to suppress flux avalanches in NbN films using copper coating, enabling the recovery of the critical state and stable shielding currents.

Findings

Copper coating suppresses thermo-magnetic instability in NbN films.

Critical state is restored in the coated part of the film.

Shielding current distribution is altered by the coating, with most flowing within the stable region.

Abstract

Thermo-magnetic instability (TMI) in superconductors is known to destroy the critical state via magnetic flux avalanches, and hence it deteriorates the ability of the superconductors to shield external magnetic field. In this work, we quantify to what extent the shielding current is affected by TMI. We recover the critical state in one half of a thermo-magnetically unstable NbN film by coating it with a thin layer of Cu. Suppression of the instability in the metal coated part is confirmed by the results of a direct real time magneto-optical imaging. A pattern of discontinuity lines in the observed flux distribution indicates that only one quarter of the shielding current flows through the whole sample, including the unstable uncoated part, while three quarters flow in loops within the stable Cu-coated part.