Imagine in a type-II superconductor a tiny tornado of circulating electric current of a radius of tens of nanometers surrounding a normal-state core (where magnetic field lines go through): You have a picture of a vortex. Such a vortex tends to repel another; it can move or become pinned by microscopic defects in the superconductor. When millions of them are generated by a strong magnetic field, vortices can—through an act of balancing the pinning effect and the intervortex repulsion—self-organize into a critical state, from which avalanche events involving the motion of many vortices can occur at the slightest magnetic or thermal trigger. Much has been learned about the critical state of the vortex matter and the dynamics of the associated avalanches, much remains to be explored and made sense of. In this experimental paper, we reveal new types of vortex dynamics through spatially resolved, real-time observations enabled by a state-of-the-art magneto-optical imaging technique.

The superconductors we investigate are thin (200-nm-thick) $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{\mathrm{x}}$ films grown to have a designed multiterraced surface structure. The magneto-optical images we obtain tell a vivid story. Bundles of vortices move along the terrace steps in the form of numerous, intermittent avalanches that grow like needles, some starting at the two film edges perpendicular to the steps and some from the deep interior of the film. Analyzing images of more than $10000$ avalanche events taken at different magnetic fields, we find that the avalanche-like motion of the vortices is essentially one-dimensional, intermittent, and self-similar and that it doesn’t require a threshold field.

What is the mechanism behind this observation? Our suggestion is the following: Linear defects, that exist and run in the direction parallel to the terrace steps, form narrow channels of confinement for the vortices. Traffic jams, or clogging, then occur along the channels in a random fashion as more and more vortices are generated in the channels and pinned by the strong pinning sites. Whenever one of the blocked parts of the channels breaks open due to excessive pressure from the incoming vortices, an avalanche occurs. This mechanism differs from that underlying the thermo-magnetic dendritic avalanches seen in many superconducting films.

This work has relevance for applications of superconducting films, where understanding the stability of vortex matter and achieving it is of crucial importance. Our results also raise fundamental questions about how the approach to the macroscopic critical state, either smooth or intermittent, is related to the physics of individual vortices in superconductors.