In a superconducting ring shaped like a washer, current sometimes flows more easily in one direction than the other. This property is called the diode effect. Usually, its direction is rigidly set by the material's structure. But in new work, physicists showed that it can be switched, and the switch is the number of magnetic vortices trapped in the ring.
Each vortex is a tiny region where superconductivity is suppressed and magnetic field penetrates. Vortices respond to the slightest irregularities in the material: here the edge is a bit thicker, there an impurity. The authors found that vortices act as a filter, amplifying certain "notes" of these irregularities and muffling others. When the number of vortices changes from even to odd, the set of amplified notes flips, and the diode effect changes sign.
Calculations on a simplified edge model and on a full two-dimensional lattice confirmed that the mechanism works flawlessly. This matters because previously similar behavior was attributed to special particles — Majorana fermions. Now it is clear that geometry and inhomogeneities alone produce the same effect. So engineers get a simple tool, a vortex switch, for superconducting electronics, where diodes are needed for logic circuits and memory.
A magnetic vortex in a superconductor carries a quantum of magnetic flux — about 2×10⁻¹⁵ webers. That's like squeezing all of Earth's magnetic field into a point the size of an atom.