An ordinary conductor carries current equally in both directions, like a door that swings both inward and outward. A diode, by contrast, is a door with a self-closing hinge: it opens easily one way but pushes back the other. Scientists have found that a superconductor based on copper oxide (Tl₂Ba₂CaCu₂O₈) behaves like such a door at 100 K (−173 °C), even though no magnet was nearby.

The researchers fabricated a microbridge from this compound and measured how it conducts current. It turned out that in one direction the current flows freely, while in the other it struggles. Moreover, the picture did not change even when a weak magnetic field of up to ±100 oersteds was applied. Such behavior indicates that time-reversal symmetry is broken inside the superconductor: if you could "rewind" the processes, the picture would not match.

This is already the second such observation: previously, the diode effect without a field was noticed in a similar compound Bi₂Sr₂CaCu₂O₈₊δ. So, time-reversal symmetry breaking may be a common property of all cuprate superconductors. And that is a serious blow to theories that explained high-temperature superconductivity without accounting for such breaking.