The crystal Ba4Ru3O10 was long considered a simple antiferromagnet: when cooled to 100 K, the spins of ruthenium atoms align in opposite directions, like rows of arrows pointing at each other. But new measurements show that at the same temperature, something more happens — a hidden charge density wave is born inside the material. Electrons, like a crowd in a stadium, begin to sway in sync, creating periodic regions of higher and lower density.

At 20 K, this wave starts to move, but not smoothly: it moves in jerks. The current through the crystal flows in steps, with thresholds below which no current flows at all, and under certain conditions the resistance becomes negative, as if a cart on a hill suddenly accelerates on its own. This resembles a zipper: the teeth catch, and you need to apply force to move the slider.

Remarkably, all these effects disappear if just 3% of the ruthenium is replaced with iridium. The crystal structure and insulating state remain, but the collective wave vanishes. So it's not about heating or defects, but about a delicate electronic tuning of the material. Ba4Ru3O10 turns out to be a rare example of a system where antiferromagnetism and a charge density wave are intertwined like threads in a tapestry.