Chunhui Rita Du

Two magnets on a table either attract or repel — depending on how their poles are oriented. Inside each piece hide tiny regions where all the magnetic arrows point in the same direction. These regions are called domains, and their arrangement determines how the material behaves in an electrical circuit. In altermagnets, a new class of materials, domains are trickier.

A compass needle trembles when an invisible current passes nearby. Physicists built a tiny compass from a single defect in diamond and brought it close to the edge of a superconductor to listen to the electric currents running there. These currents are unlike ordinary ones: they flow only along the boundary, like water along a riverbank, and do not scatter off obstacles.