A diode passes current in one direction and blocks it in the other. In superconductors, a superconducting diode would pass supercurrent in one direction while allowing resistance in the reverse direction — enabling dissipationless nonreciprocal electronics. But the mechanism has been debated: what breaks the symmetry between forward and reverse current?
de Melo Junior and colleagues (arXiv:2602.20339) answer this with patterned asymmetric antidots — holes deliberately punched into a niobium film in asymmetric arrangements. The antidots create a landscape of pinning sites for magnetic flux vortices. When current flows, the vortices' response differs depending on current direction because the pinning landscape is asymmetric. One direction pushes vortices against strong pinning sites (high critical current); the reverse pushes them against weak ones (low critical current).
Two regimes emerge depending on field direction: edge flux pinning dominates at lower fields and in-plane orientations, while bulk flux pinning governs high-field responses. The diode efficiency — how different the critical currents are in each direction — is determined by the specific pinning landscape. Time-dependent Ginzburg-Landau simulations and an analytical model provide a unified description.
The design principle is clear: engineer the asymmetry of the pinning landscape, and the diode effect follows. The material is symmetric (niobium is centrosymmetric); the asymmetry is entirely in the pattern. No exotic materials, no spin-orbit coupling, no broken inversion symmetry at the crystal level — just geometry.
The general observation: nonreciprocal behavior requires broken symmetry, but the symmetry can be broken at the level of engineered structure rather than fundamental material properties. The pattern is the diode.