Unexpected fourfold symmetry in the resistivity of patterned superconductors

We report the magneto-optical observation of a surprising fourfold symmetry of the flux penetration in a superconducting YBa 2 Cu 3 O 7 - δ thin-film disk containing a square array of antidots, leading to an angular variation of the critical current by a factor of nearly 2. This behavior is explained using a vortex channeling model. Potential applications in superconducting devices are discussed.