A Spiral of Magnet and Superconductor express

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Inside the EuRbFe4As4 crystal, layers of iron and arsenic alternate with spacers of europium and rubidium. When cooled below 15 Kelvin (–258°C), europium atoms align their magnetic moments into a spiral: each successive layer is rotated by 90 degrees, like steps of a spiral staircase. It is in this state that the strange superconducting ripple is born: electron pairs, which usually carry current without resistance equally in all directions, here bunch into stripes along one direction.

The key lies in the iron layers. They are asymmetric: the iron atom lacks an inversion center, and electrons experience Rashba spin-orbit interaction. This is like a runner feeling a crosswind: their trajectory is slightly curved depending on the spin direction. The europium magnetic spiral acts as a rotating field, and together with the Rashba effect, it creates an effective gauge field for Cooper pairs (bound electrons). This field acts selectively: it affects only electrons with a certain orbital symmetry: those that resemble dumbbells oriented along the x and y axes.

This momentum shift gives rise to spatial modulation — the superconducting order parameter oscillates in space with a period of about 8 nanometers, matching experiment. Moreover, the theory predicts that spontaneous loop currents should flow between layers — tiny vortices that could be detected by muon relaxation or scanning SQUID microscopy. If found, this would be direct confirmation of the mechanism.