Topological magnetic order and superconductivity in EuRbFe4As4

We study single crystals of the magnetic superconductor $\mathrm{Eu}\mathrm{Rb}{\mathrm{Fe}}₄{\mathrm{As}}₄$ by magnetization, electron spin resonance (ESR), angle-resolved photoemission spectroscopy, and electrical resistance in pulsed magnetic fields up to 63 T. The superconducting state below 36.5 K is almost isotropic and is only weakly affected by the development of ${\mathrm{Eu}}²⁺$ magnetic order at 15 K. On the other hand, for the external magnetic field applied along the $c$ axis the temperature dependence of the ESR linewidth reveals a Berezinskii-Kosterlitz-Thouless topological transition below 15 K. This indicates that ${\mathrm{Eu}}²⁺$ planes are a good realization of a two-dimensional $\mathit{XY}$ magnet, which reflects the decoupling of the ${\mathrm{Eu}}²⁺$ magnetic moments from superconducting FeAs layers.