Topological magnetic order and superconductivity in EuRbFe${}_4$As${}_4$

Abstract

We study single crystals of the magnetic superconductor EuRbFe${}_4$As${}_4$ by magnetization, electron spin resonance (ESR), angle-resolved photoemission spectroscopy (ARPES) and electrical resistance in pulsed magnetic fields up to 630 kOe. The superconducting state below 36.5 K is almost isotropic and only weakly affected by the development of 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 Eu${}^{2+}$-planes are a good realization of a two-dimensional XY-magnet, which reflects the decoupling of the Eu${}^{2+}$ magnetic moments from superconducting FeAs-layers.