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

TL;DR

This study investigates EuRbFe$_4$As$_4$, revealing that its Eu$^{2+}$ magnetic planes exhibit a 2D XY-magnet behavior with a Berezinskii-Kosterlitz-Thouless transition, coexisting with superconductivity below 36.5 K.

Contribution

It provides experimental evidence of topological magnetic order coexisting with superconductivity in EuRbFe$_4$As$_4$, highlighting the decoupling of magnetic and superconducting layers.

Findings

Superconductivity persists below 36.5 K with weak influence from Eu$^{2+}$ magnetic order.

Eu$^{2+}$ planes behave as a 2D XY-magnet with a BKT transition.

Magnetic and superconducting properties are decoupled in this compound.

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.