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

Abstract

We study single crystals of the magnetic superconductor EuRbFe4As4 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.

Spin waves and high-frequency response in layered superconductors with helical magnetic structure

“…On the other hand, the magnetic transition temperature 15 K is 4 K lower than in the parent 122 compound, probably due to the weaker interaction between the magnetic layers. These materials are characterized by highly-anisotropic easy-axis Eu magnetism [41,44,45]. With increasing pressure the superconducting transition temperature decreases and the magnetic transition temperature increases so that at pressures larger that ∼ 7 GPa superconducting transition already takes place in the magnetically-ordered state [46,47].…”