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 Eu2+ 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 Eu2+-planes are a good realization of a two-dimensional XY-magnet, which reflects the decoupling of the Eu2+ magnetic moments from superconducting FeAs-layers.
@article{arxiv.2010.02110,
title = {Topological magnetic order and superconductivity in EuRbFe$_4$As$_4$},
author = {M. Hemmida and N. Winterhalter-Stocker and D. Ehlers and H. -A. Krug von Nidda and M. Yao and J. Bannies and E. D. L. Rienks and R. Kurleto and C. Felser and B. Büchner and J. Fink and S. Gorol and T. Förster and S. Arsenijevic and V. Fritsch and P. Gegenwart},
journal= {arXiv preprint arXiv:2010.02110},
year = {2021}
}