Electronic Structure and Coexistence of Superconductivity with Magnetism in RbEuFe4As4

In the novel stoichiometric iron-based material RbEuFe4As4, superconductivity coexists with a peculiar long-range magnetic order of Eu 4f states. Using angle-resolved photoemission spectroscopy, we reveal a complex three-dimensional electronic structure and compare it with density functional theory calculations. Multiple super-conducting gaps were measured on various sheets of the Fermi surface. High-resolution resonant photoemission spectroscopy reveals magnetic order of the Eu 4f states deep into the superconducting phase. Both the absolute values and the anisotropy of the superconducting gaps are remarkably similar to the sibling compound without Eu, indicating that Eu magnetism does not affect the pairing of electrons. A complete decoupling between Fe-and Eu-derived states was established from their evolution with temperature, thus unambiguously demonstrating that superconducting and a long-range magnetic orders exist independently from each other. The established electronic structure of RbEuFe4As4 opens opportunities for the future studies of the highly unorthodox electron pairing and phase competition in this family of iron-based superconductors with doping. We thank Matthew Watson for his critical reading of the manuscript. We thank Diamond Light Source for access to beamline I05 (Proposal No. SI15074 and No. SI19041) that contributed to the results presented here. Work was done using equipment from the LPI Shared Facility Center. K.S.P. and V.M.P. acknowledge support by the Russian Scientific Foundation (RSF Project No. 21-12-00394). A.V.S. and A.S.U. acknowledge support by the Russian Foundation for Basic Research (Project No. 21-52-12043). E.V.C. acknowledges funding by Saint Petersburg State University project for scientific investigations (ID No. 73028629). S.V.E. acknowledges support from the government research assignment for ISPMS SB RAS (Project FWRW-2019-0032). R.V. acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG) TRR 288 (Project A05). V.B. thanks the Goethe University Frankfurt for computational resources and Daniel Guterding for providing the FS plotting software. K.K. thanks M. Valvidares, J. Herrero, H. B. Vasili, S. Agrestini, and N. Brookes for their support during the XMCD experiment at ALBA via IHR Proposal 2019063615. D.V.V. also acknowledges support from the Spanish Ministry of Economy (MAT-2017-88374-P)