Eu-containing iron-based superconductors are outstanding materials because the system shows both superconductivity in the FeAs layers and local-moment long-range magnetic order in the Eu sublattice. EuRbFe₄As₄ [K. Kawashima et al., J. Phys. Soc. Jpn. 85, 064710 (2016).] is such a compound in the absence of chemical disorder. In EuRbFe₄As₄, the neutron spin resonance which gives the crucial information on the microscopic mechanism underlying the superconductivity as well as the magnetic structure and the spin Hamiltonian of the Eu sublattice have never been reported yet by means of the microscopic techniques. To investigate the neutron spin resonance and the magnetic structure, we performed inelastic neutron scattering (INS) and neutron diffraction measurements [K. Iida et al., Phys. Rev. B 100, 014506 (2019).].

Below the superconducting transition temperature T_c = 35 K, the INS spectra exhibit the neutron spin resonance at Q_res = 1.27(2) and 1.79(3) Å^-1, which correspond to the (0.5, 0.5, 1) and (0.5, 0.5, 3) nesting wave vectors [Fig. 1(a)]. The characteristic energy of the spin resonance is E_res = 17.7(3) meV corresponding to 5.7(1)k_BT_c. Observation of the spin resonance and our random phase approximation calculations suggest the superconducting order parameter in EuRbFe₄As₄ is the s_+/- pairing symmetry. In addition to the neutron spin resonance, upon decreasing temperature below the magnetic transition temperature T_N = 15 K, the spin wave excitation originating in the long-range magnetic order of the Eu sublattice was observed in the low-energy inelastic channel [Fig. 1(b)]. Linear spin wave calculations can reproduce quantitatively the observed spin wave excitation. Single-crystal neutron diffraction measurements demonstrate that the magnetic propagation vector of the Eu sublattice is k = (0, 0, 0.25) [Fig. 1(c)], indicating the three-dimensional antiferromagnetic order. Our results show that superconductivity and long-range magnetic order coexist in EuRbFe₄As₄ whereas the coupling between them is rather weak.

In my talk, I will also review for recent results on EuRbFe₄As₄.