Abstract
Magnetic excitations and magnetic structure of EuRbFe4As4 were investigated by inelastic neutron scattering (INS), neutron diffraction, and random phase approximation (RPA) calculations. Below the superconducting transition temperature Tc=36.5 K, the INS spectra exhibit the neutron spin resonances at Qres=1.27(2) and 1.79(3)Å−1. They correspond to the Q=(0.5,0.5,1) and (0.5,0.5,3) nesting wave vectors, showing three-dimensional nature of the band structure. The characteristic energy of the neutron spin resonance is Eres=17.7(3) meV corresponding to 5.7(1)kBTc. Observation of the neutron spin resonance mode and our RPA calculations in conjunction with the recent optical conductivity measurements are indicative of the s± superconducting pairing symmetry in EuRbFe4As4. In addition to the neutron spin resonance mode, upon decreasing temperature below the magnetic transition temperature TN=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. Single-crystal neutron diffraction measurements demonstrate that the magnetic propagation vector of the Eu sublattice is k=(0,0,0.25), representing the three-dimensional antiferromagnetic order. Linear spin wave calculations assuming the obtained magnetic structure with the intra- and interplane nearest neighbor exchange couplings of J1/kB=−1.31 K and Jc/kB=0.08 K can reproduce quantitatively the observed spin wave excitation. Our results show that superconductivity and long-range magnetic order of Eu coexist in EuRbFe4As4, whereas the coupling between them is rather weak.
