Effect of electron correlations on magnetic excitations in the isovalently doped iron-based superconductor Ba(Fe_1-xRu_x) ₂As₂

Magnetic correlations in isovalently doped Ba(Fe_1-xRu _x)₂As₂ (x=0.25, T_c=14.5 K; x=0.35, T_c=20 K) are studied by elastic and inelastic neutron scattering techniques. A relatively large superconducting spin gap accompanied by a weak resonance mode is observed in the superconducting state in both samples. In the normal state, the magnetic excitation intensity is dramatically reduced with increasing Ru doping toward the optimally doped regime. Our results favor that the weakening of the electron-electron correlations by Ru doping is responsible for the dampening of the resonance mode, as well as the suppression of the normal state antiferromagnetic correlations near the optimally doped regime in this system.