Doping dependence of spin excitations and its correlations with high-temperature superconductivity in iron pnictides

Since spin excitations may be responsible for electron pairing and superconductivity in iron pnictides, it is important to determine their electron/hole-doping evolution and connection with superconductivity. Here we use inelastic neutron scattering to show that while electron doping to the antiferromagnetic BaFe$_2$As$_2$ parent compound modifies the low-energy spin excitations and their correlation with superconductivity ($<50$ meV) without affecting the high-energy spin excitations ($>100$ meV), hole-doping suppresses the high-energy spin excitations and shifts the magnetic spectral weight to low-energies. In addition, our absolute spin susceptibility measurements for the optimally hole-doped iron pnictide reveal that the change in magnetic exchange energy below and above Tc can account for the superconducting condensation energy. These results suggest that high-Tc superconductivity in iron pnictides is associated with both the presence of high-energy spin excitations and a coupling between low-energy spin excitations and itinerant electrons.