Dispersive high-energy spin excitations in iron pnictide superconductors investigated with RIXS

The discovery of iron-based high temperature superconductivity has triggered tremendous research efforts in searching for novel high-T_c superconductors. Unlike cuprates, which have long-range ordered antiferromagnetic Mott insulators as parent compounds, the parent compounds of iron-based superconductors are spin-density wave metals with delocalized electronic structure and more itinerant magnetism. Recent developments of the high-resolution resonant inelastic X-ray scattering (RIXS) technique have enabled investigations of magnetic excitations in cuprates, which show excellent agreement with results from Inelastic Neutron Scattering. In this presentation we demonstrate that RIXS can be used to measure collective magnetic excitations in iron-based superconductors despite their much stronger itinerancy compared to cuprates. The persistence of high-energy spin excitations even in optimally doped pnictide superconductors in a wide range of temperatures strongly suggests a spin-mediated Cooper pairing mechanism as proposed in cuprate superconductors.