Coexistence of Insulatorlike Paramagnon and Metallic Spin-Orbit Exciton Modes in SrIrO 3

We probe the spectrum of elementary excitations in SrIrO₃ by using heterostructured [(SrIrO₃)ₘ/(SrTiO₃)ₗ] samples to approach the bulk limit. Our resonant inelastic x-ray scattering (RIXS) measurements at the Ir L₃ edge reveal a robust low-lying collective magnetic mode with an antiferromagnetic (AFM) dispersion similar to the insulators Sr₂IrO₄ and Sr₃Ir₂O₇, albeit with a large gap and much larger linewidth. At higher energies we find the spin-orbit exciton, also strongly broadened, but with an inverted dispersion and doubled periodicity that are controlled by the charge hopping. These results demonstrate that the AFM paramagnon persists, somewhat counterintuitively, far into the metallic regime of the insulator-metal transition driven by the degree of confinement in the heterostructure. We conclude that these two excitations, which are contrasting but coexisting hallmarks of strong AFM pseudospin and charge fluctuations in a spin-orbit-coupled Mott-Slater material, are properties intrinsic to the ground state of semimetallic perovskite SrIrO₃.