Complex orbital state stabilized by strong spin-orbit coupling in a metallic iridium oxide IrO₂

Resonant x-ray diffraction experiments were performed for the metallic iridium oxide IrO₂. We observed anisotropic tensor of susceptibility (ATS) scattering, the spectrum of which shows a sharp contrast between the L₃ and L₂ edges. At the L₃ edge, resonance excitations were clearly observed from the core 2p orbitals to both the 5d t_2g and e_g orbitals. In contrast, the resonance mode associated with 5d t_2g orbitals was indiscernible at the L ₂ edge. This contrasting behavior indicates that Ir 5d t_2g orbitals are fairly close to the J_eff=1/2 state due to the strong spin-orbit coupling in 5d transition-metal ions, as in the Mott insulator Sr₂IrO₄. Our results clearly demonstrate that ATS scattering is a useful probe for investigating complex orbital states in a metallic material, where novel phenomena such as the spin Hall effect are expected.