Resonant x-ray diffraction experiments were performed for the metallic iridium oxide IrO$_{ 2}$. We observed anisotropic tensor of susceptibility (ATS) scattering, the spectrum of which shows a sharp contrast between the $L_{ 3}$ and $L_{ 2}$ edges. At the $L_{ 3}$ edge, resonance excitations were clearly observed from the core 2$p$ orbitals to both the 5$d$ $t_{ 2g}$ and $e_{ g}$ orbitals. In contrast, the resonance mode associated with 5$d$ $t_{ 2g}$ orbitals was indiscernible at the $L_{ 2}$ edge. This contrasting behavior indicates that Ir 5$d$ $t_{ 2g}$ orbitals are fairly close to the $J_{ \rm eff}$ = 1/2 state due to the strong spin--orbit coupling in 5$d$ transition metal ions, as in the Mott insulator Sr$_{ 2}$IrO$_{ 4}$. Our results clearly demonstrate that ATS scattering is a useful probe for investigating complex orbital states in a metallic state. Such states induce novel phenomena such as the spin Hall effect.
Categories: cond-mat.str-el