We investigated the electronic structure of $\mathrm{Ir}{\mathrm{O}}_2$ to address the controversy regarding spin-orbit coupling (SOC) effects in metallic $5d$ transition-metal oxides. Two issues have come to the forefront: (1) SOC effects on electronic structure and physical properties of $\mathrm{Ir}{\mathrm{O}}_2$ and (2) the possible formation of a novel ground state in this material, the $J_{\mathrm{eff}}=1/2$ state. To better understand the SOC mechanism, we grew epitaxial $\mathrm{Ir}{\mathrm{O}}_2$ films whose $dc$ resistivity values were comparable with those of a single crystal. We obtained polarization-dependent optical and x-ray absorption spectra (XAS) and compared these results with those acquired using the generalized gradient approximation (GGA) and $\mathrm{GGA}+\mathrm{SOC}$ calculations. From the optical spectra, peak structures were identified at 0.4 and 2.0 eV, which could only be explained using the $\mathrm{GGA}+\mathrm{SOC}$ calculation. This suggests that SOC plays an important role in the electronic structure of $\mathrm{Ir}{\mathrm{O}}_2$. From the polarization-dependent $\mathrm{O}1s$ XAS spectra, we observed that the empty state near the Fermi level lacks involvement of an Ir $d_{xy}$ orbital. Despite the importance of SOC in $\mathrm{Ir}{\mathrm{O}}_2$, the $J_{\mathrm{eff}}=1/2$ state does not form in metallic $\mathrm{Ir}{\mathrm{O}}_2$.

• Received 4 November 2015

DOI:https://doi.org/10.1103/PhysRevB.93.045104