Spin-orbit coupling induced band structure change and orbital character of epitaxial IrO2 films

Abstract

We investigated the electronic structure of IrO2 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 IrO2 and (2) the possible formation of a novel ground state in this material, the J(eff) = 1/2 state. To better understand the SOC mechanism, we grew epitaxial IrO2 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 GGA + SOC calculations. From the optical spectra, peak structures were identified at 0.4 and 2.0 eV, which could only be explained using the GGA + SOC calculation. This suggests that SOC plays an important role in the electronic structure of IrO2. From the polarization-dependent 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 IrO2, the J(eff) = 1/2 state does not form in metallic IrO2.