Anomalies in the electronic structure of a 5$d$ transition metal oxide, IrO$_2$

Ir-based materials have drawn much attention due to the observation of insulating phase believed to be driven by spin-orbit coupling while Ir 5$d$ states are expected to be weakly correlated due to their large orbital extensions. IrO$_2$, a simple binary material, shows metallic ground state which seems to deviate from the behavior of most other Ir-based materials and varied predictions in these material class. We studied the electronic structure of IrO$_2$ at different temperatures employing high resolution photoemission spectroscopy with photon energies spanning from ultraviolet to hard $x$-ray range. Experimental spectra exhibit a signature of enhancement of Ir-O covalency in the bulk compared to the surface electronic structure. The branching ratio of the spin-orbit split Ir core level peaks is found to be larger than its atomic values and it enhances further in the bulk electronic structure. Such deviation from the atomic description of the core level spectroscopy manifests the enhancement of the orbital moment due to the solid state effects. The valence band spectra could be captured well within the density functional theory. The photon energy dependence of the features in the valence band spectra and their comparison with the calculated results show dominant Ir 5$d$ character of the features near the Fermi level; O 2$p$ peaks appear at higher binding energies. Interestingly, the O 2$p$ contributions of the feature at the Fermi level is significant and it enhances at low temperatures. This reveals an orbital selective enhancement of the covalency with cooling which is an evidence against purely spin-orbit coupling based scenario proposed for these systems.