Optical properties from 0.2 to 6.5 eV of epitaxial ${\mathrm{NbO}}_2$ $(4d^1 \text{system})$ films in their insulating states have been investigated by spectroscopic ellipsometry. The optical spectra are compared to that of epitaxial ${\mathrm{VO}}_2$ $(3d^1 \text{system})$ in its insulating phase. Both compounds are insulators at room temperature and undergo temperature-induced metal-insulator transitions. We find a $d_{\left|\right|}-e_{\mathrm{g}}^{\pi }$ orbital splitting energy of $\sim 1.6$ eV in ${\mathrm{NbO}}_2$ compared to $\sim 1.3$ eV in ${\mathrm{VO}}_2$; orbital splitting is a key ingredient that stabilizes the cation-dimerized insulating states of both materials. The edge of the $\text{O}2p$-like valence band is estimated to be 3.2 eV below the Fermi level through x-ray photoelectron spectroscopy measurements, suggesting that electrons from $\text{O}2p$ states do not contribute to absorption below this energy. This allows us to also assign an observed optical peak at $\sim 3.0$ eV to the $\mathrm{Nb}4d_{\left|\right|}\to 4d_{\left|\right|}^{*}$ transition.

• Received 27 June 2014
• Revised 24 August 2014

DOI:https://doi.org/10.1103/PhysRevB.90.115135