We have investigated the excitation spectra of $j_{\mathrm{eff}}=\frac{1}{2}$ Mott insulator ${\mathrm{Na}}_2$${\mathrm{IrO}}_3$. Taking into account a relativistic multiplet structure of Ir ions, we have calculated the optical conductivity $\sigma \left(\omega \right)$ and resonant inelastic x-ray scattering (RIXS) spectra, which manifest different features from those of a canonical $j_{\mathrm{eff}}=\frac{1}{2}$ system ${\mathrm{Sr}}_2$${\mathrm{IrO}}_4$. Distinctly from the two-peak structure in ${\mathrm{Sr}}_2$${\mathrm{IrO}}_4$, $\sigma \left(\omega \right)$ in ${\mathrm{Na}}_2$${\mathrm{IrO}}_3$ has a broad single peak dominated by interband transitions from $j_{\mathrm{eff}}=\frac{3}{2}$ to $\frac{1}{2}$. RIXS spectra exhibit the spin-orbit (SO) exciton that has a two-peak structure arising from the crystal-field effect, and the magnon peak at energies much lower than in ${\mathrm{Sr}}_2$${\mathrm{IrO}}_4$. In addition, a small peak near the optical-absorption edge is found in RIXS spectra, originating from the coupling between the electron-hole ($e$-$h$) excitation and the SO exciton. Our findings corroborate the validity of the relativistic electronic structure and importance of both itinerant and local features in ${\mathrm{Na}}_2$${\mathrm{IrO}}_3$.

• Received 1 July 2013
• Revised 19 October 2013

DOI:https://doi.org/10.1103/PhysRevB.89.081109