The electronic structure of the honeycomb lattice iridates ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ and ${\mathrm{Li}}_2{\mathrm{IrO}}_3$ has been investigated using resonant inelastic x-ray scattering (RIXS). Crystal-field-split $d\mathrm{\text{−}}d$ excitations are resolved in the high-resolution RIXS spectra. In particular, the splitting due to noncubic crystal fields, derived from the splitting of $j_{\mathrm{eff}}=3/2$ states, is much smaller than the typical spin-orbit energy scale in iridates, validating the applicability of $j_{\mathrm{eff}}$ physics in $A_2{\mathrm{IrO}}_3$. We also find excitonic enhancement of the particle-hole excitation gap around 0.4 eV, indicating that the nearest-neighbor Coulomb interaction could be large. These findings suggest that both ${\mathrm{Na}}_2{\mathrm{IrO}}_3$ and ${\mathrm{Li}}_2{\mathrm{IrO}}_3$ can be described as spin-orbit Mott insulators, similar to the square lattice iridate ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$.

• Received 21 September 2012

DOI:https://doi.org/10.1103/PhysRevLett.110.076402