Beom Hyun Kim, G. Khaliullin, B. I. Min
We have investigated excitation spectra of the edge-shared $j_{eff}$=${1}{2}$ Mott insulator Na$_2$IrO$_3$. Taking into account strong relativistic multiplet structures of local Ir ions, we have calculated optical conductivity ($\sigma(\omega)$) and resonant inelastic x-ray scattering (RIXS) spectra, which manifest somewhat different features from those of the corner-shared $j_{eff}$=${1}{2}$ Mott insulator Sr$_2$IrO$_4$. Distinctly from two-peak structure in Sr$_2$IrO$_4$, $\sigma(\omega)$ in Na$_2$IrO$_3$ has a one-peak-like structure, the spectral weight of which comes mostly from the electron-hole ($e$-$h$) excitations of $j_{eff}$=${3}{2}$. RIXS spectra in Na$_2$IrO$_3$ exhibit the spin-orbit (SO) excitation that has a two-peak structure arising from the trigonal crystal-field effect, in addition to low energy magnetic excitation that is much narrower than that in Sr$_2$IrO$_4$. A small peak near the $e$-$h$ excitation edge in RIXS spectra originates from the coupling between the $e$-$h$ excitation and the SO excitation. Our findings corroborate the validity of relativistic electronic structure in Na$_2$IrO$_3$.
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http://arxiv.org/abs/1307.0614
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