Crystal-Field Splitting and Correlation Effect on the Electronic Structure of A 2 IrO 3

Abstract

The electronic structure of the honeycomb lattice iridates Na 2 IrO 3 and Li 2 IrO 3 has been investigated using resonant inelastic x-ray scattering (RIXS). Crystal-field-split d − 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 eff = 3 / 2 states, is much smaller than the typical spin-orbit energy scale in iridates, validating the applicability of j eff physics in A 2 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 Na 2 IrO 3 and Li 2 IrO 3 can be described as spin-orbit Mott insulators, similar to the square lattice iridate Sr 2 IrO 4 .