Relativistic GW plus BSE study of the optical properties of Ruddlesden-Popper iridates

We study the optical properties of the Ruddlesden-Popper series of iridates Sr(n+1)Ir(n)O3(n+1) (n = 1, 2, and infinity) by solving the Bethe-Salpeter equation (BSE), where the quasiparticle (QP) energies and screened interactions W are obtained by the G W approximation including spin-orbit coupling. The computed optical conductivity spectra show strong excitonic effects and reproduce very well the experimentally observed double-peak structure, in particular for the spin-orbital Mott insulators Sr-2 IrO4 and Sr3Ir2O7. However, G W does not account well for the correlated metallic state of SrIrO3 owing to a much too small band renormalization, and this affects the overall quality of the optical conductivity. Our analysis describes well the progressive redshift of the main optical peaks as a function of dimensionality (n), which is correlated with the gradual decrease of the electronic correlation (quantified by the constrained random phase approximation) towards the metallic n = infinity limit. We have also assessed the quality of a computationally cheaper BSE approach that is based on a model dielectric function and conducted on top of DFT+U one-electron energies. Unfortunately, this model BSE approach does not accurately reproduce the outcome of the full GW+BSE method and leads to larger deviations to the measured spectra.