Magnetic couplings, optical spectra, and spin-orbit exciton in 5d electron Mott insulator Sr2IrO4

Based on the microscopic model including strong spin-orbit coupling, on-site Coulomb and Hund's interactions, as well as crystal field effects, we have investigated magnetic and optical properties of Sr2IrO4. Taking into account all possible intermediate state multiplets generated by virtual hoppings of electrons, we calculated the isotropic Heisenberg, pseudodipolar, and Dzyaloshinsky-Moriya coupling constants, which describe the experiment quite well. We have also evaluated the optical conductivity {\sigma}({\omega}) by employing the exact diagonalization method on small clusters, and obtained two peaks at \sim 0.5 and \sim 1.0 eV in agreement with experiment. The two peak structure of {\sigma}({\omega}) arises from the unusual Fano-type overlap between electron-hole continuum of the Jeff = 1/2 band and the intrasite spin-orbit exciton observed recently in Sr2IrO4 by resonant x-ray scattering.