Coupled spin–orbital fluctuations in a three orbital model for 4d and 5d oxides with electron fillings n = 3, 4, 5—application to NaOsO3, Ca2RuO4 and Sr2IrO4

Abstract

A unified approach is presented for investigating coupled spin–orbital fluctuations within a realistic three-orbital model for strongly spin–orbit coupled systems with electron fillings n = 3, 4, 5 in the t 2g sector of d yz , d xz , d xy orbitals. A generalized fluctuation propagator is constructed which is consistent with the generalized self-consistent Hartree–Fock approximation where all Coulomb interaction contributions involving orbital diagonal and off-diagonal spin and charge condensates are included. Besides the low-energy magnon, intermediate-energy orbiton and spin–orbiton, and high-energy spin–orbit exciton modes, the generalized spectral function also shows other high-energy excitations such as the Hund’s coupling induced gapped magnon modes. We relate the characteristic features of the coupled spin–orbital excitations to the complex magnetic behavior resulting from the interplay between electronic bands, spin–orbit coupling, Coulomb interactions, and structural distortion effects, as realized in the compounds NaOsO3, Ca2RuO4, and Sr2IrO4.